About the year 1730, a pensioner of the name of Stephen Gray found that electric force may be produced at one end of a wire by holding an electrified body near the other. He expressed this fact by saying that the wire “conducts electricity.” He found that all bodies may be divided into conductors and non-conductors. He proved that the human body is a conductor, and created a sensation by suspending a boy on a network of silk and showing that he conducted electricity. Instead of suspending the boy on a silk net, he found that it was sufficient to let him stand on a cake of resin, which did not conduct electricity. This experimental work with the electric force was taken up in France with great interest. The French experimenters found out that when a body is electrified strongly, sparks may be drawn from it. One experimenter had himself suspended on a silken net, and got the others to electrify him. When that was done sparks could be drawn out of his body by any one who touched him. These sparks made a crackling noise and produced a pricking sensation. “I shall never forget the surprise caused by the first electric spark which was ever drawn from the human body,” exclaimed one of those who were present. So great was the interest roused by these new observations that people began to make electric machines. One of the best of these was made by Professor Bose in 1744. It consisted of a glass globe mounted in a frame and kept rotating by means of a pulley. [Illustration: Fig. 48.--Electricity produced by Friction with Glass and conducted by a Metal Tube.] One of the experimenters held his hands on the glass globe, and so electrified it. The electricity was conducted by a long tin tube suspended by silk threads. The end of the tube next the glass globe was provided with linen threads, which collected the electricity into the tube. At the other end stood an assistant on a cake of pitch or resin. He held a sword in his hand, and from the end of that sword sparks could be drawn as soon as the assistant touched the tin tube with his other hand. These sparks could be made to set fire to spirits of wine held in an iron dish by a servant who stood on the floor. About the same time another German professor used a glass cylinder instead of a glass globe, and did away with the necessity of rubbing the glass with the hand by letting a leather cushion rub against it. This leather cushion was a kind of artificial hand. But it was not sufficiently like the human hand to be very useful, and was afterwards improved by covering with a mixture of tin and mercury. Later still, glass discs were used instead of globes or cylinders.