Since the electricity thus yielded by the electrophorus is not obtained at the expense of any part of the original charge, it is a matter of some interest to inquire whence is the source from which the energy of this apparently unlimited supply is drawn; for it cannot be called into existence without the expenditure of some other form of energy. The fact is, more work is done in lifting the cover when it is charged with the positive electricity than when it is not charged; for when charged, there is the force of the electric attraction to be overcome as well as the force of gravity; this excess force is the real origin of the energy stored up in the separate charges.
Condensers; Leyden Jar.—A condenser is an apparatus for condensing a large quantity of electricity on a comparatively small surface. The form may vary considerably, but in all cases it consists essentially of two insulated conductors, separated by an insulator and the working depends on the action of induction.
A form of condenser generally used in making experiments on static electricity is the Leyden jar, so named from the town of Leyden where it was invented. It consists of a glass jar coated inside and out to a certain height with tinfoil, having a brass rod terminating in a knob passed through a wooden stopper, and connected to the inner coat by a loose chain, as shown in fig. 30.
The jar may be charged by repeatedly touching the knob with the charged plate of the electrophorus or by connecting the inner coating to one knob of an electrical machine and the outer coating to the other knob.
The discharge of a condenser is effected by connecting the plates having an opposite charge. This may be done by use of a wire or a discharger, as shown in fig. 31; the connection is made between the outer coat and the knob.
When the knob of the discharger is sufficiently close to the knob of the jar, a bright spark will be observed between the knobs. This discharge occurs whenever the difference of potential between the coats is great enough to overcome the resistance of the air between the knobs.
Let a charged jar be placed on a glass plate so as to insulate the outer coat. Let the knob be touched with the finger. No appreciable discharge will be noticed. Let the outer coat be in turn touched with the finger. Again no appreciable discharge will appear. But if the inner and outer coatings be connected with the discharger, a powerful spark will pass.
Electric Machines.—Various machines have been devised for producing electric charges such as have been described. The ordinary “static” or electric machine, is nothing but a continuously acting electrophorus.