Count Rumford, Faraday readings

Count Rumford, Faraday readings - 8 THE DEATH OF CERTAINTY:...

Info iconThis preview shows pages 1–7. Sign up to view the full content.

View Full Document Right Arrow Icon
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 2
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 4
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 6
Background image of page 7
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: 8 THE DEATH OF CERTAINTY: SCIENCE AND WAR % 8.1 Count Benjamin Thompson Rumford, An Experimental Inquiry Concerning the Source of the Heat which is Excited by Friction Count Rumford (Benjamin Thompson) (1753—1814) was born in North Wolmrn, Massa- chusetts and received a spotty education. He was a man with a wildly varied career that included spying for Britain, service to the Elector of Bavaria (who created him Count Rumford of the Holy Roman Empire), building a better fireplace, marry— ing Lavoisier’s widow, and overseeing the produc— tion of cannons. It was Rumford’s observation of cannon-boring equipment that led him to reject Lavoisier’s substance of heat (caloric). He observed that a dull drill bit, although spinning indefinitely, could not cut through the metal of a cannon, but would continue to produce heat. In 1798, he pull— lished An Experimental Inquiry Concerning the Source of Heat Excited by Friction, which became a classic in physics. AN INQUIRY CONCERNING THE SOURCE OF HEAT WHICH Is EXCITED BY FRICTION It frequently happens that in the ordinary affairs and occupations of life, opportunities present themselves of contemplating some of the most curious opera tions of Nature; and very interesting philosophical experiments might Often be made, almost without trouble or expense, by means of machinery con- trived for the mere mechanical purposes of the arts and manufactures. I have frequently had occasion to make this observation; and am persuaded that a habit of 292 keeping the eyes Open to everything that is going on in the ordinary course of the business of life has oftener led, as it were by accident, or in the playful excursions of the imagination, put into action by contemplating the most common appearances, to useful doubts and sensible schemes for investigation and improvement, than all the more intense medita- tions of philosophers in the hours expressly set apart for study. It was by accident that I was led to make the experiments of which I am about to give an account; and, though they are not perhaps of sufficient impor- tance to merit so formal an introduction, I cannot help flattering myself that they will be thought curious in several respects, and worthy of the honour of being made known to the Royal Society. Being engaged lately in superintending the boring of cannon in the workshops of the military arsenal at Mimich, l was struck with the very con- siderable degree of Heat which a brass gun acquires in a short time in being bored, and with the still more intense Heat (much greater than that of boiling water, as I found by experiment) of the metallic chips separated from it by the borer. The more I meditated on these phenomena, the more they appeared to me to be curious and inter- esting. A thorough investigation of them seemed even to bid fair to give a farther insight into the hidden nature of Heat; and to enable us to form some reasonable conjectures respecting the exis~ fence, or non-existence, of an igneous fluid—a subject on which the opinions of philosophers have in all ages been much divided. In order that the Society may have clear and dis- tinct ideas of the speculations and reasonings to which these appearances gave rise in my mind, and also of the specific objects of philosophical investiga- BENJAMIN RUMFORD, THE SOURCE OF THE HEAT WHICH IS EXCITED BY FRICTION 293 tion they suggested to me, I must beg leave to state them at some length, and in such manner as I shall think best suited to answer this purpose. From whence comes the Heat actually produced in the mechanical operation above mentioned? Is it fLu'nished by the metallic chips which are separated by the borer from the solid mass of metal? If this were the case, then, according to the modern doctrines of latent Heat, and of caloric, the capacity for Heat of the parts of the metal, so reduced to chips, ought not only to be changed, but the change undergone by them should be sufficiently great to account for all the Heat produced. But no such change had taken place; for I found, upon taking equal quantities, by weight, of these chips, and of thin slips of the same block of metal separated by means of a fine saw, and putting them at the same temperature (that of boiling water) into equal quantities of cold water (that is to say, at the temperature of 59 1/2° F), the portion of water into which the chips were put was not, to all appearance, heated either less or more than the other portion in which the slips of metal were put. This experiment being repeated several times, the results were always so nearly the same that I could not determine whether any, or what change had been produced in the metal, in regard to its capac— ity for Heat, by being reduced to chips by the borer.1 From hence it is evident that the Heat produced could not possibly have been furnished at the expense of the latent Heat of the metallic chips. But, not being willing to rest satisfied with these trials, however conclusive they appeared to me to be, I had recourse to the following still more decisive experiment. Taking a cannon (a brass six—pounder), cast solid, and rough as it came from the foundry, and fixing it (horizontally) in the machine used for boring, and at the same time finishing the outside of the cannon by turning (see Fig. 2), I caused its extremity to be cut off, and, by turning down the metal in that part, a solid cylinder was formed, 71 inches in diameter, and 9 8/10 inches long, which, When finished, remained joined to the rest of the metal (that which, properly speaking, constituted the cannon) by a small cylindrical neck, only 2 1/ 6 inches in diameter, and 3 8/ 10 inches long. This short cylinder, which was supported in its horizontal position and turned round its axis by means of the neck by which it remained united to the cannon, was now bored with the horizontal borer used in boring cannon; but its bore, which was 3.7 inches in diameter, instead of being continued through its whole length (9.8 inches) was only 7.2 inches in length; so that a solid bottom was left to this hollow cylinder, which bottom was 2.6 inches in thickness. This cavity is represented by dotted lines in Fig. 2; as also in Fig. 3, where the cylinder is represented on an enlarged scale. This cylinder being designed for the express purpose of generating Heat by friction, by having a blunt borer forced against its solid bottom at the same time that it should be turned round its axis by the force of horses, in order that the Heat accumu~ lated in the cylinder might from time to time be measured, a small round hole 0.37 Of an inch only in diameter, and 4.2 inches in depth, for the purpose of introducing a small cylindrical mercurial thermome- ter, was made in it, on one side, in a direction per- pendicular to the axis of the cylinder, and ending in the middle of the solid part of the metal which formed the bottom of its bore. The solid contents of this hollow cylinder, exclu- sive of the cylindrical neck by which it remained united to the cannon, were 385 8/4 cubic inches, English measure, and it weighed 113.13 1b., avoirdu- pois; as I found on weighing it at the end of the course of experiments made with it, and after it had been separated from the cannon with which, during the experiments, it remained connected. Experiment NO. 1 This experiment was made in order to ascertain how much Heat was actually generated by friction, when a blunt steel borer being so forcibly shoved (by means of a strong screw) against the bottom of the bore of the cylinder, that the pressure against it was equal to the weight of about 10,000 lb., avoirdupois, the cylin— der was turned round on its axis (by the force of horses) at the rate of about 32 times in a minute. This machinery, as it was put together for the experiment, is represented by Fig. 2 [not shown]. W is a strong horizontal iron bar, connected with _ proper machinery carried round by horses, by means Of which the cannon was made to turn round its axis. To prevent, as far as possible, the loss of any 294 THE DEATH OF CERTAINTYI SCIENCE AND WAR part of the Heat that was generated in the experi— ment, the cylinder was well covered up with a fit coating of thick and warm flannel, which was care- fully wrapped round it, and defended it on every side from the cold air of the atmosphere. This cover— ing is not represented in the drawing of the appara- tus, Fig. 2. I ought to mention that the borer was a flat piece of hardened steel, 0.63 of an inch thick, 4 inches long, and nearly as wide as the cavity of the bore of the cylinder, namely, 3 1/2 inches. Its corners were rounded off at its end, so as to make it fit the hollow bottom of the bore; and it was firmly fastened to the iron bar (711) which kept it in its place. The area of the surface by which its end was in contact with the bottom of the bore of the cylinder was nearly 2 1/ 3 inches. This borer, which is distinguished by the letter 71, is represented in most of the figures. At the beginning of the experiment, the temper— ature of the air in the shade, as also that of the cylin— der, was just 60° F. At the end of 30 minutes, when the cylinder had made 960 revolutions about its axis, the horses being stopped, a cylindrical mercurial thermometer, whose bulb was 32/100 of an inch in diameter, and 3 1/4 inches in length, was introduced into the hole made to receive it, in the side of the cylinder, when the mercury rose almost instantly to 130”. Though the Heat could not be supposed to be quite equally distributed in every part of the cylin— der, yet, as the length of the bulb of the thermometer was such that it extended from the axis of the cylin- der to near its surface, the Heat indicated by it could Thus at the end of 4. minutes not be very different from that of the mean tempera- ture of the cylinder; and it was on this accormt that a thermometer of that particular form was chosen for this experiment. To see how fast the Heat escaped out of the cylinder (in order to be able to make a probable con- jecture respecting the quantity given off by it during the time the Heat generated by the friction was accu- mulating), the machinery standing still, I suffered the thermometer to remain in its place near three quarters of an hour, observing and noting down, at small intervals of time, the height of the temperature indicated by it. Having taken away the borer, I now removed the metallic dust, or, rather, scaly matter, which had been detached from the bottom of the cylinder by the blunt steel borer, in this experiment; and, having carefully weighed it, I found its weight to be 837 grains, Troy. Is it possible that the very consid- erable quantity of Heat that was produced in this experiment (a quantity which actually raised the temperature of above 113 lb. of gim-metal at least 70 degrees of Fahrenheit’s thermometer, and which, of course, would have been capable of melting 6 1/2 lb. of ice, or of causing near 5 lb. of ice—cold water to boil) could have been furnished by so inconsiderable a quantity of metallic dust? and this merely in consequence of a change of its capacity for Heat? As the weight of this dust (837 grains, Troy) amounted to no more than 1 /947 the part of that of the cylinder, it must have lost no less than 948 The Heat. as shown b the thermometer, waxy . . . . . 126° after 5 minutes, always reckoning from the first observation at the end of 7 minutes . l 1 “ . 14 “I . 16 u 20 " . 24. “ . 28 “ . 31 a . 3+“-- u ' . and when 41 minutes had elapsed . . . . . 125 . . . . 123 . . . . . 120 . . . . 119 . . .. . . 118 . . . . 116 . . . _ . . 115 . . . . H4 . . . . . 113 . . _ . . 112 0 u . ‘ n . III e - e , H0 BENJAMIN RUMFORD, THE SOURCE OF THE HEAT WHICH IS EXCITED BY FRICTION 295 degrees of Heat, to have been able to have raised the temperature of the cylinder 1 degree; and conse~ quently it must have given off 66,360 degrees of Heat to have produced the effects which were actually found to have been produced in the experiment! But without insisting on the improbability of this supposition, we have only to recollect, that from the results of actual and decisive experiments, made for the express purpose of ascertaining that fact, the capacity for Heat of the metal of which great guns are cast is not sensibly changed by being reduced to the form of metallic chips in the operation of boring cannon; and there does not seem to be any reason to think that it can be much changed, if it be changed at all, in being reduced to much smaller pieces by means of a borer that is less sharp. If the Heat, or any considerable part of it, were produced in consequence of a change in the capacity for Heat of a part of the metal of the cylinder, as such change could only be superficial, the cylinder would by degrees be exhausted; or the quantities of Heat produced in any given short space of time would be found to diminish gradually in successive experi- ments. To find out if this really happened or not, I repeated the last—mentioned experiment several times with the utmost care; but I did not discover the smallest sign of exhaustion in the metal, notwith— standing the large quantities of Heat actually given off. Finding so much reason to conclude that the Heat generated in these experiments, or excited, as I would rather choose to express it, was not fur- nished at the expense of the latent Heat or combined caloric of the metal, I pushed my inquiries a step farther, and endeavoured to find out whether the air did, or did not, contribute anything in the gen— eration of it. Experiment No. 2 As the bore of the cylinder was cylindrical, and as the iron bar (171), to the end of which the blunt steel borer was fixed, was square, the air had free access to the inside of the bore, and even to the bottom of it, where the friction took place by which the Heat was excited. As neither the metallic chips produced in the ordinary course of the operation of boring brass cannon, nor the finer scaly particles produced in the last—mentioned experiments by the friction of the blunt borer, showed any signs of calcination, I did not see how the air could possibly have been the cause of the Heat that was produced; but, in an investigation of this kind, I thought that no pains should be spared to clear away the rubbish, and leave the subject as naked and open to inspection as possible. In order, by one decisive experiment, to deter- mine whether the air of the atmosphere had any part, or not, in the generation of the Heat, I contrived to repeat the experiment under circumstances in which it was evidently impossible for it to produce any efl'ect whatever. By means of a piston exactly fitted to the mouth of the bore of the cylinder, through the middle of which piston the square iron bar, to the end of which the blunt steel borer was fixed, passed in a square hole made perfectly air—tight, the access of the external air to the inside of the bore of the cylinder was effectually prevented. I did not find, however, by this experiment, that the exclusion of the air diminished, in the small— est degree, the quantity of Heat excited by the friction. There still remained one doubt, which, though it appeared to me to be so slight as hardly to deserve any attention, I was however desirous to remove. The piston which closed the mouth of the bore of the cylinder, in order that it might be air-tight, was fitted into it with so much nicety, by means of its collars of leather, and pressed against it with so much force, that, notwithstanding its being oiled, it occasioned a considerable degree of friction when the hollow cylinder was turned round its axis. Was not the Heat produced, or at least some part of it, occasioned by this friction of the piston? [A]nd, as the external air had free access to the extremity of the bore, where it came in contact with the piston, is it not possible that this air may have had some share in the generation of the Heat produced? Experiment N 0. 3 A quadrangular oblong deal box, watertight, 11 1/ 2 _ English inches long, 9 4/ 10 inches wide, and 9 6/ 10 inches deep (measured in the clear), being provided with holes or slits in the middle of each of its ends, just large enough to receive, the one the square iron rod to the end of which the blunt steel borer was fas- 296 THE DEATH OF CERTAINTY: SCIENCE AND WAR tened, the other the small cylindrical neck which joined the hollow cylinder to the cannon; when this box (which was occasionally closed above by a wooden cover or lid moving on hinges) was put into its place, that is to say, when, by means of the two vertical openings or slits in its two ends (the upper parts of which openings were occasionally closed by means of narrow pieces of wood sliding in vertical grooves), the box (g, 11, i, k, Fig. 3) was fixed to the machinery in such a manner that its bottom (i, It) being in the plane of the horizon, its axis coincided with the axis of the hollow metallic cylinder; it is evident, from the description, that the hollow metal— lic cylinder would occupy the middle of the box, without touching it on either side (as it is repre- sented in Fig. 3); and that, on pouring water into the box, and filling it to the brim, the cylinder would be completely covered and surrounded on every side by that fluid. And farther, as the box was held fast by the strong square iron rod (in) which passed in a square hole in the centre of one of its ends (a, Fig. 4), while the rotmd or cylindrical neck, which joined the hollow cylinder to the end of the cannon, could turn \ round freely on its axis in the round hole in the centre of the other end of it, it is evident that the machinery could be put in motion without the least danger of forcing the box out of its place, throwing the water out of it, or deranging any part of the apparatus. Everything being ready, I proceeded to make the experiment I had projected in the following manner. The hollow cylinder having been previously cleaned out, and the inside of its bore wiped with a clean towel till it was quite dry, the square iron bar, with the bltmt steel borer fixed to the end of it, was put into its place; the mouth of the bore of the cylin— der being closed at the same time by means of the circular piston, through the centre of which the iron bar passed. This being done, the box was put in its place, and the joinings of the iron rod and of the neck of the cylinder with the two ends of the box having been made watertight by means of collars of oiled leather, the box was filled with cold water (viz. at the temperature of 60°) and the machine was put in motion. Quenti of ioeoeold water which, with tith’given quantity of Heat, 01" the Heat excited there appears to have will“ hm be“ hm“ '8" 5" armada to boil. been actually accumulated, - 1 In avoid“de weight- In the water contained in the wooden box, 182- lb., avoirdupois, heated 150 degrees, m namely, from 60° to 210° F. . . 15.2 In 113.13 lb. of gun-metal (the hollow cylinder), heated l 50 degrees; and, as the capacity for Heat of this metal is to that of water as 0.1 too to 1.0000, this quantity of Heat would have heated 12; lb. of water the same number of degrees . 10.37 In 36.75 cubic inches of iron (being that part of the iron bar to which the bore:- was fixed which entered the box), heated 150 degrees; which may be reckoned equal in ca- pacity for Heat to 1.21 lb. of water . . . . . . _ 1.01 N. B. No estimate is here made of‘the Heat accumulated ' in the wooden box, nor of that dispersed during the experi- ment. ' Total quantity of ice-cold water which, with the Heat actually generated by friction, and accumulated in 2 hours and 30 minutes, might have been heated 180 degrees, or -— made to boil . . . . . . . . 26.58 BENJAMIN RUIVIFORD, THE SOURCE OF THE HEAT WHICH IS EXCITED BY FRICTION 297 The result of this beautiful experiment was very striking, and the pleasure it afforded me amply repaid me for all the trouble I had had in contriving and arranging the complicated machinery used in making it. The cylinder, revolving at the rate of about 32 times in a minute, had been in motion but a short time, when I perceived, by putting my hand into the water and touching the outside of the cylinder, that Heat was generated; and it was not long before the water which surrounded the cylinder began to be sensibly warm. At the end of 1 hour I formd, by plunging a thermometer into the water in the box (the quantity of which fluid amounted to 18.77 1b., avoirdupois, or 21 wine gallons), that its temperature had been raised no less than 47 degrees; being now 107° of Fahrenheit’s scale. When 30 minutes more had elapsed, or 1 hour and 30 minutes after the machinery had been put in motion, Heat of the water in the box was 142". At the end of 2 hours, reckoning from the begin- ning the experiment, the temperature of the water was to be raised to 178°. At 2 hours 20 minutes it was at 200°; and at 2 hours 30 minutes it ACTUALLY BOILEDI aea-x- It remains for me to give an account of one experi- ment more, which was made with this apparatus. I found, by the experiment No. 1, how much Heat was generated when the air had free access to the metallic surfaces which were rubbed together. By the experiment No. 2, I found that the quantity of Heat generated was not sensibly diminished when the free access of the air was prevented; and by the result of No. 3, it appeared that the generation of the Heat was not prevented or retarded by keeping the apparatus immersed in water. But as, in this last-mentioned experiment, the water, though it surrounded the hollow metallic cylinder on every side, externally, was not suffered to enter the cavity of its bore (being prevented by the piston), and consequently did not come into contact with the metallic surfaces where the Heat was generated; to see What effects would be produced by giving the water free access to these surfaces, I now made the Experiment No. 4 The piston which closed the end of the bore of the cylinder being removed, the blunt borer and the cylinder were once more put together; and the box being fixed in its place, and filled with water, the machinery was again put in motion. There was nothing in the result of this experi- ment that renders it necessary for me to be very par- ticular in my account of it. Heat was generated as in the former experiments, and, to all appearance, quite as rapidly; and I have no doubt but the water in the box would have been brought to boil, had the exper- iment been continued as long as the last. The only circumstance that surprised me was, to find how little difference was occasioned in the noise made by the borer in rubbing against the bottom of the bore of the cylinder, by filling the bore with water. This noise, which was very grating to the ear, and some— times almost insupportable, was, as nearly as I could judge of it, quite as loud and as disagreeable when the surfaces rubbed together were wet with water as when they were in contact with air. By meditating on the results of all these experi~ ments, we are naturally brought to that great ques— tion which has so often been the subject of specula- tion among philosophers; namely— What is Heat? Is there any such thing as an igneous fluid? Is there anything that can with propri- ety be called caloric? We have seen that a very considerable quantity of Heat may be excited in the friction of two metallic surfaces and given off in a constant stream or flux in all directions without interruption or intermission, and without any signs of diminution or exhaustion. From whence came the Heat which was contin- ually given off in this manner in the foregoing exper— iments? Was it furnished by the small particles of metal, detached from the larger solid masses, on their being rubbed together? This, as we have already seen, could not possibly have been the case. Was it furnished by the air? This could not have been the case; for, in three of the experiments, the machinery being kept immersed in water, the access of the air of the atmosphere was completely prevented. Was it furnished by the water which sur- rounded the machinery? That this could not have been the case is evident: first, because this water was continually receiving Heat from the machinery, and could not at the same time be giving to, and receiving Heat from, the same body; and, secondly, because there was no chemical decomposition of any part of this water. Had any such decomposition taken place (which, indeed, could not reasonably have been expected), one of its component elastic fluids (most probably inflammable air) must at the same time have been set at liberty, and, in making its escape into the atmosphere, would have been detected; but though I frequently examined the water to see if any air-bubbles rose up through it, and had even made preparations for catching them, in order to examine them, if any should appear, I could perceive none; nor was there any sign of decomposition of any kind whatever, or other chemical process, going on in the water. Is it possible that the Heat could have been sup— plied by means of the iron bar to the end of which the blunt steel borer was fixed? or by the small neck of gun—metal by which the hollow cylinder was united to the cannon? These suppositions appear more improbable even than either of those before mentioned; for Heat was continually going off, or out of the machinery, by both these passages, during the whole time the experiment lasted. And, in reasoning on this subject, we must not forget to consider that most remarkable circum— stance, that the source of the Heat generated by fric— tion, in these experiments, appeared evidently to be inexhaustible. It is hardly necessary to add, that anything which any insulated body, or system of bodies, can continue to furnish without limitation, cannot possi- bly be a material substance; and it appears to me to be extremely difficult, if not quite impossible, to form any distinct idea of anything capable of being excited and communicated in the manner the Heat was excited and communicated in these experi~ ments, except it be MOTION. I am very far from pretending to know how, or by what means or mechanical contrivance, that par- ticular kind of motion in bodies which has been sup- posed to constitute Heat is excited, continued, and propagated; and I shall not presru'ne to trouble the Society with mere conjectures, particularly on a subject which, during so many thousand years, the most enlightened philosophers have endeavoured, but in vain, to comprehend. But, although the mechanism of Heat should, in 298 THE DEATH OF CERTAINTYZ SCIENCE AND WAR fact, be one of those mysteries of nature which are beyond the reach of human intelligence, this ought by no means to discourage us or even lessen our ardour, in our attempts to investigate the laws of its operations. How far can we advance in any of the paths which science has opened to us before we find ourselves enveloped in those thick mists which on every side bound the horizon of the human intellect? But how ample and how interesting is the field that is given us to explore! Nobody, surely, in his sober senses, has ever pretended to understand the mechanism of gravita- tion; and yet what sublime discoveries was our immortal Newton enabled to make, merely by the investigation of the laws of its action! The effects produced in the world by the agency of Heat are probably just as extensive, and quite as important, as those which are owing to the tendency of the particles of matter towards each other; and there is no doubt but its operations are, in all cases, determined by laws equally inunutable. Note 1 As these experiments are important, it may perhaps be agreeable to the Society to be made acquainted with them in their details. One of them was as follows— To 4590 grains of water, at the temperature of 591/2n F (an allowance as compensation, reckoned in water, for the capacity for Heat of the containing cylindrical tin vessel being included), were added 1016 1/8 grains of gun-metal in thin slips, separated from the gun by means of a fine saw, being at the temperature of 210° F. When they had remained together 1 minute, and had been well stirred about, by means of a small rod of light wood, the Heat of the mixture was found to be = 63°. From this experiment the specific Heat of the metal, calculated according to the rule given by Dr. Crawford, turns out to be : 0.1100, that of water being 2 1.0000. An experiment was afterwards made with the metallic chips as follows: To the same quantity of water as was used in the experiment above mentioned, at the same tempera- ture (viz. 591/2”), and in the same cylindrical tin vessel, were now put 1016 1 / 8 grains of metallic chips of gunvmetal bored out of the same gun from which the slips used in the foregoing experiment were taken, and at the same temperature (210“). The Heat of the mixture at the end of 1 minute was just 63°, as before; consequently the specific Heat of these metallic chips was = 0.1100. Each of the above experi- ments was repeated three times, and always with nearly the same results. 8.2 Michael Faraday, Experimental Researches in Electricity Michael Faraday (1791-1867) was the son of a blacksmith and was largely self-taught. Faraday began his scientific work in chemistry as an assis— tant to Sir Humphry Davy, but he shifted his work more and more away from matter and towards forces. He was convinced that electricity, magnet- ism, light, heat, and chemical affinity were all aspects of the same phenomenon and that this phe- nomenon, rather than being based on some kind of fluid movement, was really aform of vibration. His work helped establish the physical study of electro— magnetism and created field theory. ON SIMPLE VOLTAIC CHzCLEs 875. The great question of the source of electricity in the voltaic pile has engaged the attention of so many eminent philosophers, that a man of liberal mind and able to appreciate their powers would probably conclude, although he might not have studied the question, that the truth was somewhere revealed. But if in pursuance of this impression he were induced to enter upon the work of collating results and conclusions, he would find such contradictory evidence, such equilibrium of opinion, such varia- tion and combination of theory, as would leave him in complete doubt respecting what he should accept as the true interpretation of nature: he would be forced to take upon himself the labour of repeating and examining the facts, and then use his own judge- ment on them in preference to that of others. 876. This state of the subject must, to these who have made up their minds on the matter, be my apology for entering upon its investigation. The views I have taken of the definite action of electricity in decomposing bodies, and the identity of the power so used with the power to be overcome, founded not on a mere opinion or general notion, but on facts which, being altogether new, were to my mind precise and conclusive, gave me, as I con- MICHAEL FARADAY, EXPERIMENTAL RESEARCHES IN ELECTRICITY 299 ceived, the power of examining the question with advantages not before possessed by any, and which might compensate, on my part, for the superior clearness and extent of intellect on theirs. Such are the considerations which have induced me to suppose I might help in deciding the question, and be able to render assistance in that great service of removing doubtful knowledge. Such knowledge is the early morning light of every advancing science, and is essential to its development; but the man who is engaged in dispelling that which is deceptive in it, and revealing more clearly that which is true, is as useful in his place, and as necessary to the general progress of the science, as he who first broke through the intellectual darkness, and opened a path into knowledge before unknown to man. 877. The identity of the force constituting the voltaic current or electrolytic agent, with that which holds the elements of electrolytes together, or in other words with chemical affinity, seemed to indicate that the electricity of the pile itself was merely a mode of exertion, or exhibition, or existence of true chemical action, or rather of its cause; and Ihave consequently already said that I agree with those who believe that the supply of electricity is due to chemical powers. 878. But the great question of whether it is orig- inally due to metallic contact or to chemical action, i.e. whether it is the first or the second which origi— nates and determines the current, was to me still doubtful; and the beautiful and simple experiment with amalgamated zinc and platina, which I have described minutely as to its results, did not decide the point; for in that experiment the chemical action does not take place without the contact of the metals, and the metallic contact is inefficient without the chemical action. Hence either might be looked upon as the determining cause of the current. 879. I thought it essential to decide this question by the simplest possible forms of apparatus and experiment, that no fallacy might be inadvertently admitted. The well known difficulty of effecting decomposition by a single pair of plates, except in the fluid exciting them into action, seemed to throw insurmountable obstruction in the way of such experiments; but I remembered the easy decompos— ability of the solution of iodide of potassium and seeing no theoretical reason, if metallic contact was not essential, why true electro-decomposition should not be obtained without it, even in a single circuit, I persevered and succeeded. 880. A plate of zinc, about eight inches long and half an inch wide, was cleaned and bent in the middle to a right angle. A plate of platina, about three inches long and half an inch wide, was fas- tened to a platina wire, and the latter bent as in the figure I]. These two pieces of metal were arranged together as delineated, but as yet without the vessel c and its contents, which consisted of dilute sul- phuric acid mingled with a little nitric acid. At x a piece of folded bibulous paper, moistened in a solu- tion of iodide of potassium, was placed on the zinc, and was pressed upon by the end of the platina wire. When under these circumstances the plates were dipped into the acid of the vessel c, there was an immediate effect at x, the iodide being decomposed, and iodine appearing at the anode, i.e., against the end of the platina wire. 881. As long as the lower ends of the plates remained in the acid the electric current continued, and the decomposition proceeded at x. On reproving the end of the wire from place to place on the paper, the effect was evidently very powerful; and on placing a piece of turmeric paper between the white paper and zinc, both papers being moistened with the solutions of iodide of potassium, alkali was evolved at the cathode against the zinc, in proportion to the evolution of iodine at the anode. Hence the decomposition was perfectly polar, and decidedly dependent upon a current of electricity passing from the zinc through the acid to the platina in the vessel c and back from the platina through the solution to the zinc at the paper x. 882. That the decomposition at x was a true elec— trolytic action, due to a current determined by the state of things in the vessel c and not dependent upon any mere direct chemical action of the zinc and platina on the iodide, or even upon any current which the solution of iodide might by its action on those metals tend to form at x, was shown, in the first place, by removing the vessel c and its acid from the plates, when all decomposition at x ceased, and in the next by connecting the metals, either in or out of the acid, together, when decomposition of the iodide at x occurred, but in a reverse order; for now alkali appeared against the end of the platina wire, and the iodine passed to the zinc, the current being the con— trary of what it was in the former instance, and pro- duced directly by the difference of action of the solu— tion in the paper on the two metals. The iodine of course combined with the zinc. THE DEATH OF CERTAINTYI SCIENCE AND WAR 883. When this experiment was made with pieces of zinc amalgamated over the whole surface, the results were obtained with equal facility and in the same direction, even when only dilute sulphuric acid was contained in the vessel c. Whichsoever end of the zinc was immersed in the acid, still the effects were the same: so that if, for a moment, the mercury might be supposed to supply the metallic contact, the inversion of the amalgamated piece destroys that objection. The use of nnmnalgamated zinc (880.) removes all possibility of doubt.1 >f'>(->(- 910. I have already expressed the view which I take of the decomposition in the experimental place, as being the direct consequence of the superior exertion at some other spot of the same kind of power as that to be overcome, and therefore as the result of an antagonism of forces of the same nature (891. 904.). Those at the place of decomposition have a re—action upon, and a power over, the exerting or determining set proportionate to what is needful to overcome their own power; and hence a curious result of resist— ance offered by decompositions to the original deter— mining force, and consequently to the current. This is well shown in the cases where such bodies as chlo- ride of lead, iodide of lead, and water would not decompose with the current produced by a single pair of zinc and platina plates in sulphuric acid, although they would with a current of higher inten- sity produced by stronger chemical powers. In such cases no sensible portion of the current passes (967); the action is stopped; and I am now of opinion that in the case of the law of conduction which I described in the Fourth Series of these Researches, the bodies which are electrolytes in the fluid state cease to be such in the solid form, because the attrac- tions of the particles by which they are retained in combination and in their relative position, are then too powerful for the electric current, The particles retain their places; and as decomposition is pre~ vented, the transmission of the electricity is pre— vented also; and although a battery of many plates may be used, yet if it be of that perfect kind which allows of no extraneous or indirect action, the whole of the affinities concerned in the activity of that battery are at the same time also suspended and counteracted. 911. But referring to the resistance of each single MICHAEL FARADAY, EXPERIMENTAL RESEARCHES IN ELECTRICITY 301 case of decomposition, it would appear that as these differ in force according to the equities by which the elements in the substance tend to retain their places, they also would supply cases constituting a series of degrees by which to measure the initial intensities of simple voltaic or other currents of electricity, and which, combined with the scale of intensities deter- mined by different degrees of acting force (909), would probably include a sufficient set of differences to meet almost every important case where a refer— ence to intensity would be required. *>t>l- 915. Returning to the consideration of the source of electricity (878 &c.), there is another proof of the most perfect kind that metallic contact has nothing to do with the production of electricity in the voltaic circuit, and further, that electricity is only another mode of the exertion of chemical forces. It is, the production of the electric spark before any contact of metals is made, and by the exertion of pure and unmixed chemical forces. The experiment, which will be described further on (956), consists in obtaining the spark upon making contact between a plate of zinc and a plate of copper plunged into dilute sul— phuric acid. In order to make the arrangement as elementary as possible, mercurial surfaces were dis~ missed and the contact made by a copper wire con— nected with the copper plate, and then brought to touch a clean part of the zinc plate. The electric spark appeared, and it must of necessity have existed and passed before the zinc and the copper were in contact. 917. As volta-electro—generation is a case of mere chemical action, so volta—electro—deconzposition is simply a case of the preponderance of one set of chemical affinities more powerful in their nature, over another set which are less powerful: and if the instance of two opposing sets of such forces (891) be considered, and their mutual relation and depend- ence borne in mind, there appears no necessity for using, in respect to such cases, any other term than chemical affinity, (though that of electricity may be very convenient) or supposing any new agent to be concerned in producing the results; for we may con- sider that the powers at the two places of action are in direct communion and balanced against each other through the medium of the metals (891), in a manner analogous to that in which mechanical forces are balanced against each other by the inter- vention of the lever (1031). 918. All the facts show us that that power com- monly called chemical affinity, can be communicated to a distance through the metals and certain forms of carbon; that the electric current is only another form of the forces of chemical affinity; that its power is in proportion to the chemical affinities producing it; that when it is deficient in force it may be helped by calling in chemical aid, the want in the former being made up by an equivalent of the latter; that, in other words, the forces termed chemical afinity and electricity are one and the same. 937. As the opposition of electric—motive pairs of plates produces results other than those due to the mere difference of their independent actions, I devised another form of apparatus, in which the action of acid and alkali might be more directly com~ pared. A cylindrical glass cup, about two inches deep within, an inch in internal diameter, and at least a quarter of an inch in thickness, was cut down the middle into halves. Abroad brass ring, larger in diam— eter than the cup, was supplied with a screw at one side; so that when the two halves of the cup were within the ring, and the screw was made to press tightly against the glass, the cup held any fluid put into it. Bibulous paper of deferent degrees of perme- ability was then cut into pieces of such a size as to be easily introduced between the loosened halves of the cup, and served when the latter were tightened again to form a porous division down the middle of the cup, sufficient to keep any two fluids on opposite sides of the paper from mingling, except very slowly, and yet allowing them to act freely as one electrolyte. The two spaces thus produced Iwill call the cells A and B. This instrument I have found of most general application in the investigation of the relation of fluids and metals amongst themselves and to each other. By combining its use with that of the galvanometer, it is easy to ascertain the relation of one metal with two fluids, or of two metals with one fluid, or of two metals and two fluids upon each other. 938. Dilute sulphuric acid, sp. gr. 1.25 was put _ into the cell A, and a strong solution of caustic potassa into the cell B; they mingled slowly through the paper, and at last a thick crust of sulphate of potash formed on the side of the paper next to the alkali. A plate of 302 THE DEATH OF CERTAINTYZ SCIENCE AND WAR clean platina was put into each cell and connected with a delicate galvanometer, but no electric current could be observed. Hence the contact of acid with one platina plate, and alkali with the other, was unable to produce a current; nor was the combination of the acid with the alkali more effectual (925). 939. When one of the platina plates was removed and a zinc plate substituted, either amalga- mated or not, a strong electric current was produced. But, whether the zinc were in the acid whilst the platina was in the alkali, or whether the reverse order were chosen, the electric current was always from the zinc through the electrolyte to the platina, and back through the galvanometer to the zinc, the current seeming to be strongest when the zinc was in the alkali and the platina in the acid. 940. In these experiments, therefore, the acid seems to have no power over the alkali, but to be rather inferior to it in force. Hence there is no reason to suppose that the combination of the oxide formed with the acid around it has any direct influence in producing the electricity evolved, the whole of which appears to be due to the oxidations of the metal (919). 941. The alkali, in fact, is superior to the acid in bringing a metal into what is called the positive state; for if plates of the same metal, as zinc, tin, lead, or copper, be used both in the acid or alkali, the elec— tric current is from the alkali across the cell to the acid, and back through the galvanometer to the alkali, as Sir Humphry Davy formerly stated. This current is so powerful, that if amalgamated zinc, or tin, or lead be used, the metal in the acid evolves hydrogen the moment it is placed in communication with that in the alkali, not fi‘om any direct action of the acid upon it, for if the contact be broken, the action ceases, but because it is powerfully negative with regard to the metal in the alkali. 942. The superiority of alkali is further proved by this, that if zinc and tin be used, or tin and lead, whichsoever metal is put into the alkali becomes positive, that in the acid being negative. Whichso~ ever is in the alkali is oxidized, whilst that in the acid remains in the metallic state, as far as the electric current is concerned. 943. When sulphuretted solutions are used (930) in illustration of the assertion, that it is the chemical action of the metal and one of the ions of the associ— ated electrolyte that produces all the electricity of the voltaic circuit, the proofs are still the same. Thus, as Sir Humphry Davy has shown, if iron and copper be plunged into dilute acid, the current is from the iron through the liquid to the copper; in solution of potassa it is in the same direction, but in solution of sulphured of potassa it is reversed. In the two first cases it is oxygen which combines with the iron, in the latter sulphur which combines with the copper, that produces the electric current; but both of these are ions, existing as such in the electrolyte, which is at the same moment suffering decomposition; and, what is more, both of these are anions, for they leave the elec- trolytes at their anodes, and act just as chlorine, iodine, or any other anion would act which might have been previously chosen as that which should be used to throw the voltaic circle into activity. >e>e>+ 946. There is no point in electrical science which seems to me of more importance than the state of the metals and the electrolytic conductor in a simple voltaic circuit before and at the moment when metal- lic contact is first completed. If clearly understood, I feel no doubt it would supply us with a direct key to the laws under which the great variety of voltaic excitements, direct and incidental, occur, and open out new fields of research for our investigation. 947. We seem to have the power of deciding to a certain extent in numerous cases of chemical affinity, (as of Zinc with the oxygen of water, &c. &c.) which of two modes of action of the attractive power shall be exerted (996). In the one mode we can transfer the power onwards, and make it produce elsewhere its equivalent of action (867, 917); in the other it is not transferred, but exerts wholly at the spot. The first is the case of volta—electric excitation, the other ordi— nary chemical affinity: but both are chemical actions and due to one force or principle. 948. The general circumstances of the former mode occur in all instances of voltaic currents, but may be considered as in their perfect condition, and then free from those of the second mode, in some only of the cases; as in those of plates of zinc and platina in solution of potassa, or of amalgamated zinc and platina in dilute sulphuric acid. >(->(->l- 950. Practically, the state of tension is best relieved by dipping a metal which has less attraction for oxygen than the zinc, into the dilute acid, and making it also touch the zinc. The force of chemical affinity, which has been influenced or polarized in the particles of the water by the dominant attraction of the zinc for the oxygen, is then transferred, in a most extraordi- nary manner, through the two metals, so as to re- enter upon the circuit in the electrolytic conductor, which, unlike the metals in that respect, cannot convey or transfer it without suffering decomposi- tion; or rather, probably, it is exactly balanced and neutralized by the force which at the same moment completes the combination of the zinc with the oxygen of the water. The forces, in fact, of the two particles which are acting towards each other, and which are therefore in opposite directions, are the origin of the two opposite forces, or directions of force, in the current. They are of necessity equivalent to each other. Being transferred forward in contrary directions, they produce what is called the voltaic current: and it seems to me impossible to resist the idea that it must be preceded by a state of tension in the fluid, and between the fluid and the zinc; the first consequence of the affinity of the zinc for the oxygen of the water. *fi-X- 956. There is, however, one beautiful experimental proof of a state of tension acquired by the metals and the electrolyte before the electric current is pro— truded, and before contact a the different metals is made (915); in fact, at that moment when chemical forces only are efficient as a cause of action. I took a voltaic apparatus, consisting of a single pair of large plates, namely, a cylinder of amalgamated zinc, and a double cylinder of copper. These were put into a jar containing dilute sulphuric acid,2 and could at pleasure be placed in metallic communication by a copper wire adjusted so as to dip at the extremities into two cups of mercury connected with the two plates. 957. Being thus arranged, there was no chemi- cal action whilst the plates were not connected. On making the connexion, a spark was obtained,3 and the solution was immediately decomposed. On breaking it, the usual spark was obtained, and the decomposition ceased. In this case it is evident that the first spark must have occurred before metallic contact was made, for it passed through an interval of air; and also that it must have tended to pass MICHAEL FARADAY, EXPERIMENTAL RESEARCHES [N ELECTRICITY 303 before the electrolytic action began; for the latter could not take place until the current passed, and the current could not pass before the spark appeared. Hence I think there is sufficient proof, that as it is the zinc and water which by their mutual action produce the electricity of this appa— ratus, so these by their first contact with each other, were placed in a state of powerful tension (951), which, though it could not produce the actual decomposition of the water, was able to make a spark of electricity pass between the zinc and a fit discharger as soon as the interval was rendered suf- ficiently small. The experiment demonstrates the direct production of the electric spark from pure chemical forces. 959. With reference to the other set of cases, namely, those of local action (947) in which chemical affinity being exerted causes no transference of the power to a distance where no electric current is produced, it is evident that forces of the most intense kind must be active, and in some way balanced in their activity, during such combinations; these forces being directed so immediately and exclusively towards each other, that no signs of the powerful electric current they can produce become apparent, although the same final state of things is obtained as if that current had passed. It was Berzelius, I believe, who considered the heat and light evolved in cases of combustion as the consequences of this mode of exertion of the electric powers of the combining par~ ticles. But it will require a much more exact and extensive knowledge of the nature of electricity, and the manner in which it is associated with the atoms of matter, before we can understand accurately the action of this power in thus causing their union, or comprehend the nature of the great difference which it presents in the two modes of action just distin- guished. We may imagine, but such imaginations must for the time be classed with the great mass of doubtful knowledge (876) which we ought rather to strive to diminish than to increase; for the very extensive contradictions of this knowledge by itself shows that but a small portion of it can ultimately prove true. 304 THE DEATH OF CERTAINTY: SCIENCE AND WAR 965. If the present paper be accepted as a correct expression of facts, it will still only prove a confir‘ mation of certain general views put forth by Sir Humphry Davy in his Bakerian Lecture for 1806 and revised and re-stated by him in another Bacter— ial Lecture, on electrical and chemical changes, for the year 1826. His general statement is, that "chemical and electrical attractions were produced by the same cause, acting in one case on particles, in the other on masses, of matter; and that the same property, under ferent modifications, was the cause of all the phenomena exhibited by different voltaic combinations." This state— ment I believe to be true; but in admitting and sup— porting it, I must guard myself from being supposed to assent to all that is associated with it in the two papers referred to, or as admitting the experiments which are there quoted as decided proofs of the truth of the principle. Had I thought them so, there would have been no occasion for this investigation. It may be supposed by some that I ought to go through these papers, distinguishing what I admit from what I reject, and giving good experimental or philosoph— ical reasons for the judgment in both cases. But then I should be equally botmd to review, for the same purpose, all that has been written both for and against the necessity of metallic contact—for and against the origin of voltaic electricity in chemical action—a duty Which I may not undertake in the present papers. Notes 1 The following is a more striking mode of making the above elementary experiment. Prepare a plate of zinc, ten or twelve inches long and two inches wide, and clean it thoroughly: provide also two discs of clean platina, about one inch and a half in diameter: dip three or four folds of bibulous paper into a strong solution of iodide of potassium, place them on the clean zinc at one end of the plate, and put on them one of the platina discs: finally clip similar folds of paper or a piece of linen cloth into a mixture of equal parts nitric acid and water, and place it at the other end of the zinc plate with the second platina disc upon it. In this state of things no change at the solu- tion of the iodide will be perceptible; but if the two discs be connected by a platina (or any other) wire for a second or two, and then that over the iodide be raised, it will be found that the whole of the surface beneath is deeply stained with evolved iodine. —Dec. 1838. 2 When nitro—sulphuric acid is used, the spark is more powerful, but local chemical action can then com- mence, and proceed without requiring metallic contact. 3 It has been universally supposed that no spark is produced on making the contact between a single pair of plates, 1 was led to expect one from the con— siderations already advanced in this paper. The wire of communication should be short; for with a long wire, circumstances strongly affecting the spark are introduced. 8.3 James Clerk Maxwell, A Dynamical Theory of Electromagnetic Field Iames Cleric Maxwell (1831—79) was born in Edin~ burgh to a middle—class family. Maxwell was not an early prodigy like Newton, but his insight and math- ematical skills led to the recognition of his talents by the time he graduated in 1854 from Trinity College, Cambridge. He went on to a series of senior aca- demic appointments at Cambridge, ultimately becoming the first Cavendish Professor of Experi— mental Physics in 1871, when he was responsible for the design of the new Cavendish Laboratory. Although he worked on a number of physical prob— lems, he is best remembered for his work on electric- ity and magnetism, which established the mathemat— ical foundation for their behaviour. Part I INTRODUCTORY (1) The most obvious mechanical phenomenon in electrical and magnetical experiments is the mutual action by which bodies in certain states set each other in motion while still at a sensible distance from each other. The first step, therefore, in reducing these phenomena into scientific form, is to ascertain the magnitude and direction of the force acting between the bodies, and when it is found that this force depends in a certain way upon the relative position of the bodies and on their electric or magnetic condi~ tion, it seems at first sight natural to explain the facts by assuming the existence of something either at rest or in motion in each body, constituting its electric or magnetic state, and capable of acting at a distance according to mathematical laws. In this way mathematical theories of statical JAMES CLERK MAXWELL, A DYNAMIC/XL THEORY OF ELECTROMAGNETIC FIELD 305 electricity, of magnetism, of the mechanical action between conductors carrying currents, and of the induction of currents have been formed. In these the- ories the force acting between the two bodies is treated with reference only to the condition of the bodies and their relative position, and without any express consideration of the surrounding medium. These theories assume, more or less explicitly, the existence of substances the particles of which have the property of acting on one another at a dis tance by attraction or repulsion. The most complete development of a theory of this kind is that of M.W. Weber, who has made the same theory include elec- trostatic and electromagnetic phenomena. In doing so, however, he has found it necessary to assume that the force between two electric parti- cles depends on their relative velocity, as well as on their distance. This theory, as developed by MM W. Weber and C. Neumann, is exceedingly ingenious, and wonder- fully comprehensive in its application to the phe- nomena of statical electricity, electromagnetic attrac- tions, induction of currents and diamagnetic phenomena; and it comes to us with the more authority, as it has served to guide the speculations of one who has made so great an advance in the practical part of electric science, both by introducing a consistent system of units in electrical measure— ment, and by actually determining electrical quanti— ties with an accuracy hitherto unknown. (2) The mechanical difficulties, however, which are involved in the assumption of particles acting at a distance with forces which depend on their veloci- ties are such as to prevent me from considering this theory as an ultimate one, though it may have been, and may yet be useful in leading to the coordination of phenomena. I have therefore preferred to seek an explana— tion of the fact in another direction, by supposing them to be produced by actions which go on in the surrounding medium as well as in the excited bodies, and endeavouring to explain the action between distant bodies without assuming the exis- tence of forces capable of acting directly at sensible distances. (3) The theory I propose may therefore be called a theory of the Electromagnetic Field, because it has to do with the space in the neighbourhood of the electric or magnetic bodies, and it may be called a Dynamical Theory, because it assumes that in that space there is matter in motion, by which the observed electromagnetic phenomena are produced. (4) The electromagnetic field is that part of space which contains and surrounds bodies in electric or magnetic conditions. It may be filled with any kind of matter, or we may endeavour to render it empty of all gross matter, as in the case of Geissler’s tubes and other so-called vacua. There is always, however, enough of matter left to receive and transmit the undulations of light and heat, and it is because the transmission of these radi- ations is not greatly altered when transparent bodies of measurable density are substituted for the so- called vacuum, that we are obliged to admit that the undulations are those of an ethereal substance, and not of the gross matter, the presence of which merely modifies in some way the motion of the ether. We have therefore some reason to believe, from the phenomena of light and heat, that there is an ethereal medium filling space and permeating bodies, capable of being set in motion and of trans— mitting that motion from one part to another, and of communicating that motion to gross matter so as to heat it and affect it in various ways. (5) Now the energy commimicated to the body in heating it must have formerly existed in the moving medium, for the undulations had left the source of heat some time before they reached the body, and during that time the energy must have been half in the form of motion of the medium and half in the form of elastic resilience. From these con- siderations Professor W. Thomson has argued, that the medium must have a density capable of compar— ison with that of gross matter, and has even assigned an inferior limit to that density. (6) We may therefore receive, as a datum derived from a branch of science independent of that with which we have to deal, the existence of a pervading medium, of small but real density, capable of being set in motion, and of transmitting motion from one part to another with great, but not infinite, velocity. Hence the parts of this medium must be so con- nected that the motion of one part depends in some way on the motion of the rest; and at the same time these connections must be capable of a certain kind of elastic yielding, since the communication of motion is not instantaneous, but occupies time. The medium is therefore capable of receiving and storing up two kinds of energy, namely, the “actual” energy depending on the motions of its ...
View Full Document

This note was uploaded on 08/24/2008 for the course HIST 302 taught by Professor Brooks during the Spring '08 term at NMSU.

Page1 / 7

Count Rumford, Faraday readings - 8 THE DEATH OF CERTAINTY:...

This preview shows document pages 1 - 7. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online