Physics for university students (Volume 2) online

motive force and the current was first enunciated by Dr.
G. S. Ohm, of Berlin, in. 1827. It has since been known
as Ohm's La'.''.

If E be the E.M.F. between two points of a conductor
and I the current flowing through it, 'then if suitable
units be chosen,



where R is a quantity called the resistance of the con-
ductor; it is independent of the value and direction of the
current flowing, and depends only on the material of
the conductor, its length and sectional area, its tempera-
ture and state of strain.

The above equation is an expression of Ohm's law ; it is
usually written in the equivalent form.

'-5-

If the practical units now adopted internationally be
employed, this law may be expressed by saying that the
number of amperes flowing through a circuit is equal to
the number of volts of electromotive force divided by the
number of ohms of resistance.

When this formula is applied to the entire circuit, which
may contain several sources of E.M.F. of different signs,



274 ELECTRICITY AND MAGNETISM.

and both metallic and electrolytic resistances, it is not
quite so simple to apply. There are then several electro-
motive forces, some tending to produce a flow in one
direction and some in the other ; and a number of different
resistances each obstructing the flow, whether it takes place
in one direction or the other. Then



J_

~



Each E.M.F. must be taken with its proper sign. Resist-
ance is not a directed quantity. If, for example, there are
several voltaic cells in the circuit, some of them may be
connected in the wrong direction so that they oppose
the current ; or ' the circuit may include electrolytic or
storage cells or motors, which offer resistance in the form
of a counter E.M.F. All such electromotive forces must
be reckoned as negative.

. 220. Resistance. Resistance is that property of a
conductor in virtue of which the energy of a current is
converted into heat. It is independent of the direction
of the current, and the transformation into heat occasioned
by it is an irreversible one ; that is, there is no tendency for
the heat-energy to revert to the energy of an electric
current.

The practical unit of resistance is the ohm. It is repre-
srntcd by the resistance offered to an unvarying ele,c,tiic
current by a column of mercury at the temperature of
melting ice, 14.4521 grammes in mass, of a constant cross-
sectional area and of a length of 106.3 centimetres. This
statement is equivalent to a cross-sectional area of one
square millimetre.



OHM'S LAU' AND ITS APPLICATIONS. 275

221. Laws of Resistance. The resistances of diverse
conductors are found to conform to the following laws :

(1) The resistance of a uniform conductor is directly
proportional to its length.

(2) The resistance of a uniform conductor is inversely
proportional to its cross-sectional area. The resistances of
round wires are therefore inversely proportional to the
squares of their diameters.

(3) The resistance of a uniform conductor of given
length and cross-section depends upon the material of
which it consists. This property is called its specific re-
Distance.

222. Specific Resistance. A definite meaning may
be given to specific resistance by conceiving the material
to be in the form of a centi-
metre cube (Fig. Ill), a

cube whose edges are 1 era.
iu length. The specific re-
sistance is the resistance
which this cube opposes to Fig m

the passage of a current

from one face a to the opposite one b. If the conductor
is a cylinder 1 cm. long with parallel ends of one square
centimetre area, the resistance from a to b is the same
as that of the cube. The specific resistance may be rep-
resented by s. Then the following formula expresses all
the laws of resistance :

Is
r = -,

a

where I is the length of the conductor in centimetres, and
a its sectional area in square centimetres. A table of
specific resistances is given in the Appendix, Table IV.

/




276 ELECTRICITY AND MAGNETISM.

223. Conductivity. - - The inverse of a resistance is
called conductivity, or sometimes conductance. A conductor
whose resistance is r ohms has a conductivity equal to 1/r.
When a number of conductors are joined in parallel with




one another, the conductivity of the whole is the sum of
their several conductivities. Let two conductors of resist-
ances-ai|^tnd r.> be joined in parallel between the points A
anal* (Fig. 112). Let V l and V. 2 be the potentials of
A and B respectively. Then since P, V 2 equals the
E.M.F., we have by Ohm's law

_ht _ " _i_ -E
r iv .r/

The first member of this equation is the total current,
which is equal to the sum of the currents through the two
branches ; and r is the combined resistance of the two con-
ductors in parallel. Hence



Similar reasoning applies to any number of parallel con-
ductors.

From the last equation,



224. Effect of Heat on Resistance. - The resistance
of metallic conductors in general increases when the tern-



OHM'S LAW AND ITS APPLICATIONS. 277

perature rises. If R,) is the resistance of a conductor at
C. and R t at t C., then the equation



expresses the relation between the two through a consid-
erable range of temperature. The constant a is called the
temperature coefficient. For most pure metals it is about
(L_per cent for one degree C., or ^0 per cent for a range
of 100 degrees of temperature. The temperature coeffi-
cient for pure copper between 20 and 250 C. was found
by Kennelly and Fessenden to be 0.00406. Dewar and
Fleming have measured the resistances of pure metals in
liquid oxygen at temperatures of 182 and _-.197 C.,
and have shown that the resistance of all of them decreases
with fall of temperature as if it would become zero at
273 C., the zero of the absolute scale. They would
then offer no obstruction to the passage of a current, how-
ever great. Pure copper is the best known conductor, but
it is only slightly better than silver.

The temperature coefficient of alloys is smaller than
that of pure metals. German silver has a coefficient only
about one-tenth as great as that of copper; while that of
platinoid is only one-half as great as that of German silver.
Manganin, an alloy of manganese, copper, and nickel, has at
certain temperatures a small negative temperature coeffi-
cient ; that is, its resistance diminishes slightly as the 1
temperature rises. * {

The resistance of carbon and of electrolytes decreases
when the temperature rises. Thus, the resistance of an
incandescent lamp filament is only about half as great at
normal incandescence as when cold. Solutions of ZnS0 4
and of CuSO^ have a temperature coefficient somewhat
over 0.02, or 2 per cent for one degree C.



278 ELECTRICITY AND MAGNETISM.

225. Loss of Potential proportional to Resistance.
If V } arid V\ are the potentials of two points A and B
on a conductor, then by Ohm's law



It is obvious from this equation that the potential differ-
ence between any two points on a conductor through
which a constant current is flowing is proportional to the
resistance between them, provided the conductor is not
the seat of an E.M.F. Even when electromotive forces
are encountered, the loss of potential, when a given cur-

rent flows through a resistance,
is proportional to that resistance.
If another point be taken be-
tween A and B so situated that
the resistance between it and B
is one-half the resistance be-
tween A and B, then the poten-
tial difference between this point




' r and B is also reduced in the

Fig 113.

same ratio.

Let the distances measured along Or represent resist-
ances (Fig. 113), and those along Ov, potentials. Then
AP equals FJ and BQ, V z \ also PQ stands for the resist-
ance R between the points A and B on the conductor.
Join A and B and let BC be drawn parallel to Or; then
will AO be equal to Fi K, the potential difference
between the points A and B. The slope of the line AB
represents the rate at which the potential drops along the
resistance R. Moreover, since



it is evident that the tangent of the angle of slope equals
the strength of the current.



OHM'S LAM' AXD ITS APPLICATIONS.



279



The principle that the loss of potential is proportional
to the resistance passed over, when the current is constant,
is one of very frequent application in electrical measure-
ments.

226. Wheatstone's Bridge. The instrument known
as a Wheatstone's bridge illustrates the use made of the
principle of the last article. It is a combination of resist-
ances more commonly used than any other method for
the comparison of two of them. It consists of six conduc-
tors connecting four points ; in one of these conductors is
a source of E.M.F., and in another branch is a galvanom-
eter, or sensitive current detector.

Let A, B, (7, D (Fig. 114),
be the four points, B' the bat-
tery, and 6r the galvanometer.
Then since the fall of poten-
tial between A and D is the
same by the path ABD as by
ACD, there must be a point
B on the former which has
the same potential as the
point C on the latter. If the
circuit through the galvanometer is made to connect these
two equipotential points, no current will floAv through it.

Let /, be current through R { ; it will also be the current
through R-, because none flows through the galvanometer,
and the same quantity of electricity flows toward B as
away from it. Also, let I, be the current through the
branch ACD. Then, the potential difference between A
and B being the same as that between A and (7, we have
by Ohm's law (219)

(a)




Fig. 114.



280 ELECTRICITY AND MAGNETISM.

Similarly, R, I, = RJ, (6)

Dividing (a) by (6), ~ - .
This equation may be written

or R, : R 2 :: R, : R 4 .

When therefore the resistances are so adjusted that no
current flows through the galvanometer, the four form a
proportion. In practice three of the resistances are fixed,
and the adjustment for a balance is made by varying the
fourth. It is necessary to know only the ratio R^ / R A , for
example ; then the equation gives the relation between
R } and R 2 .

227. Cells joined in Series. Let there be n similar
voltaic cells, each having an electromotive force E and an
internal resistance between the terminals of the cell equal
to r. Then if R is the external resistance, by Ohm's law

_JP

: +

The n cells may be joined in series by connecting the
negative of the first with the positive of the second ; the
negative of the second with the positive of the third, and
so on. Then the total E.M.F. between the positive of the
first and the negative of the last will be nE, and the entire
internal resistance will be nr. Hence

/- nE

R + nr

If R is small in comparison with r, then 1= E/r nearly, or



OHM'S LA\V AM) ITS APPLICATIONS.



281



the current is no greater than could be obtained from one
cell. But if R is large in comparison with r, or even wr,
then the current is nearly n times as great as one cell alone

will yield.

228. Graphical Representation of Potentials for
Cells in Series. Let there be three cells in series ; and
let AB (Fig. 115) represent 3r, the internal resistance of
the three. Also let
BO equal the exter-
nal resistance R on
the same scale. Be-
ginning at A, erect a
perpendicular Al
equal to E, the E.^I.
F. of one of the cells.
Suppose the E.M.F.
to originate at the
surface of the zinc.
Then as the current flows across through the liquids over
the resistance r there will be a fall of potential represented
by the sloping line be. At , the zinc of the second cell, there
is a sudden rise of potential <?r7, equal to Ab, and then a fall
from d to e ; at e there is a third rise, represented by ef ;
then another drop from/ to g over the internal resistance
of the last cell. The potential difference between the ter-
minals of the battery is then Bg, and this is the loss of
potential over the external resistance R.

The line AD represents 3^7, and DF is the loss of poten-
tial in the three cells on account of their internal resist-
ance. Then

37? E'




Fig. 115.



282 ELECTRICITY AND MAGNETISM.

Since the tangent of the angle of slope is the numerical
value of the strength of current, it is evident that the
lines be, de, and/<? must slope at the same angle as DC, or
must be parallel to one another, because the current lias
the same value in every part of the circuit.

If the external resistance were made infinite by opening
the circuit, the line D would become horizontal, and the
current zero. Also, with any given external resistance,
the less the internal resistance the less the difference
between ZE and E'.

229. Cells joined in Parallel. A battery is said to
be connected in parallel, or in multiple, when all the pos-
itive terminals are joined together, and likewise all the
negatives. The chief object aimed at is the reduction
of the internal resistance. In the case of storage cells,
which have a very low resistance, they may be joined in
parallel when it is desired to use a larger current than the
normal discharge current for one cell. With several cells
in parallel, the current through the external circuit is
divided among them.

If n similar cells are connected in parallel, the E.M.F.
is the same as for a single cell, but there are n internal
paths of equal resistance through the cells, and the result-
ant internal resistance is r/n. Hence



n

In case R is small in comparison with r, the reduction of
the internal resistance secured by joining the n colls in
parallel results in a larger current, but no such result
follows for a large external resistance. For the latter
condition the cells should be in series.



OHM'S LAir AM) ITS APPLICATORS. 283

23O. Cells in Multiple Series. Let there be m series
of n cells each, the m series being joined in parallel. The X
whole number of cells is then mn. The current will be

/- nE E



m n m

To find the condition for a maximum current it may be
remarked that the product of the two terms in the denomi-
nator of the last expression is Rr/nm, a constant. R and
r are assumed to be constant, and nm is the whole number
of cells. But when the product of two terms is a con-
stant, their sum is least when they are equal to each other,
or when R/n r/m. For this condition



m

But R is the external resistance and nr/m is the internal
resistance. For the greatest steady current, therefore, the
cells should be so arranged that the resulting internal
resistance shall be equal to the external resistance. The
efficiency may then be said to be 50 per cent, since half
the energy is wasted internally and half may be utilized
externally. This relation does not hold if there is a
counter E.M.F. in the circuit.

231. Variation of Internal Resistance with Current.
- The internal resistance of a given cell is not a fixed
quantity. It changes with the operation of the cell, on
account of the chemical changes going on which alter the
composition of the liquids. It is also dependent on the
current drawn from the cell. The larger the current,
the smaller is the measured internal resistance. The



284



ELECTRICITY AND MAGNETISM.



curves of Fig. 116 represent graphically the relation be-
tween the internal resistance and the current for two
particular cells. The lower curve was made from obser-



Amperes



.02



.04 .06



.08



.10 .12 .14.
Fig. 116.



.16



.20



vations on an old " dry cell," and the upper one from
observations on a Daniell cell. The scale for the internal
resistance of the latter is twice as large as for the former.
The dry cell showed a most remarkable fall in the resist-
ance as the current increased.



PROBLEMS.

1. Three Daniell cells are connected in series; the E.M.F. of
each cell is 1.1 volts and the internal resistance 2 ohms; if the
external resistance is 5 ohms, find the current.

2. Two Leclanche cells are joined in parallel ; each has an E.M.F.
of 1.5 volts and an internal resistance of 4 ohms. If the external
resistance consists of two parallel conductors of 2 and 3 ohms
respectively, find the current through each branch



OHM'S LAW AM) ITS APPLICATION >. !>">

3. Deduce the formula for the resistance of three conductors in
parallel.

4. Three Bunsen cells are connected in series with one another
and with one copper oxide cell, the latter with its poles set the wrong
way round. If the internal resistance of the Bunsens is 0.5 ohm
each and that, of the other cell 0.2, find the current through an
external resistance of 3 ohms (201).

5. Two equal masses of copper are drawn into wire, one 10
metres long and the other 15 metres. If the resistance of the shorter
piece is 0.4 ohm, find that of the longer.

6. Three wires are joined in parallel ; their resistances are 30,
20, and 60 ohms. Find the resultant resistance.

7. The resistance between two points A and B of a circuit is
25 ohms ; on joining another wire in parallel between A and B the
resistance becomes 20 ohms. Find the resistance of the second wire.

8. The terminals of a battery of five Grove cells in series, the
total K.M.F. of which is 9.5 volts, are connected by three wires,
each of 12 ohms resistance. If the current through each wire is
one-third of an ampere, find the internal resistance of each cell.

9. Given 24 cells, each of 1 volt E.M.F. and 0.5 ohm internal
resistance. How should they be connected to give a maximum cur-
rent through an external resistance of 3 ohms? What will be the
current ?

10. What is the resistance of a column of mercury 212.6 cms.
long and 0.5 of a square millimetre in cross-section, at a tempera-
ture of 25 C. ? Temperature coefficient of mercury, 0.072 per cent
per degree C.



286 ELECTRICITY AND MAGNETISM.



CHAPTER XIX.

THERMAL RELATIONS.

232. Conversion of Electric Energy into Heat. -
Electric energy is readily convertible into other forms.
If an electric current encounters a back E.M.F. anywhere
in the circuit, work will be done by the passage of the
current against this opposing E.M.F. Such is the case in
electrolysis and in the storage battery. All the energy of
an electric current not so converted, or stored up in some
form of stress, is dissipated as heat. Heat appears
wherever the circuit offers resistance to the current. In a
simple circuit containing no devices for transforming and
storing energy, all of it is frittered away as heat. Part
of it disappears in heating the battery or other generator,
and the remainder in heating the external circuit.

The heat evolved by dissolving 33 gms. of zinc in sul-
phuric acid Favre found to be 18,682 calories. When the
same weight of zinc was consumed in a Smee cell, the heat
evolved in the entire circuit was 18,674 calories. These
operations were conducted by introducing the vessel con-
taining the zinc and acid in the first case, and the Smee cell
and its circuit in the second case, into a large calorimeter.
The two quantities are nearly identical, or the heat evolved
is the same whether the solution of the zinc produces a
current or not. When the electric current was employed
to do work in lifting a weight, the heat generated in the
circuit was diminished by the exact thermal equivalent of



THERM A L KEL A TIONS.



287



the work done. When, therefore, a definite amount of
chemical action takes place in a battery and no work is
done, the distribution of the heat is altered, but not its
amount.

233. Laws of the Development of Heat. The laws
of the development of heat in an electric circuit were dis-
covered experimentally by Joule and

Lenz. The latter experimented with a
simple calorimeter represented in Fig. 117.
A thin platinum wire, joined to two stout
conductors, was enclosed in a wide-
mouthed bottle containing alcohol. A
thermometer t was passed through a hole
in the insulating stopper of the bottle.
The resistance of the fine wire was known,
and the observations consisted in measur-
ing the current and noting the rise of
temperature. Joule found that the num-
U-r of units of heat generated in a con-
ductor is proportional

(1) To its resistance.

(2) To the square of the strength of the current.

(3) To the length of time the current flows.

234. The Heat Equivalent of a Current. Let the
potentials of two points A and B of a conductor be \\ and

i nd let Q units of electricity be transferred from A to
B in the time t. Then the work done, expressed in ergs,
will be




Fig. 117.



If all this work is converted into heat, W^JH, by (86) ;
if the current of strength T flows for time /, the quantity



288 ELECTRICITY AND MAGNETISM.

Q = It, since the strength of current is the quantity passing
any section of the conductor in one second; also Fj F* =
RL Substituting,

JH= PRt,

jr PRt PRt

and H=



J 4.19 xlO 7
The current strength and the resistance are expressed in
C.G.S. electromagnetic units (294). An ampere is 10~ l
C.G.S. unit ; an ohm, 10'*. If the measurements are made
in amperes and ohms, then for I- must be substituted
7 2 xlO~' 2 , and for 7, R x 10". Thy equation then becomes

ff= PU x :- 10 - t = PRt x 0.24.
4.19xlO :

The energy expended per second is the product of the cur-
rent strength and the electromotive force. If Zbe measured
in amperes and E in volts (a volt is 10 s C.G.S. units), then

W= IE x 10- 1 x 10 s = IEx 10 7 ergs per second,
or IE watts (I., 43). But

ff= I'Rx 0.24 = IE x 0.24 calories per second.
Therefore one watt is equivalent to 0.24 calorie per second.

235. Counter E.M.F. in a Circuit. The total activ-
ity, or rate at which a generator is supplying energy to
the circuit, is represented in part by the heat evolved in
accordance with Joule's law and in part by work done, such
as chemical decomposition by electrolysis, the mechanical
work of a motor, etc. In every case of doing work the
energy absorbed is a function of the current strength in-
stead of its square. We may therefore write for the whole
energy transformed in time t



THERMAL RELATION*. 289

The first term of the second member of this equation is the
waste in heat ; the second, the work done; A is a constant.
Dividing through by It and transposing,

T _E-A

~^r

R is the entire resistance of the circuit. It is evident from
the form of the equation that the quantity A is of the
nature of an E.M.F. Since it is affected by the negative
sign it is a counter E.M.F. The effective E.M.F. produc-
ing the current is the applied E.M.F. less the back E.M.F.
This counter E.M.F. is" a necessary phenomenon in every
case in which work is done by an electric current.

236. Division of the Energy in a Circuit. If the
counter E.M.F. be represented by E\ the equation for the
current by Ohm's law is

I= E-E'

II
But the heat waste in watts is

PR = I (E-E') = IE- IE'.

Now IE is the total activity in the portion of the circuit
considered. The heat generated in this same portion of
the circuit of resistance R is less than the entire activity
by IE' watts. Hence the energy spent per second in doing
work is the product of the current strength and the counter
E.M.F.

The ratio of the work done to the heat waste is
IE' E'



EE''

The efficiency with which electric energy is converted into
work increases therefore with the counter E.M.F.



290 ELECTRICITY AND MAGNETISM.

237. Applications of the Heating Effect of a Current.
Of the various applications of heating, the following are
some of the more important:

1. Electric Cautery. A thin platinum wire heated to
incandescence is sometimes employed in surgery instead
of a knife. Platinum is used because it is infusible, except
at a high temperature, and is not corrosive.

2. Safety Fuses. Advantage is taken of the low tem-
perature of fusion of some alloys, in which lead is a large
constituent, for the purpose of automatically interrupting
the circuit when for any reason the current becomes ex-
cessive. Some of these alloys, notably those containing
zinc, may oxidize on heating ; and if the current be in-
creased slowly the fused metal may become encased in
the oxide as an envelope, and be heated to redness with-
out breaking the circuit. Safety fuses should be mounted
on non-combustible bases; their length should be pro-
portioned to the voltage employed on the circuit in which
they are placed. Provision is sometimes made for an
automatic blast, produced by the explosive vaporization
of the metal, to blow out the arc which is formed between
the terminals when the fuse " blows."

3. Electric Heating. Electric street-cars are sometimes
heated by a current through suitable iron-wire resistance
embedded in cement, asbestos, or enamel. Similar de-
vices for cooking have now become articles of commerce.