=Chemical Change.= When bodies unite so as to satisfy affinity, or to bring about the freeing of thermal or other energy, the union is usually accompanied by sensible heat or light. Sulphuric acid added to water produces heat; a match in burning produces light. Another form of chemical change is decomposition or separation (the reverse of combination), such as takes place in the voltaic-battery, the electro-plating bath, and other forms of electrolysis. This is not accompanied by heat or light, but by the evolution of electricity. =Chemical Element.= (_See_ Element, Chemical.) =Chemistry.= The science which treats of the atomic and molecular relations of the elements and their chemical compounds. Chemistry is divided into many departments, but electro-chemistry treats only of the science wherein electricity plays an active part, such as batteries, electro-plating, and electro-metallurgy. =Choking-coil.= (=See= Coil, Choking.) =Circle, Magic.= A form of electro-magnet. It is a thick circle of round iron used in connection with a magnetized coil to illustrate electro-magnetic attraction. =Circuit.= A conducting-path for electric currents. Properly speaking, a complete circuit has the ends joined, and includes a source of current, an apparatus, and other elements introduced in the path. When the circuit is complete it is called active. The term circuit is also applied to portions of a true circuit--as, an internal or external circuit. =Circuit, Astatic.= A circuit so wound, with reference to the direction of the currents passing through it, that the terrestrial or other lines of force have no directive effect upon it. =Circuit-breaker.= Any apparatus for opening and closing a circuit, such as switches, automatic cut-outs, lightning-arresters, and the like. A ratchet-wheel engaged with a spring, or wire, which rests against the teeth. The current passes through the wire, the wheel, and axle. The wheel is revolved by a crank, and as the ratchets pass the spring, or wire, an instantaneous make-and-break occurs. The speed of the wheel regulates the frequency of the interruptions. =Circuit, External.= A portion of the circuit not included within the generator, such as a secondary telegraph key and sounder. =Circuit, Grounded.= A circuit in which the ground is used as a conductor. This is common in telegraph and telephone lines, particularly for short distances where the conductivity of the earth does not offer too much resistance. =Circuit, Incandescent.= A circuit in which incandescent lamps are installed. =Circuit Indicator.= A pocket-compass, galvanometer, or other device for indicating or detecting the condition of a wire, whether it is active or dead, and, if active, in which direction the current is flowing. It may also give a general idea of its strength. =Circuit, Internal.= That portion of an electric circuit which is included within the generator.