=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. =Coil.= A strand of wire wound in circular form about a spool, a soft-iron core, or in layers, as a coil of rope. An electro-magnetic generator. A helix. (_See also_ Induction, Resistance, Magnetizing.) =Coil, Choking.= A form of resistance to regulate the flow of current. Any coil of insulated wire wound upon a laminated or divided iron core forms a choking-coil. In alternating-current work special choking-coils are used. They have a movable iron core, and by thrusting it in or out the power is increased or diminished, thus raising or lowering the lights, the same as gas is regulated. =Coil, Faradic.= The name given to a medical induction-coil or faradic machine.