amount of stretch under the same pull?
30. A pieceof No? 17 and a pieceof No. 26 iron wire
stretch the same amount under the same pull. What are
the relative lengths?
31. If a force of 2 lbs. stretches 2 mm. a wire which is
I m. long and . i sq. mm. in cr

a lens is the line throughB (Fig.360)normal to the planesurface.
FIG. 360. " A converging lens FIG. 361. " A diverging lens,
with one plane surface.
It will be observed that the axis of a lens is normal to each
surface of the lens at the point where it pa

8. How great must be the force that can be resolved
into two forces,the smaller of which, 50 lbs.,acts N., and
the larger,80 lbs.,acts E. In what directionmust it
act?
9. Two forces which are in the ratio of 2 : 3 act dpeicrpuelan-rly
to each other upon a

GENERAL PHYSICS
spending elements in two successive apertures is n\, the light
which then reaches P in maximum intensityis said to be the
"spectrum of the wth order." So long,therefore,as the diencnitwave
front is parallelto the grating,the condition that

between us and the moon it appears to be rigidly
true that rays of all colors travel with the same speed; for
when a white star is occulted by the moon, all the various colors
in the star's lightdisappearat once; and when the star emerges
from behind the

increased pressure about the source is,in general,to slightly
increase the wave length.
(v) The Zeeman Effect. " When a source of lightis placed
in a strong magnetic field,each individual line in its spectrum
will,in general,become a triplet, or a stillmo

1. In any given conductor, the heat developed by an electric
current in any given time varies directlyas the square of the
current.
2. In any given conductor,the heat developed by any given
current is directlyproportional to the time during which the creu

~
m2i\
' Eq. 150
395. These two laws of Faraday would seem to indicate that
there is associated with each atom of matter a certain definite
charge of electricitwyhich is the same for all atoms of the
same valencyand is quiteindependent of the "atomic weig

F = HLi dynes
as the mechanical force requiredto drive the inductor through
the field. If i be measured in amperes instead of C.G. S. units,
then
F = HLi x 10-1 dynes, Eq. 156
which is the second fundamental equationof the dynamo.
The student of electric

is its sp. gr.
6 PROBLEMS IN PHYSICS
9. The weight of a given mass of substance is 660 g. ;
its volume is 33 c.c. What is its sp. gr. ?
V 10. A certain volume of sulphuricacid of density1.84
weighs82.8 g. The same volume of another liquidweighs
32 g. Wha

one of the vertical sides may be just as great as the entire
pressure on the box before the tube was inserted ?
y 23. A pond 100 ft. wide and 20 ft. deep is kept in place
by a dam slopingat an angle of 45"to the horizon. What is
the pressure on the dam ?

which this single
value for the angle
of incidence can be
FIG. 369." Refraction through a prism of small angle, obtained IS by imangkij
= i'2;from which it follows that rl = rz and hence that
the ray inside the prism is symmetrical with respect to the
two

"/ 63. A diver with his suit on weighs 200 lbs. It takes
* 3*5of a cu. ft. of lead to justsink him. How many cu. ft.
of water does he displacewhen in water
64. A cube of pine 10 cm. on a side and of sp. gr. .5
has a hole 2 cm. in diameter and 5 cm. deep

Total Reflection
451. Up to this point we have been consideringthe passage
of a ray of lightfrom a medium where it travels with a certain
speed to one where it travels less rapidly. Let us now reverse
456 GENERAL PHYSICS
the sense in which the lightmoves

in alcohol. Half of its length is composed of wood of
sp* gr. .3. What is the sp. gr. of the other half?
115. If the densityof brass is 8.5, what will be the
weightin air of a mass of brass which weighs in vacuo 500
g. ? The weightof i literof air is 1.2

center of the slit,the number of zones in the half slit will pass
alternatelyfrom even to odd, and the illumination will change
alternatelyfrom darkness to
brightness. This effect is
easilyobserved by placinga
FIG. 382,-Fresnei's diffraction phenomenarrow

45*with the beam. From the upper end of the beam is
suspended a weight of 500 lbs. (a)What is the pressure
on the beam ? (U)What is the tension on the rope ? Dreigs-ard
the weightof the beam.
FORCE AND ACCELERATION 37
63. A square lot has a post at each c

lbs.,and No. 30 steel wire with a pullof 18 lbs.,what is the
relative tenacityof brass and steel ?
10. What is the relative tenacityof brass and steel if No.
26 steel wire can sustain a load of 50 lbs.? [Use data from
Problem 7.]
11. What is the relative

that might be introduced.
A long wire of soft iron " say 70 centimeters in lengthand
1 or 2 millimeters in diameter " may be stronglyimazegdn;etbut
if it receive even a slightjar,its magnetization will
almost entirelydisappear. This also would lead us to

'
" Illustratingthe i-elation between
once been described by draw- iines of force and equipotentialsurfaces.
ELECTROSTATICS 351
ingits equipotentialsurfaces,it becomes a matter of great ease to
also describe it in terms of lines of force. For we have mere

ft.longand 8 ft. high. What must be the force of friction
to keep it from slidingdown the plane?
34 PROBLEMS IN PHYSICS
43. There is a uniform gate 4 ft. long and 3 ft. high
which weighs 50 lbs. It is attached to a post by hinges at
the top and bottom, (a

12. Prove that in the case of the ordinarypocket lens" double convex
magnifying glass" the magnificationis D/f, where D is the distance of
distinct vision.
13. Two parallelwalls are 20 ft. apart. It is desired to projectupon
one wall,with a magnificationo

the dimensions of the body, and to see that no third
body influences the gold leaves.
It need hardly be added that the size of the electroscope
must be small in comparison with that of the body whose potenELECTROSTATICS
353
tial is to be measured, otherwi

Thus, at the center of
the screen P0, the atpuererwill
contain at
most one Fresnel zone ?
n = 1 ; the light from
every part of the slit
reaches P0 in nearly
FIG. 380. the same phase. Hence
the center of the screen is nearly always bright. If rc = 3,
BC =

apertures.
Then a + d will be what
is called "the grating
space,"that is, the dtains-ce
between any two
corresponding elements
of the wave surface in
two consecutive atpuerer-s.
If the source of light
is placedin the principal
focus of the collimator
lens

process is known as discharging.
The followingtwo features distinguishthe storage battery
from most other Voltaic cells : "
(a) The fact that the platesare prepared,i.e. made cichaelm-ly
different,by electrical means.
(i) The fact that they are capable of

that this assumptionleads to results the same as those obtained
above. As before let L (Fig.357) be the luminous pointwhose
image is sought. Let AIP be the positionwhich any spherical
wave front startingfrom L would have assumed at an instant
",if the gla

In brief the incident wave front from the source S is splitup
by the mirror R into two wave fronts which enter the eye in a
condition for interference. The wave front from the fixed
mirror F behaves exactlyas if it came from its image in the
mirror jR,i.e

perhaps the accurate measurement of small distances. As the
name would imply,the principleinvolved is that of the fienrtenr-ce
of lightwaves. But, as we shall see presently,the
interference is not between two single point sources or two
singlelinear sourc