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.