THE ELECTRIC CURRENT
In the previous chapters we have dealt with electricity in charged
bodies, or static electricity, and now we must turn to electricity in
motion, or current electricity. In Chapter I. we saw that if a metal
rod is held in the hand and rubbed, electricity is produced, but it
immediately escapes along the rod to the hand, and so to the earth.
In other words, the electricity flows away along the conducting path
provided by the rod and the hand. When we see the word “flow” we at
once think of a fluid of some kind, and we often hear people speak of
the “electric fluid.” Now, whatever electricity may be it certainly is
not a fluid, and we use the word “flow” in connexion with electricity
simply because it is the most convenient word we can find for the
purpose. Just in the same way we might say that when we hold a poker
with its point in the fire, heat flows along it towards our hand,
although we know quite well that heat is not a fluid. In the experiment
with the metal rod referred to above, the electricity flows away
instantly, leaving the rod unelectrified; but if we arrange matters so
that the electricity is renewed as fast as it flows away, then we get a
continuous flow, or current.
Somewhere about the year 1780 an Italian anatomist, Luigi Galvani,
was studying the effects of electricity upon animal organisms, using
for the purpose the legs of freshly killed frogs. In the course of
his experiments he happened to hang against an iron window rail a
bundle of frogs’ legs fastened together with a piece of copper wire,
and he noticed that the legs began to twitch in a peculiar manner. He
knew that a frog’s leg would twitch when electricity was applied to
it, and he concluded that the twitchings in this case were caused in
the same way. So far he was quite right, but then came the problem
of how any electricity could be produced in these circumstances, and
here he went astray. It never occurred to him that the source of the
electricity might be found in something quite apart from the legs, and
so he came to the conclusion that the phenomenon was due to electricity
produced in some mysterious way in the tissues of the animal itself.
He therefore announced that he had discovered the existence of a kind
of animal electricity, and it was left for his fellow-countryman,
Alessandro Volta, to prove that the twitchings were due to electricity
produced by the contact of the two metals, the iron of the window rail
and the copper wire.
[Illustration: FIG. 6.--Voltaic Pile.]
Volta found that when two different metals were placed in contact
in air, one became positively charged, and the other negatively.
These charges however were extremely feeble, and in his endeavours
to obtain stronger results he hit upon the idea of using a number of
pairs of metals, and he constructed the apparatus known as the Voltaic
pile, Fig. 6. This consists of a number of pairs of zinc and copper
discs, each pair being separated from the next pair by a disc of
cloth moistened with salt water. These are piled up and placed in a
frame, as shown in the figure. One end of the pile thus terminates in
a zinc disc, and the other in a copper disc, and as soon as the two
are connected by a wire or other conductor a continuous current of
electricity is produced. The cause of the electricity produced by the
voltaic pile was the subject of a long and heated controversy. There
were two main theories; that of Volta himself, which attributed the
electricity to the mere contact of unlike metals, and the chemical
theory, which ascribed it to chemical action. The chemical theory is
now generally accepted, but certain points, into which we need not
enter, are still in dispute.
There is a curious experiment which some of my readers may like to try.
Place a copper coin on a sheet of zinc, and set an ordinary garden
snail to crawl across the zinc towards the coin. As soon as the snail
comes in contact with the copper it shrinks back, and shows every sign
of having received a shock. One can well imagine that an enthusiastic
gardener pestered with snails would watch this experiment with great
glee.
[Illustration: FIG. 7.--Simple Voltaic Cell.]
Volta soon found that it was not necessary to have his pairs of metals
in actual metallic contact, and that better results were got by
placing them in a vessel filled with dilute acid. Fig. 7 is a diagram
of a simple voltaic cell of this kind, and it shows the direction of
the current when the zinc and the copper are connected by the wire.
In order to get some idea of the reason why a current flows we must
understand the meaning of electric potential. If water is poured into
a vessel, a certain water pressure is produced. The amount of this
pressure depends upon the level of the water, and this in turn depends
upon the quantity of water and the capacity of the vessel, for a given
quantity of water will reach a higher level in a small vessel than
in a larger one. In the same way, if electricity is imparted to a
conductor an electric pressure is produced, its amount depending upon
the quantity of electricity and the electric capacity of the conductor,
for conductors vary in capacity just as water vessels do.
This electric pressure is called “potential,” and electricity tends to
flow from a conductor of higher to one of lower potential. When we say
that a place is so many feet above or below sea-level we are using the
level of the sea as a zero level, and in estimating electric potential
we take the potential of the earth’s surface as zero; and we regard a
positively electrified body as one at a positive or relatively high
potential, and a negatively electrified body as one at a negative
or relatively low potential. This may be clearer if we think of
temperature and the thermometer. Temperatures above zero are positive
and represented by the sign +, and those below zero are negative and
represented by the sign -. Thus we assume that an electric current
flows from a positive to a negative conductor.
[Illustration: PLATE I.
_By permission of_ _Dick, Kerr & Co. Ltd._
HYDRO-ELECTRIC POWER STATION.]
In a voltaic cell the plates are at different potentials, so that
when they are connected by a wire a current flows, and we say that
the current leaves the cell at the positive terminal, and enters it
again at the negative terminal. As shown in Fig. 7, the current moves
in opposite directions inside and outside the cell, making a complete
round called a _circuit_, and if the circuit is broken anywhere the
current ceases to flow. If the circuit is complete the current keeps on
flowing, trying to equalize the electric pressure or potential, but it
is unable to do this because the chemical action between the acid and
the zinc maintains the difference of potential between the plates. This
chemical action results in wasting of the zinc and weakening of the
acid, and as long as it continues the current keeps on flowing. When we
wish to stop the current we break the circuit by disconnecting the wire
joining the terminals, and the cell then should be at rest; but owing
to the impurities in ordinary commercial zinc chemical action still
continues. In order to prevent wasting when the current is not required
the surface of the zinc is coated with a thin film of mercury. The zinc
is then said to be amalgamated, and it is not acted upon by the acid so
long as the circuit remains broken.
The current from a simple voltaic cell does not remain at a constant
strength, but after a short time it begins to weaken rapidly. The
cell is then said to be polarized, and this polarization is caused by
bubbles of hydrogen gas which accumulate on the surface of the copper
plate during the chemical action. These bubbles of gas weaken the
current partly by resisting its flow, for they are bad conductors,
and still more by trying to set up another current in the opposite
direction. For this reason the simple voltaic cell is unsuitable for
long spells of work, and many cells have been devised to avoid the
polarization trouble. One of the most successful of these is the
Daniell cell. It consists of an outer vessel of copper, which serves
as the copper plate, and an inner porous pot containing a zinc rod.
Dilute sulphuric acid is put into the porous pot and a strong solution
of copper sulphate into the outer jar. When the circuit is closed,
the hydrogen liberated by the action of the zinc on the acid passes
through the porous pot, and splits up the copper sulphate into copper
and sulphuric acid. In this way pure copper, instead of hydrogen, is
deposited on the copper plate, no polarization takes place, and the
current is constant.
Other cells have different combinations of metals, such as silver-zinc,
or platinum-zinc, and carbon is also largely used in place of one
metal, as in the familiar carbon-zinc Leclanché cell, used for ringing
electric bells. This cell consists of an inner porous pot containing
a carbon plate packed round with a mixture of crushed carbon and
manganese dioxide, and an outer glass jar containing a zinc rod and a
solution of sal-ammoniac. Polarization is checked by the oxygen in the
manganese dioxide, which seizes the hydrogen on its way to the carbon
plate, and combines with it. If the cell is used continuously however
this action cannot keep pace with the rate at which the hydrogen is
produced, and so the cell becomes polarized; but it soon recovers after
a short rest.
The so-called “dry” cells so much used at the present time are not
really dry at all; if they were they would give no current. They are in
fact Leclanché cells, in which the containing vessel is made of zinc
to take the place of a zinc rod; and they are dry only in the sense
that the liquid is taken up by an absorbent material, so as to form
a moist paste. Dry cells are placed inside closely fitting cardboard
tubes, and are sealed up at the top. Their chief advantage lies in
their portability, for as there is no free liquid to spill they can be
carried about and placed in any position.
We have seen that the continuance of the current from a voltaic cell
depends upon the keeping up of a difference of potential between the
plates. The force which serves to maintain this difference is called
the electro-motive force, and it is measured in volts. The actual flow
of electricity is measured in amperes. Probably all my readers are
familiar with the terms volt and ampere, but perhaps some may not be
quite clear about the distinction between the two. When water flows
along a pipe we know that it is being forced to do so by pressure
resulting from a difference of level. That is to say, a difference
of level produces a water-moving or water-motive force; and in a
similar way a difference of potential produces an electricity-moving
or electro-motive force, which is measured in volts. If we wish to
describe the rate of flow of water we state it in gallons per second,
and the rate of flow of electricity is stated in amperes. Volts thus
represent the pressure at which a current is supplied, while the
current itself is measured in amperes.
We may take this opportunity of speaking of electric resistance. A
current of water flowing through a pipe is resisted by friction against
the inner surface of the pipe; and a current of electricity flowing
through a circuit also meets with a resistance, though this is not due
to friction. In a good conductor this resistance is small, but in a
bad conductor or non-conductor it is very great. The resistance also
depends upon length and area of cross-section; so that a long wire
offers more resistance than a short one, and a thin wire more than a
thick one. Before any current can flow in a circuit the electro-motive
force must overcome the resistance, and we might say that the volts
drive the amperes through the resistance. The unit of resistance is the
ohm, and the definition of a volt is that electro-motive force which
will cause a current of one ampere to flow through a conductor having a
resistance of one ohm. These units of measurement are named after three
famous scientists, Volta, Ampère, and Ohm.
[Illustration: FIG. 8.--Cells connected in Parallel.]
A number of cells coupled together form a battery, and different
methods of coupling are used to get different results. In addition to
the resistance of the circuit outside the cell, the cell itself offers
an internal resistance, and part of the electro-motive force is used
up in overcoming this resistance. If we can decrease this internal
resistance we shall have a larger current at our disposal, and one way
of doing this is to increase the size of the plates. This of course
means making the cell larger, and very large cells take up a lot of
room and are troublesome to move about. We can get the same effect
however by coupling. If we connect together all the positive terminals
and all the negative terminals of several cells, that is, copper to
copper and zinc to zinc in Daniell cells, we get the same result as
if we had one very large cell. The current is much larger, but the
electro-motive force remains the same as if only one cell were used, or
in other words we have more amperes but no more volts. This is called
connecting in “parallel,” and the method is shown in Fig. 8. On the
other hand, if, as is usually the case, we want a larger electro-motive
force, we connect the positive terminal of one cell to the negative
terminal of the next, or copper to zinc all through. In this way we add
together the electro-motive forces of all the cells, but the amount
of current remains that of a single cell; that is, we get more volts
but no more amperes. This is called connecting in “series,” and the
arrangement is shown in Fig. 9. We can also increase both volts and
amperes by combining the two methods.
[Illustration: FIG. 9.--Cells connected in Series.]
A voltaic cell gives us a considerable quantity of electricity at low
pressure, the electro-motive force of a Leclanché cell being about 1½
volts, and that of a Daniell cell about 1 volt. We may perhaps get some
idea of the electrical conditions existing during a thunderstorm from
the fact that to produce a spark one mile long through air at ordinary
pressure we should require a battery of more than a thousand million
Daniell cells. Cells such as we have described in this chapter are
called primary cells, as distinguished from accumulators, which are
called secondary cells. Some of the practical applications of primary
cells will be described in later chapters.
Besides the voltaic cell, in which the current is produced by chemical
action, there is the thermo-electric battery, or thermopile, which
produces current directly from heat energy. About 1822 Seebeck was
experimenting with voltaic pairs of metals, and he found that a
current could be produced in a complete metallic circuit consisting
of different metals joined together, by keeping these joinings at
different temperatures. Fig. 10 shows a simple arrangement for
demonstrating this effect, which is known as the “Seebeck effect.” A
slab of bismuth, BB, has placed upon it a bent strip of copper, C. If
one of the junctions of the two metals is heated as shown, a current
flows; and the same effect is produced by cooling one of the junctions.
This current continues to flow as long as the two junctions are kept at
different temperatures. In 1834 another scientist, Peltier, discovered
that if a current was passed across a junction of two different metals,
this junction was either heated or cooled, according to the direction
in which the current flowed. In Fig. 10 the current across the heated
junction tends to cool the junction, while the Bunsen burner opposes
this cooling, and keeps up the temperature. A certain amount of the
heat energy is thus transformed into electrical energy. At the other
junction the current produces a heating effect, so that some of the
electrical energy is retransformed into heat.
[Illustration: FIG. 10.--Diagram to illustrate the Seebeck effect.]
[Illustration: FIG. 11.--Diagram to show arrangement of two different
metals in Thermopile.]
A thermopile consists of a number of alternate bars or strips of two
unlike metals, joined together as shown diagrammatically in Fig. 11.
The arrangement is such that the odd junctions are at one side, and the
even ones at the other. The odd junctions are heated, and the even
ones cooled, and a current flows when the circuit is completed. By
using a larger number of junctions, and by increasing the difference of
temperature between them, the voltage of the current may be increased.
Thermopiles are nothing like so efficient as voltaic cells, and they
are more costly. They are used to a limited extent for purposes
requiring a very small and constant current, but for generating
considerable quantities of current at high pressure they are quite
useless. The only really important practical use of the thermopile
is in the detection and measurement of very minute differences of
temperature, which are beyond the capabilities of the ordinary
thermometer. Within certain limits, the electro-motive force of a
thermopile is exactly proportionate to the difference of temperature.
The very slightest difference of temperature produces a current,
and by connecting the wires from a specially constructed thermopile
to a delicate instrument for measuring the strength of the current,
temperature differences of less than one-millionth of a degree can be
detected.