=Shunt-winding.= A dynamo or motor is shunt-wound when the field-magnet winding is parallel with the winding of the armature. =Silver-bath.= A solution of a salt of silver used in the electro-plating process. =Silver-plating.= Depositing a coating of silver on a metallic surface by the acid of electro-metallurgy. =Silver-stripping Bath.= An acid solution used for stripping silver from a metallic surface before re-plating it. =Simple Circuit.= (_See_ Circuit, Simple.) =Simple Immersion.= (_See_ Immersion, Simple.) =Simple Magnet.= (_See_ Magnet, Simple.) =Single-trolley System.= A trolley system employing only one overhead conducting wire, the track and ground serving as the return-circuit. =Single-wound Wire.= Wire insulated by winding or overlaying with but a single layer of material. =Sliding-condenser.= (_See_ Condenser, Sliding.) =Snap-switch.= A switch so contrived as to give a quick break. A spiral spring is fastened between the handle and arm in such a manner that when the handle is drawn back the spring operates and quickly draws a knife-bar from the keeper, breaking the contact instantly and without the formation of an arc. =Socket.= A receptacle for an incandescent lamp or plug. =Solenoid.= A helical coil of wire of uniform diameter or cylindrical in shape. It is useful in experiments with electro-magnetism. =Solution.= A fluid composed of dissolved salts; a mixture of liquids and fluids. =Sound Waves.= Waves produced in an elastic medium by sonorous vibration, as in wireless telegraphy. =Sounder.= In telegraphy, the instrument operated on by the key at the other end of a line. Various devices are employed to increase their resonance--as, for instance, hollow boxes. Sounders are generally placed on local circuits and are actuated by relays. =Sounder, Repeating.= A telegraphic instrument which repeats a message into another circuit. =S-P.= An abbreviation for single pole. =Spark-arrester.= A screen of wire-netting fitted around the carbons of arc-lamps to prevent the chips or hot sparks from flying. =Spark-coil.= A coil for producing a spark from a source of comparatively low electro-motive force. The induction-coil is an example. =Spark, Electric.= The phenomenon observed when a disruptive charge leaves an accumulator or induction-coil and passes through an air gap. =Spark-gap.= The space left between the ends of an electric resonator across which the spark springs. =Sparking.= The production of sparks at the commutator, between the bars and the brushes of dynamos and motors. They are minute voltaic arcs, and should not be allowed to occur, as they cut away the metal and score the surface of the commutator. =Spark-tube.= A tube used as a gauge to determine when the exhaustion of the vacuum chamber, or bulb, of an incandescent lamp is sufficiently high.