CHAPTER X. The Electric Current

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.

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.

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.