Out of this experiment came the Leyden jar, which for a century and a half was of no practical use, but which now forms an important part of every wireless telegraph equipment. The Leyden jar is simply a glass bottle or jar coated with tin-foil both inside and outside (Fig. 16). When charged with electricity the jar will hold its charge until the two coatings are connected by a metal wire or other good conductor of electricity. A person may receive a strong shock by holding the jar in one hand and touching a knob connected to the inner coating with the other hand. [Illustration: FIG. 16--A LEYDEN JAR] Popular interest in electricity was aroused by this discovery. The friction electrical machine and the Leyden jar were simple and easy to make. People of fashion found them interesting and amusing, the more so because of the shock felt on taking through the body the discharge from the "wonderful bottle," and the fact that several persons could receive the shock at the same instant. On one occasion the Abbé Nollet discharged a Leyden jar through a line composed of all the monks of the Carthusian Monastery in Paris. As the line of serious-faced monks a mile in length jumped into the air, the effect was ridiculous in the extreme. Conductors and Insulators About this time other great electrical discoveries were made. Early in the century, Stephen Gray discovered that some objects conduct electricity and others do not. He discovered that, when a glass tube is electrified by rubbing, it will attract and repel light objects. In the same way a comb or penholder of rubber may be electrified by rubbing it on the sleeve. A bit of paper which touches the comb becomes electrified. Electricity can be transferred from one object to another. Gray discovered further that contact is not necessary, that a hempen thread or a wire will carry an electric charge from one object to another. A silk thread will not carry the electric charge. "Some things convey electricity," he said, "and some do not, and those which do not can be used to prevent the electricity escaping from those which do." Could this obscure inventor have seen a modern telegraph line with the glass insulators on the poles, which prevent the electric current escaping from the telegraph wire, he might have realized the importance of his discovery. He set up a line of hempen thread six hundred and fifty feet long, and with an electrical machine at one end of the line electrified a boy suspended from the other end. Two Kinds of Electric Charge