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384 SOUND MOTION PICTURES
and capacitance. Total reactance equals inductive reactance minus capacitive reactance.
Reactor: An inductor of large value, usually having an iron core.
Rectifier: An apparatus which converts alternating current (A. C.) into pulses of direct current (D. C). Tungar, Rectigon, and Kenetron apparatus are employed for rectifying purposes.
Relay: A device by means of which electric power in one circuit controls electric power in another circuit.
Resistance: Opposition to the flow of an electric current through a conducting medium. All metals have more or less electrical resistance. Copper is used universally for both electrical and radio work on account of its low resistance, comparatively low cost, and ready supply. Silver is a better conductor, but it is too expensive. The unit of resistance is the ohm.
Resistor: A unit or element in which resistance is prominent.
Resonance: A very important function of radio circuits. Resonance in a given circuit is said to exist when its natural frequency has the same value as the frequency of the alternating electromotive force introduced in it. The current is then in tune with the natural period of vibration of the circuit. The theory of electrical resonance is the same as that of acoustics, readily demonstrated by the tuning forks, where one tuning fork will not respond to another unless it is of the same key or pitch. Bringing a circuit into resonance means bringing it into tune with another circuit or transmitter.
Rheostat: A variable resistance usually employed to control or regulate current flow.
Secondary: The output circuit or coil of a transformer.
Series: The tandem, or successive connection of several electrical devices in one circuit, especially of battery cells having the positive of one cell connected to the negative of the next, throughout the battery.
Shield: A plate or casing, usually connected to ground for preventing changes in capacitance.