=Secondary Plates.= The plates of a secondary battery or storage-battery. When charged, the negative plate should be brown or deep reddish in color, and the positive slate-colored. =Self-excited.= Electrified by its own current. =Self-winding Clock.= A clock which automatically winds itself by electricity. It is operated by a small electro-magnetic motor which obtains its current from an outside source. =Semaphore, Electric.= An apparatus for exhibiting signals. Used in the railway block system. =Series.= Arranged in succession. When incandescent lamps are installed so that the current goes in and out of one lamp, and so on to the next and the succeeding ones, they are said to be arranged in series. It takes high E-M-F and current, or amperage, to operate such lamps. Series batteries are arranged with the zinc pole of one connected to the carbon pole of the next. =Series Arc Cut-out.= A device by means of which a short circuit is established past a defective lamp, thereby securing the undisturbed operation of all the other lamps in the circuit. =Series Distribution.= A distribution of electricity in which the receptive devices are arranged in successive order upon one conductor, extending the entire length of the circuit. =Series Dynamo.= A series-wound dynamo. =Series Incandescent Lamp.= An incandescent lamp adapted for service in a series circuit. =Series Motor.= A motor adapted for use in a series circuit; a motor whose field-coil winding is in series with the armature. =Series, Multiple.= An arrangement of electric apparatus in which the parts are grouped in sets in parallel, and these sets are connected in series. =Series Winding.= A method of winding a generator or motor in which one of the commutator brush connections is joined to the field-magnet winding. The other end of the magnet winding is connected with the outer circuit, and the second armature brush is coupled with the remaining terminal of the outer circuit. =Service Wires.= Wires connected to the supply circuit or main wires, and which run into buildings to supply current for heat, light, and power. =Shellac.= A resin gum, gathered from certain Asiatic trees. It is soluble in alcohol, and is used extensively in electric work as an insulator. =Shifting Magnetic Field.= (_See_ Magnetic Field, Shifting.) =Shock, Electric.= The effect upon the animal system of the discharge of an electric current of high potential difference. The voltage is the main element in a shock. =Shoe.= As applied to electric railways, the casting employed to bear on the third rail to take in positive current and electro-motive force. The cast-iron plate of an electric break, which, by magnetism, adheres to another iron surface. =Short Circuit.= (_See_ Circuit, Short.) =Shunt-box.= A resistance-box designed for use as a galvanometer shunt. The box contains a series of resistance-coils which can be plugged in or out as required.