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88 RICHARDSON'S &LUEBOOK OF PROJECTION
or (equivalent chemicals, and a large piece of aluminum foil which has been rolled or crimped to occupy the smallest possible space. The liquid, which is a conductor, constitutes one of the "plates," to which external contact is made by means of a carbon rod or other suitable conductor. The aluminum is the other plate of the condenser. The insulating layer consists of an extremely thin chemical film formed on the aluminum by the action of the condenser liquid. The capacitance of any condenser may be increased if its insulating material can be made more thin. The chemically formed layer in the electrolytic condenser is far thinner than paper or mica, and small electrolytic condensers accordingly have very high capacitances.
(82) The common type of electrolytic condenser can be used with d.c. only, and must always be connected in correct polarity as indicated by markings on the device. The signs, "-{-" and " — " may be used to designate the terminals, or the manufacturer may prefer a simple color code in which the terminal to be connected to positive polarity is colored red and the negative terminal any other color. The insulating layer is formed in the factory by connecting d.c. to the condenser in proper polarity ; if the condenser be wrongly connected the insulation will break down at once. That is why the device cannot be used with a.c, for which paper or mica condensers are required. Special forms consisting essentially of a double electrolytic condenser mounted in one case are made for use with a.c, but are not very common.
(83) The capacitance of condensers is measured either in micro-farads or micro-microfarads. The color code for rating resistors is often used for condensers, which are likely to be marked with three small colored dots and an arrow showing the order in which the colors are to be read. They have the same numerical significance as in the case of resistors (see page 80). Condensers also are rated in terms of the voltage their insulation will withstand, usually in terms of the d.c. working voltage. When used with a.c. the effective or "r.m.s." voltage to which they are exposed must, as already explained (see page