=Resistance, Standard.= A known resistance employed to determine unknown resistances by comparison. =Resistance, True.= The true resistance measured in ohms as distinguished from counter-electro-motive force. =Resonator, Electric.= A small, open electric circuit with ends nearly touching. When exposed to electric resonance, or to a sympathetic electric oscillating discharge, a spark passes across the gap. The spark is due to inductance in the resonator. =Retentiveness.= That property which enables steel to retain its magnetism. =Return.= A line or conductor which carries current back to its starting-point after it has traversed a circuit. The best definition of a return is a circuit on which no new apparatus is installed. =Return-circuit.= (_See_ Circuit, Return.) =Return-circuit, Railway.= A grounded circuit used in trolley systems for ground returns through the tracks, they being joined by links or flexible wires so as to form perfect conductors. It is the negative side of the system, the positive being in the overhead or underground feed-wire or rail. =Reversibility.= The principle by which any form of generator for producing a given form of energy may be reversed to absorb energy. The dynamo of the reversible type driven to generate current may be reversed and will develop power if a current is run through it. =Rheostat.= An adjustable resistance. An apparatus for changing the resistance, without opening the circuit, by throwing a switch-bar across contact points. =Rod Clamp.= A clamp used in the lamp rod of an arc-light to hold the carbon. =Röntgen Effects.= Phenomena obtained by the use of the X or Röntgen rays. =Röntgen-ray Screen.= A screen whose surface is covered with fluorescent material for the purpose of receiving and displaying the Röntgen image. =Röntgen Rays.= A peculiar form of light radiation discovered by Röntgen, and which is emitted from that portion of a high vacuum tube upon which the kathode rays fall. =Rotary Magnetic Field.= (_See_ Magnetic Field, Rotary.) =Ruhmkorff Coil.= (_See_ Coil, Ruhmkorff.) S =Safety Fuse.= A device to prevent overheating of any portion of a circuit by excessive current. It generally consists of a strip of fusible metal which, if the current attains too great strength, melts and opens the circuit. =Salt.= A chemical compound containing two atoms or radicals which saturate each other. One is electro-positive, the other electro-negative. Salts are decomposed by electrolysis, and in separating they combine to form new molecules. =Saturated.= A liquid is said to be saturated when it has dissolved all the salts it will take up. =Search-light.= An apparatus for producing a powerful beam of light and projecting it in any desired direction. =Secondary.= A term applied to the secondary coil of a transformer or induction-coil. =Secondary Battery.= (_See_ Battery, Secondary.)