5. If a sharp tack be dropped point up on the plate of an electroscope the latter is quickly discharged. Explain. (5) ELECTROSTATIC GENERATORS =234. Static Electric Machines.=--Many machines have been invented to produce larger quantities of static electricity than we have used in the experiments previously described. One of the earlier of these was the _plate friction machine_ in which a large circular glass plate was rotated while a pad of some material was held against it. This machine was capable of producing powerful effects, but it took much work to turn it, and it has been abandoned for a more efficient device, the _static induction machine_. =235. The electrophorus= is the simplest static induction generator, consisting simply of a flat circular plate of some insulating material, as paraffine, shellac, or rosin contained in a metal pan, and a flat circular metal _disc_ having an insulating handle. [Illustration: FIG. 212.--An electrophorus.] _The electrophorus is used as follows_: The plate is first electrified by rubbing or beating with fur or a woolen cloth. The plate will be found to be charged negatively. The metal disc is placed upon the plate by holding the insulating handle. The upper surface of the charged body is slightly uneven so that the disc touches but a few high points. The greater part of the charged surface is separated from the metal disc by air, a good insulator. The charge therefore acts inductively upon the disc _repelling negative_ electricity to the upper surface of the disc, leaving the lower surface charged positively (Fig. 212). If now the finger is touched to the disc the repelled negative charge escapes and the whole disc is left positively charged. The disc is now removed (Fig. 213) and the charge upon it may be tested or used in any desired manner. The disc may be recharged many times without rubbing the plate again. [Illustration: FIG. 213.--Electrophorus charged.] These electrical charges possess _energy_. What is the source of this energy? The answer may be determined by the following experiment. Place the disc upon the charged plate. Touch the disc with the finger to remove the repelled charge. Connect an electroscope to the disc by a fine wire. Nothing appears on the electroscope, since the disc has been connected to the earth, and is therefore at zero potential. If now the disc is lifted slowly, the leaves of the electroscope gradually separate, showing that a charge of electricity appears when the disc is being lifted against the force of attraction between the two charges. Just as potential energy is developed in a weight when it is lifted against the earth's attraction so electrical energy appears in the disc while it is being separated from the plate. The electrical energy of the charge is therefore due to the work done in separating the two charges. This electrical energy appears as heat and light, when the disc is discharged. It may be employed to ignite gas, gunpowder, etc.