On account of the changes in length caused by teumreperavariations,
engineersgenerallyrest one end of a girder
bridge,not directlyupon the pier,but upon an iron roller as
shown in Fig. 184.
Many accurate experiments by methods whi
be well cleaned and have a little mercury rubbed over it until
it is thoroughlyamalgamated, the gas will no longerbe given off
from the zinc. But under these circumstances it is found that
the zinc is still "eaten up" by the acid. Zinc sulphate is
6. A cubical box 12 cm. on a side is filled with mercury.
What is the entire pressure on the box 1
7. In the previous problem, the box has a tube i cm.
square extending through a hole in the top, and rising tveicr-ally
10 cm. above the top of the box. The
as nearlyas can be observed, at the same instant. From
this and evidence to be adduced later it appears that heat waves,
lightwaves, and electric waves all travel with the stupendous
270 GENERAL PHYSICS
exhausted vessel. At each end of the bar is attached
of rays passingthrough it; hence its power is said to
vary inverselyas its focal length.
1 Definingequation _, _
Jr = " " for power of a Jiiq. J. " o
A lens whose focal lengthis 1 meter is said to have a power
of 1 dioptric. The dioptricis the uni
which flows firstin one drei-ction,
then in the other.
FIG. 274. " An electric current produced by imnogva
coil in a magnet field.
(i) A current which changes its direction in a regularand continuous
manner is called an "alternating"current.
(ii)But if th
a luminous particlea,nd DCUFthe positionof the front of a
sphericalwave at any instant t. Let 5, 5, 6, b denote particles
of the medium through which this wave has passed on its
way from A to its present position. Then at some instant
earlier than ", each
figureswill be distorted. A lens so corrected is said
to be "rectilinear."
For specialpurposes, such as portraitor landscapeprahophtoy,git
is obviouslyunnecessary that the lens should possess
all of these four qualities.
II. The Human Eye
486. The eye, fr
a pieceof newspaper, the lightwhich falls upon it is scattered
in all directions. Such lightis said to be diffused; and the
process is called diffuse reflection.
But some of the lightwhich is incident upon a body will
penetrate the body ; this we know bec
SOME SPECIAL PROPERTIES OF MATTER
133. In the preceding pages we have considered four eprotip-es
of matter which are perfectlygeneral. It is an emexnpetarilfact
that matter possessingany one of these,
The image of a pointwould no longerbe a point.
Up to this pointwe have assumed that p is a constant, but if
we use lightof more than one color we find that /* varies from
color to color and that the image of a luminous pointon the
axis is drawn out into a
mainly in the good conductor, that is,in the copper. As the
current is increased this expectationis justifiedf,or we now
"observe that the copper wire is the first to turn red and melt.
In the laboratoryit is well to remember this principle; for
when a se
96. There is a uniform rod 6 ft.long and 4 in. square of
sp. gr. .5. What must be the sp. gr. of a* cube of metal 4 in.
on a side,which when attached to the lower end, will mseubr-ge
the rod in water ?
v/ 96. There is a wooden rod of sp. gr. .5, 2 cm. squ
the piano and guitar,it would be impossible to pass to other
keys without enormously complicating the instrument. To
avoid this difficultyBach (1685-1750) invented the so-called
tempered scale, in which the octave is divided into twelve equal
sets itself so as to include as many lines of force as psiobs-le
in the right-handedscrew direction. A slack wire of gold
tinsel carrying a current shows the same tendency to wrap
itself about a straightmagnet, that lines of force have to wrap
magnetic field in such a way that 106 lines of force are uniformly added
to the circuit in 2 sec. Find the average E.M.F. generated. If the
R = 8 OHMS
FIG. 286, E.
ELECTRIC CURRENTS 395
resistance of the circuit is jj^ ohm, what current will be produced d
convex, and plane refractingsurfaces.
1. Find the refractive index when the anglesof incidence and refraction
at a plane surface are 45" and 30", respectively.
2. The critical angle for a certain medium is 45". Find it'srefractive
other parts of space. The iron filingsarrange themselves in
curved lines ; and these curved lines remain the same, however
many times we repeat the experiment.
uWe next introduce a method of conceivingand describingmagnetic
actions which was invented and
of its resistivity.In like manner the conductance of a body is
defined as the reciprocalof its resistance.
383. Indeed, it has been adopted as a generalprinciplein electrical mneo-nclature
that specificproperties,propertiesof substances as distinguished
175. There is a rod 70 cm. long,10 cm. square, of sp. gr.
.5, that rests on the bottom of a vessel 50 cm. deep. What
will be the tension on a string10 cm. long,which is attached
to the bottom of the vessel and to the lower end of the rod,
when in the dish
which he had alreadyproved by analysis.
The majesticphenomenon of the rainbow he showed to be
merely a case of refraction in raindrops.
Another Illustration of .Refraction
471. The laws of refraction may be illustrated by means of
the "opticaldisk," one f
the flame is displacedby the colder
water, which is heavier, and therefore
sinks to the bottom, as indicated by
the arrows. The potentialenergy of
the system is thus less than if the cold
water remained on top.
This cold water, in turn, becomes FlG- 178."
air and 2 kg.in water. Find radius and sp. gr.
160. The weightof a literof hydrogen at o" C. and 760
mm. pressure is .0896 g. Nitrogenis 14 times as heavyand
oxygen 16 times as heavy as hydrogen. Air is composed of
a mixture of 4 parts nitrogenand i part
or from any polishedsurface,when viewed through a ltionuer,mahas
its intensitychanged in a marked manner as thetourmaline
is rotated in front of the eye. He showed also that
for each substance there is a certain angle at which, if the light
In other words, if we measure temperature from a zero which
is 273" C. below the melting pointof ice and denote these ptemr-atures
by jT,we find that the pressure and the volume each
vary directlyas the temperature. A gas which obeys Boyle's
133. A bottle when empty weighs 40 g., when full of cmeur-y
543.2 g., and when 20 g. of mercury have been taken
out and their place filled with another liquid it weighs
524.229 g. (a)What is the volume of the bottle? (b)
What is the sp. gr. of the
the coin is changed,it will
be seen that the Coin is
Fi"- ^- -Coin is lifted iuto view by the water.
lifted into view, as indicated in Fig. 346. The change in
direction of the ray PS, when it leaves the water and passes
into air,is called refraction.
404. Experiment2. If a current be passed through a coil of
wire (i.e.through a pileof loops),the lines of force distribute
themselves about the coil
very much as indicated in
Fig. 302. Such a coil is
called a "helix." And by
use of a compass needle
In the other, proposed by Reaumur, the volume between the
freezingpointand the boilingpointof water is divided into 80
degrees,the former of these two pointsbeing employed as zero.
It is importantto notice that these three thermometers, Celsius,
The machine shown in Fig. 316 is the type
Here only a part of the
" series wound."
used for lightingincandescent lamps,
current from the armature is
diverted to excite the field.
These dynamos are said to be
"shunt wound." When both