=Circuit Loop.= A minor circuit introduced, in series, into another circuit by a switch or cut-out, so that it becomes a part of the main circuit. =Circuit, Main.= a circuit or main line, includes the apparatus supplying current to it. Thus distinguished from a local circuit. =Circuit, Metallic.= A circuit in which the current outside the generator passes through metal parts or wire, but not through the ground. Electric light and power lines are always metallic circuits. An electro-plating apparatus may be properly termed a metallic circuit, although a part of the circuit is formed by the electrolyte in the bath. The essential meaning of the words metallic circuit is that the earth does not form a part of the return circuit. =Circuit, Open.= A circuit in which a switch has been opened to prevent the continuous flow of current, such as an electric-bell circuit, which normally remains open, and which is active only when the push-button is pressed, thereby closing the circuit and operating the bell. An open-circuit battery is one that remains inactive when the circuit is open. =Circuit, Parallel.= A term signifying a multiple circuit. =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.