CH. XXIX.

BENJAMIN FRANKLIN.

253

CHAPTER XXIX.

SCIENCE OF THE EIGHTEENTH CENTURY (CONTINUED). Benjamin Franklin, born 1706-His Early Life-Du Faye discovers two kinds of Electricity-Franklin proves that Electricity exists in all bodies, and is only developed by Friction-Positive and Negative Electricity-Franklin draws down Electricity from the Sky Invents Lightning-conductors-Discovery of Animal Electricity by Galvani-Controversy between Galvani and Volta - Volta proves that Electricity can be produced by the Contact of two Metals-Electrical Batteries-The Crown of Cups-The Voltaic

Benjamin Franklin, born 1706.-He Investigates the Nature of Electricity, 1746.-Benjamin Franklin, the printer and man of science, was born at Boston, in America, in the year 1706. He was the son of a tallow-chandler, and had so many hard struggles in his early life that he does not seem to have turned his thoughts to science till he was nearly forty years of age. His father intended him for the Church, but there was not enough money to pay for his education, so he was apprenticed to his brother, who was a printer. Here he worked very hard, yet he used to snatch every spare moment to read any books which came within his reach ; but his brother being unkind and harsh to him, a quarrel sprang up between them, and Benjamin at last ran away to New York, and from there to Philadelphia. In this last place he got a little work, but hoping to do better in England he came to London, where he learnt many of the

newest improvements in printing. After a time he went back to Philadelphia, and from that time he began to succeed as a printer and became a well-known and respected man.

It was in the year 1746 that he first began to pay attention to the experiments in electricity which were being made in England and France. A great deal had been learnt about this science since the time when Otto Guericke made the first electrical machine in 1600, and a Frenchman named Du Faye had shown that two different kinds of electricity could be produced by rubbing different substances. You will remember that a pith-ball, when filled with electricity from a stick of electrified sealing-wax, draws back, and will not approach the sealing-wax again (see p. 124). But Du Faye discovered that if you rub the end of a glass rod with silk, and bring it near to this ball, it will draw the ball towards itself, showing that the electricity in the glass rod has exactly the opposite effect to that in the sealing-wax. In other words, while Guericke had shown that substances filled with the same kind of electricity repel each other, Du Faye showed that substances filled with different kinds of electricity attract each other. Both these men thought that electricity was a fluid which was created by the rubbing, and which was not in bodies at other times; when Franklin, however, began to make his experiments he saw clearly that this was not as they had supposed, but that all bodies have more or less electricity in them, which the rubbing only brings out.

The way in which he proved this is very interesting; but to understand it you must first know that any body which is to be filled with electricity requires to be so placed that the electricity cannot pass away from it into the earth. The best way to do this is to stand it upon a stool with glass legs, because electricity does not pass easily along glass.

CH. XXIX.

EXPERIMENTS IN ELECTRICITY.

255

You must also know that when any substance is full of electricity, if you bring your finger or a piece of metal near to it, a spark will pass between the electrified substance and your finger or the metal.

You will now, I think, be able to follow Franklin's experiments. He put a person, whom we will call A, upon a glass stool, and made him rub the metal cylinder of an electrical machine with one hand and place his other hand upon it to receive the electricity. Now, he said, if electricity is created by the rubbing, this person must be filled with it, for he will be constantly taking it from the machine, and it cannot pass away, because of the glass legs under the stool. But he found that A had no more electricity in him after rubbing the cylinder than he had before, neither could any sparks be drawn out of him. He then took two people, A and B, and placing each of them on a glass stool, made a rub the cylinder, and B touch it, so as to receive the electricity. Now notice carefully what happened. B was soon so full of electricity that when Franklin touched him, sparks came out at all points; but what was still more curious, when Franklin went to A and touched him, sparks came out between them just as they had done between him and B.

Α

This he explained as follows: A, B, and myself,' he said, 'have all our natural quantity of electricity. Now when A rubbed the tube, he gave up some of his electricity to it, and this B took, so that A had lost half his electricity and B had more than his share. I then touched B, and his extra charge of electricity passed into me and ran away into the earth. I now went to A, and I had more electricity in me than he had, because he had lost half his natural quantity, and so part of my electricity passed into him, prodɩ cing the sparks as before.'

This Franklin believed to be the case with all electricity, namely, that every body contains its own amount of it, but that when for any reason it is distributed unequally, those which have no more than they can well carry, give some up to those which have less, till they have each their right quantity. And this explained at once why a man cannot electrify himself, for so long as he has no one else from whom he can procure electricity, he is only taking back with one hand what he gives out with the other. Those who had too much electricity were called by Franklin positively electrified, and those who had too little, negatively electrified, and from this come the terms positive and negative electricity, which are now used,

I should tell you here that it is now believed that electricity is composed of two different kinds existing together in all substances. These two kinds are supposed to remain at rest as long as they are equally balanced, but when a body contains too much of one kind, it is always trying either to give it up or to get some of the other kind to balance it. This theory explains some facts which Franklin's theory does not; but it is not yet really known what electricity is, only it is certain that Franklin was right in saying that it is not created when we see its effects, but only drawn out of bodies which contain it.

Franklin draws down Lightning from the Sky-It was in 1749, when he had already made most of his experiments upon electricity, that Dr. Franklin began to consider how many of the effects of thunder and lightning were the same as those which he could produce with his electrical machines. Lightning travels in a zigzag line, said he, and so does an electric spark; electricity sets things on fire, so does lightning; electricity melts metals, so does lightning.

CH. XXIX.

FRANKLIN'S KITE.

257

Animals can be killed by both, and both cause blindness; electricity always finds its way along the best conductor, or the substance which carries it most easily, so does lightning; pointed bodies attract the electric spark, and in the same way lightning strikes spires, and trees, and mountain tops. Is it not most likely that lightning is nothing more than electricity passing from one cloud to another just as an electric spark passes from one substance to another?

Franklin communicated these ideas to the Royal Society in London, suggesting at the same time that, if he was right, it would be possible to prevent a great deal of the harm done by lightning by fixing upright rods of iron near high buildings so that the electricity might run down from the clouds into the earth without doing any harm. But this notion seemed so absurd, even to clever men, that they could not help laughing when his papers were read, and did not even think them worth printing. You will easily understand that after this Franklin was ashamed to speak of an experiment he meant to make by which he hoped to bring down electricity from the sky. So we find that he told no one but his son, whom he took with him upon this strange expedition.

Franklin's idea was that if he could send an iron rod up into the clouds to meet the lightning, it would become charged with the electricity, which he believed was there, and would send it down a thread attached to it, so that he might be able to feel it. He took, therefore, two light strips of cedar fastened crossways, upon which he stretched a silk handkerchief tied by the corners to the end of the cross, and to the top of this kite he fixed a sharp-pointed iron wire more than a foot long. He then put a tail and a string to his kite, and at the end of the string near his hand he tied