THE ELECTRIC CURRENT
THE electric machine was used for extracting what was called
the “electric fluid” out of a body by rubbing. The fluid in its passage
through the air made a spark and a noise. It pricked the finger. In
larger quantities it gave a severe shock, sufficient to kill small
animals. In the form of lightning it killed men.
Franklin called the fluid the “electric fire,” and supposed that it
was a very subtle and delicate fluid distributed through all nature,
and was the reason and cause of all electric happenings. When he found
that two silk threads when charged with electricity repelled each
other, he came to the conclusion that one portion of the electric fluid
repels any other portion. When one body was rubbed against another he
supposed that some of the fluid was extracted from one and absorbed by
the other, but that after the exchange the two bodies attracted each
other in proportion to the amount of electric fluid which had been
extracted. There was, therefore, both an electric attraction and an
electric repulsion. The attraction existed between the electric fluid
and a body which had lost its ordinary quantity of that fluid. The
repulsion took place between the portion of the fluid itself, and also
between two portions of matter which had both been deprived of some
of their electric fluid. Franklin believed that the whole science of
electricity could be built up by studying the motion and distribution
of the electric fluid.
Franklin also tried to find out whether a body, after losing some
of its natural quantity of electric fluid, weighs less than before,
but he could not find any difference in weight. This result made him
think that the electric fluid had no weight. He knew the difference
between conductors of electricity and bodies which did not conduct
it. He supposed that conductors could take up a great quantity of
the fluid, and store it away in the interior of their substance. He
knew that not more than a certain quantity of electricity could be
extracted from a body however hard it was rubbed. He supposed that
the amount of electricity in a body was limited, and that after some
rubbing the limit was arrived at. The fact that the electric machine
gave a constant supply of electricity he explained by supposing that
the electricity returns to it through the air. There was, therefore, a
constant circulation of electricity, a kind of electric current. But in
Franklin’s time there was no way of producing a really steady current
of electricity.
If we compare the electric current with a current of water, we may
express the case as follows:--Lightning and the electric spark are like
splashes of water falling from a great height, and making a great deal
of noise and disturbance. The electric machine gives a kind of current
which may be compared to a brook running along a stony bed, and making
a great deal of noise with but little water. The next great discovery
in electricity was how to produce a steady current, which we may
compare with the full, steady, and noiseless flow of a good water-tap.
This discovery was made in a very roundabout way.
Towards the end of the eighteenth century, it was found that certain
curious fish called “torpedoes,” or cramp fish, were capable of
giving their enemies a severe electric shock. When this was proved,
men of science inclined to the idea that all movements of muscles are
electric. One of them, who made experiments with frogs, ventured the
guess that a kind of electric vapour moves through all muscles and
nerves, and produces not only the movements of animals, but also their
sensations. Great efforts were made to discover this electric vapour or
“animal electricity.”
It was, however, not till the year 1790 that the first actual
observation of something like animal electricity was made by a
professor in Bologna of the name of Galvani. We are told that his
wife, being in poor health, was ordered to eat frogs’ legs, and that
the professor experimented with a few of those; but in his own account
there is nothing about his wife or about frogs’ legs considered as
food. He simply relates how he placed the hind legs of a frog together
with a piece of its backbone upon a table on which his electric machine
stood, but without the conductor of the machine touching the frog. One
of his assistants happened to touch the nerves of the legs with a
knife, and he observed to his surprise that a kind of shudder seized
the legs of the dead frog. Another assistant thought that this only
took place when a spark was coming from the electric machine. Galvani
says:--“Astonished at this new observation he drew my attention to it.
Although I was engaged in some quite different work, I was inflamed
by an incredible zeal and desire to examine it and bring to light the
cause that was hidden in it. I, therefore, touched one or other of the
nerves with a knife, and at the same moment one of the assistants drew
a spark from the machine. The effect was always the same. In every
case the muscles of the legs were violently contracted at the moment
at which the spark burst, just as if the animal had been seized with a
cramp.”
Professor Galvani was quite certain that he had discovered animal
electricity. He found that whenever the frogs’ legs were touched with
two different metals, they gave a jerk. He tried combinations of iron
with brass, and lead with silver, and finally concluded that silver is
the best conductor of animal electricity. Galvani upheld his theory of
animal electricity until the time of his death, which occurred eight
years after his discovery. But his theory was attacked by another
Italian whose name was Volta.
Volta was distinguished already by his invention of a simple little
machine which gave a continual supply of electricity. It consisted of a
cake made of resin mixed with turpentine and wax, which was cast into
a flat mould, and allowed to cool until it was hard. A flat tin plate
was cut large enough just to cover the cake, and was provided with a
glass handle. The cake was beaten with a piece of catskin or woollen
cloth. The tin plate was placed on top of the cake and touched with
the finger, which drew a small spark. After that the plate was lifted
off the cake by the handle, and was then found to have a strong charge
of electricity. By repeatedly putting the plate on the cake, touching
it lightly, and then taking it off by means of the glass handle, any
amount of electricity could be obtained without renewing the charge
on the cake by another beating. That little instrument was called the
“electrophorus.”
Volta was all his life the greatest opponent of Galvani’s theory of
animal electricity, and at the end of a long series of brilliant
investigations and experiments he succeeded in demolishing the theory
entirely. He was helped by a curious experiment. It consisted in taking
a plate of lead and a plate of silver, and putting one on the top of
his tongue and the other under his tongue. As soon as the edges of
the two plates touched each other he perceived a queer taste. There
was no such taste when the pieces did not touch, and two different
tastes could be got by putting one or the other plate on the top of the
tongue. No effect was obtained if the two pieces consisted of the same
metal. He therefore concluded that the effect observed by Galvani was
really due, not to animal electricity, but to the contact between two
pieces of different metals. The frogs’ legs in Galvani’s experiment, or
the tongue in his own experiment, simply acted as a conductor, for the
electricity produced as soon as the two metals touched each other.
Volta experimented with a great many different metals, and he soon was
able to show that the electricity produced by contact can be proved
to exist by one of the measuring instruments called electroscopes,
which he had himself done much to make more delicate and sensitive. He
found also that metals behaved very differently, some of them giving
much electricity, and others little. Zinc and copper were found to be
a good combination. The zinc was found after contact with copper to
have the same kind of electrical condition as glass when rubbed with
silk. That kind of electricity was at that time usually called glassy
or “vitreous” electricity. The copper was found to be in the same kind
of electric state as sealing wax or resin after they have been rubbed
with wool or catskin. This was called “resinous electricity.” The
two kinds of electricity were known to attract each other, and when
they were allowed to combine, they rushed together in a spark, and
completely disappeared, the one just neutralising the other. Following
the practice of arithmetic and algebra, the two electricities were also
called positive and negative.
Franklin thought that the positive electricity was the real electric
fluid, and that the negative electricity was simply the want of that
fluid. It was a mere guess of his, and he might have guessed just the
other way. If he had done so, he would have been much more correct,
for it has since been found out that the resinous electricity is best
considered as the real fluid, and vitreous electricity as due to a
want or shortage of that fluid. However, at that time there was no
way of telling what was the real electric fluid, and even the number
of electric fluids which existed. Volta was sure of the existence of
vitreous and resinous electricity, but in addition to this Galvani
maintained the existence of a third fluid, which he called animal
electricity.
After many more experiments, Volta was able to arrange the chief metals
in a row or series, in such a manner that any metal, when in contact
with any other metal further on in the series, would acquire vitreous
electricity. This series was the following: Zinc, lead, tin, iron,
copper, silver, gold. He found that zinc could not be made to acquire
resinous electricity by contact, nor could gold be made to acquire
vitreous electricity. The strongest effect was obtained by the contact
of metals as far apart in the series as possible. The strongest effect
of all was obtained by touching zinc with gold.
[Illustration: Fig. 50.--Volta’s Pile.]
It occurred to Volta that perhaps the strongest effect might be
obtained by combining several pairs of the same metals in succession.
He usually used zinc and copper, because silver and gold were too
expensive. He fixed four glass rods in an upright position, and cut a
large number of round plates of zinc, copper, and leather or cardboard.
The leather or cardboard he soared in salt water. He laid a plate of
copper at the bottom between the four glass rods. On this he placed a
plate of zinc, then a piece of cardboard, then a plate of copper, then
a plate of zinc, then another piece of cardboard, and so on until he
had made a big pile of such combinations. This pile is known to this
day as Volta’s Pile.
Volta found that such a pile gave a little spark on attaching a wire
to the top of it, and bringing the other end of the wire in contact
with the bottom of the pile.
[Illustration: Fig. 51.--Volta’s Crown of Caps.]
He therefore called such a pile an “artificial electric organ,” in
order to direct attention to its similarity to the electric organ of a
cramp fish. The pile did not last long. It either dried up, or if there
was too much water, the water ran down the sides and interfered with
the proper contact of the metals. Volta, however, improved it by using
salt water in a row of cups in order to conduct the electricity from
one pair of metals to the next. Instead of using round plates, he used
the metals in the shape of flat bands, soldering together a copper band
and a zinc band, and arranging the cups in the form of a crown.
The spark obtained with the pile, or crown of cups, was not sufficient
to prove that the electricity produced by it was the same as that
furnished by the electric machine. But the proof was not long in
coming. In the year 1789, water had been separated into the two gases
of which it is composed by conducting the discharge of a Leyden jar
through the water a great many times. The same effect was found to
result also from the action of the electric current obtained from one
of Volta’s piles. This coincidence was discovered accidentally.
A man named Nicholson had attached a wire to the bottom plate of one
of the piles, and brought the other end into contact with the top
plate. In order to improve the contact, he put a drop of water on the
plate just where the wire touched it. He then observed that very small
bubbles appeared in the drop of water, and although the quantity of
the gas produced was very small, he thought it smelt of hydrogen. He
at once tried the experiment of attaching two brass wires to the upper
and lower end of the pile, and bringing them side by side into water,
without touching each other. He then found that small bubbles were
given off at one wire, and that the other was eaten away. He collected
the bubbles of gas, and found that when it was mixed with air it
exploded on bringing a lighted match to it. This proved that the water
had indeed been broken up into two gases called oxygen and hydrogen. It
was the first service to which the newly discovered electric current
was put.