Capacity, Storage. The quantity of electricity in a secondary battery when fully charged, usually reckoned in ampere hours. Carbon. A material, like coke, ground or crushed, and formed into sticks or plates by molding or compression. It requires a high heat to melt or burn, and is used as electrodes for arc lamps and for battery elements. It has poor conductivity, and for arc lamps is coated with copper to increase its conductivity. Cell, Electrolytic. A vessel containing an electrolyte for electroplating purposes. Charge. The quantity of electricity on the surface of a body or conductor. Chemical Change. When a current passes through electrodes in a solution, a change takes place which is chemical in its character. Adding sulphuric acid to water produces heat. If electrodes of opposite polarity are placed in such an acid solution, a chemical change is produced, which is transformed into electricity. Choking Coil. An instrument in a circuit which by a form of resistance regulates the flow of the current, or returns part of it to the source of its generation. Counter-electromotive Force. Cells which are inserted in opposition to a battery to reduce high voltage. Circuit, Astatic. A circuit in an instrument so wound that the earth's magnetism will not affect it. Circuit Breaker. Any instrument in a circuit which cuts out or interrupts the flow of a current. Circuit, External. A current flows through a wire or conductor, and also along the air outside of the conductor, the latter being the _external circuit._ Circuit Indicator. An instrument, like a galvanometer, that shows the direction in which a current is flowing through a conductor. Circuit, Return. Usually the ground return, or the negative wire from a battery. Circuit, Short. Any connection between the mains or parallel lines of a circuit which does not go through the apparatus for which the circuit is intended. Coherer. A tube, or other structure, containing normally high resistance particles which form a path or bridge between the opposite terminals of a circuit. Coil. A wire, usually insulated, wound around a spool. Coil, Induction. One of a pair of coils designed to change the voltage of a current of electricity, from a higher to a lower, or from a lower to a higher electro-motive force.