_Current Reverse._—A current in the opposite direction to the normal current. _Decimetre._—The tenth part of a metre. _Deflection._—The angle or number of degrees through which the needle of a galvanometer moves when a current is passing through its coils. _Diaphragm._—A porous division between two liquids through which electric current passes. _Duplex Cut-out._—An instrument which enables a spare fuse to be immediately substituted for that melted. _Duty._—A term used to denote the economy of any motor. _Dynamo._—A name given to machines which produce electricity for commercial purposes. _Dynamometer._—An instrument for ascertaining the horse-power absorbed by any machine. _Dyne._—The Unit of force which gives a velocity of 1 centimetre per second to 1 gramme weight after acting for 1 second. _Direct-Current Dynamo._—An electric generator producing currents passing in one direction. _Earth._—A term for the return circuit, which for economy is formed through the earth in telegraph work. A return conductor common to many circuits is sometimes called “earth.” _Electrodes._—A term for the poles or plates leading the current into and out of a cell. _Electrolysis._—The act of decomposition by the electric current. _Electrolyte._—The liquid in a cell. _Electrometer._—An instrument for measuring electric potential. _Electro-motive Force_ (_E. M. F._) (E).—The electric force tending to produce electric current. The Unit is the volt. _Erg._—The C. G. S. Unit of energy. The work of moving a body through 1 centimetre against the force of a dyne. _Extra Current._—The induced current of higher E. M. F., which appears in a wire wound in a helix when the current is broken. _Farad._—The Unit of capacity: a coulomb divided by a volt. _Field of Force._—The space between or around the poles of a magnet. _Filament._—That part of an incandescent lamp which gives out the light. _Field-Magnets._—In a dynamo the magnets between which the armature revolves. _Foot Pound._—The British Unit of work, or 1 lb. raised 1 foot high. _Galvanometer._—An instrument for measuring current. _Generator._—Another term for a dynamo. _Governor._—An apparatus for controlling the speed of any motor. _Horse-Power_ (_HP._)—indicated HP.—The Unit is 33,000 lbs. lifted 1 foot high per minute. The nominal HP. of any motor is generally fixed considerably less than the indicated. 2(A P R S) Ind. HP. of any engine = ————————— 33,000