Circuit, Quadruple. A single circuit capable of having four messages transmitted over it simultaneously—two in one direction, and two in the other.
Circuit, Return. In telegraphy the ground is used as the return circuit. It is also that portion of a circuit which leads from an apparatus back to the terminal of a dynamo or battery, usually the negative wire.
Circuit, Short. A connection between two parts of a circuit, causing the current to skip a great part of its appointed path. Short-circuits prevent the proper working of any electrical apparatus.
Circuit, Simple. A circuit containing a single generator, the proper wire for carrying the current, and a switch to operate it. An electric-bell line, a single telegraph line, or a direct telephone line are all simple circuits.
Clamp. A tool for grasping and holding the ends of wires while joining them.
The appliance for holding the carbon-pencils in arc-lamps.
Cleats. Blocks of wood, porcelain, or other insulating material used to hold wires against a wall or beam. They have one, two, and three notches at one side, for single, double, and three wire systems.
Clutch, Electric. A form of magnetic brake applied to car-wheels, the armatures of motors, and other revolving mechanism, whereby the current, passing through a coil, magnetizes a mass of cast-iron, and brings it to bear frictionally upon the moving parts of the mechanism.
Code, Cipher. A set of disconnected words which, in accordance with a prearranged key, stand for whole sentences and phrases. Commercially the system is used as a short-cut—ten words perhaps meaning what otherwise it would take forty or fifty words to express. It is used extensively in telegraphy, both as an abbreviated message and as a means for securing secrecy.
Coherer. Conducting particles constituting a semi-conducting bridge between two electrodes, and serving to detect electro-magnetic waves. The coherer in wireless telegraphy is understood to mean that form of radio-receiver which, being normally at high resistance, is, under the influence of Hertzian-waves, changed to a low resistance, thus becoming relatively a conductor. Tubes of various kinds have been used for this purpose. Within them is a filling of carbon granules, copper filings, nickel and silver filings, and other substances. Marconi’s coherer consists of a tube one and one-half inches long and one-twelfth inch internal diameter. This is filled with filings—90 per cent. of nickel, 10 per cent. of silver. A globule of mercury coats the outer surface of each grain with a thin film of the quicksilver. Into both ends a piece of pure silver wire is plugged. These latter are a quarter of an inch long, and fit the tube very accurately. The tube is thus sealed, and it is considered preferable to have a slight vacuum within it.