=Galvanometer, Tangent.= A galvanometer provided with two magnetic needles differing in length, the shorter one serving to measure tangents, the longer being used for sine measurements of current strength. =Galvanoscope.= An instrument, generally of the galvanometer type, used to ascertain whether a current is flowing or not. =Generator.= An apparatus for maintaining an electric current, such as a dynamo, a faradic machine, a battery, etc. =German-silver.= An alloy of copper, nickel, and zinc. Used chiefly in resistance-coils, either in the form of wire or in strips of the sheet-metal. =Gold-bath.= A solution of gold used for depositing that metal in the electro-plating bath. =Graphite.= A form of carbon. It occurs in nature as a mineral, and also is made artificially by the agency of electric heat. =Gravity Battery.= (_See_ Battery, Gravity.) =Grounded Circuit.= (_See_ Circuit, Grounded.) =Ground-plate.= (_See_ Plate, Ground.) =Ground-wire.= The contact of a conductor, in an electric circuit, with the earth. It permits the escape of current if another ground-wire exists. =Guard Tube.= A tube inserted in a wooden or brick partition to insulate wires that may pass through it. These tubes are made of porcelain, gutta-percha, compositions of a non-conducting nature, and fibre. =Gutta-percha.= Caoutchouc treated with sulphur to harden it; sometimes called vulcanized rubber or vulcanite. It is a product obtained from tropical trees, and when properly treated it is a valuable insulator in electrical work, particularly in submarine cables, since it offers great resistance to the destructive agencies of the ocean’s depths. H =Hand Generator.= A magneto-generator driven by hand for the generation of light currents. =Harmonic Currents.= (_See_ Currents, Harmonic.) =Harmonic Receiver.= A receiver containing a vibrating reed acted on by an electro-magnet. Such a reed answers only to impulses tuned to its pitch. =Heat.= One of the force agents of nature. It is recognized in its effects through expansion, fusion, evaporation, and generation of energy. =Heat, Electric.= Caused by a resisting medium, such as carbon or German-silver, when too much current is forced through it. The principle of the car-warmers, electric iron, electric chafing-dish, etc. =Helix.= A coil of wire. Properly a coil of wire so wound as to follow the outlines of a screw without overlaying itself. =Horse-power, Electric.= Meaning the same as in mechanics. Referred to when speaking of the working capacity of a motor or the power required to drive a dynamo. =Horse-power Hour.= A unit or standard of electrical work theoretically equal to that accomplished by one horse during one hour. =Horseshoe Magnet.= (_See_ Magnet, Horseshoe.) =H-P.= Abbreviation for horse-power. =Hydrometer.= An instrument employed to determine the amount of moisture in the atmosphere.