=Knife Switch.= A switch with a narrow and deep, movable blade, or bar of copper or brass, which resembles the blade of a knife. It is forced between two spring-clamps attached to one terminal so as to make perfect contact. L =Laminated.= Made up of thin plates, as an armature-core. =Laminated Core.= (_See_ Core, Laminated.) =Lamp-Arc.= A lamp in which the light is produced by a voltaic arc. Carbon electrodes are used, and a special mechanism operates and regulates the space between the carbons so that a perfect arc may be maintained. =Lamp, Incandescent.= A lamp in which the light is produced through heating a filament to whiteness by the electric current. It consists of a glass bulb from which the air is exhausted and sealed, after the filament is enclosed. The ends of the filament are attached to platinum wires, which in turn are made fast to the contact-plates at the head of the lamp, so as to connect with the current. =Lamp-socket.= A receptacle for an incandescent lamp. It is generally made of brass and provided with a key-switch to turn the current on and off. =Latent Electricity.= (_See_ Electricity, Latent.) =Lead.= (Not the metal.) An insulated conductor which leads to and from a source of power; an insulated conductor to and from a telegraph or telephone instrument; a circuit, a battery, or a station. Not a part of the line circuit. That part of an electric light or power circuit which leads from the main to the lamps or motors. =Leading-in Wires.= The wires which lead into a building from an aerial circuit. The wires which lead in and out from a lamp, battery, or instrument. =Leak.= An escape of electrical energy through leakage. This is more liable to occur in bare than in insulated wires. The escape of current from bare trolley wires is much greater than that from the insulated conductors, particularly in damp or rainy weather. =Leclanché Battery.= (_See_ Battery, Leclanché.) =Leyden-jar.= A type of static condenser. Its usual form is a glass jar. Tin-foil is pasted about its inner and outer surfaces covering about half the wall. The balance of the glass is painted with shellac or insulating varnish. The mouth is closed with a cork stopper, and through its centre a brass rod is passed which, by a short chain, is connected with the interior coating of the jar. The top of the rod is provided with a brass knob or ball, and from this last the spark is drawn. =Lightning.= The electro-static discharge of clouds floating in the atmosphere. It is the highest form of frictional electricity, uncontrollable and very dangerous, since the strength of a single flash may run into hundreds of thousands of volts. =Lightning-arrester.= An apparatus for use with electric lines to carry off to earth any lightning discharges that such lines may pick up; or it may be a form of fuse which burns out before the current can do any harm to the electrical mechanism. =Line-insulator.= An insulator serving to support an aerial line.