_Polarity._—The distinct features of the two separate poles of a magnet. _Poles._—The two ends of a magnet. The wires, plates, &c., leading from a battery. _Positive._—In a machine the wire proceeding to the lamp. In a battery the zinc plate. Sign +. _Potential._—A word used to indicate a condition for work. Difference of potential is a difference of electrical condition. Potential of a battery means its E. M. F. _Power_ (P).—The rate of doing work. When an ampère passes through an ohm, the unit power, called a watt, is required. _Quantity_ (Q).—The Unit is the Coulomb. _Relay._—An electro-magnet which, receiving its current from a distance, closes the circuit of a local battery so as to produce the required effect of strength. _Resistance_ (R).—The opposition presented by the circuit to the development of the current. The Unit of resistance is the Ohm. _Rheostat._—An instrument for inserting resistances. A valuable artificial resistance employed for measuring unknown resistances. _Return Current._—The current in the wire leading to the machine. _Rigger._—The pulley or wheel by which power is transmitted. _Secondary Battery._—Wrongly termed an accumulator, is an appliance for storing energy in such a form that it shall be available for the reproduction of electric currents. _Secondary Generator._—A transformer of a current of high potential into a current of less E. M. F. _Series._—The plan of connecting lamps so that the current passes one after the other. _Shunt._—A coil of wire arranged to take a certain proportion of any current. _Solenoids._—Helices of wire which act like magnets. _Switch._—An apparatus for changing one circuit on to another. _Spectrum._—The elongated figure of the prismatic colours. _Torque._—Term used to express the strain on a shaft due to electro-magnetic action. _Units._—The various bases of any system of measurement. _Volt._—The Unit of electro-motive force and potential. An ampère multiplied by an ohm. (See Electrical Measurements.) _Voltameter._—An apparatus for measuring the current by its chemical action. _Voltmeter._—An instrument used for measuring E. M. F. _Watt._—The Unit of power. A volt-ampère. The horse-power electrical, taken as 746 B A watts, is equivalent to only 736 true watts. The horse-power electrical is equal to 756 B A watts, which is equal to 746 true watts. The “force de cheval,” or horse-power in use abroad, is defined as 75 kilogrammetres, and is, therefore, 736 true watts. _Work_ (W.)—Is a volt multiplied by a coulomb, or (amp.² × sec × ohm) or (amp. × sec × volt). The Unit is the Joule.