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466 RICHARDSON'S BLUEBOOK OF PROJECTION
constantly flowing into one and out of the other, reversing this procedure so many times a second, according to the frequency of the alternating supply. The result is a very slight but endless surging of current in the wires leading to the switch.
(3) Now if the switch be partly closed, this slight flow of current will be increased, because a greater number of negatrons will be attracted to the negative blade and a greater number of them repelled from the positive blade. The closer the blades, the greater the flow of current will be, though in any event it is of course too weak to be detected by anything but a very delicate galvanometer.
Sometimes it is desirable to include in a circuit a device that will permit the effect of alternating current flowing, while at the same time preventing the passage of any direct current. Such a device is an "open switch," constructed of two pieces of metal placed very closely together, but kept from touching by some efficient insulation such as mica or paper. The thinner the insulation— the more nearly the "switch" is closed — the greater the amount of alternating current that will move in the wires, although as just explained negatrons do not in fact move through the insulation. A still greater flow of alternating current is obtained by increasing the dimensions or surface area of the conductors at the point where they face each other, because the larger this area the greater the number of negatrons that will be attracted into the negative and repelled from the positive plate.
The Electrical Condenser
(4) A device of this kind is called a condenser. Its action is precisely the same as that of an open switch in an a. c. line. In commercial form it consists of very large areas of metal or tin-foil, separated from each other by extremely thin layers of either treated paper or of mica. (5) Another commercial form uses a plate of aluminum separated from a conducting liquid by a chemical layer deposited on the aluminum. This latter