. Transactions . variations of current cause thediaphragm or diaphragms to vibrate in unison with the sound-vibrationsreceived by the transmitter ; when used as a transmitter the sound-wavevibrations cause the diaphragm or diaphragms in their rapid advances toand retrogressions from the magnet or core to induce currents capable ofacting on another receiver like the variations caused by a microphone,though not generally to so great an extent. Generally, the reproducedspeech is not quite so loud, although the simplicity of the magnetic tele-phone only makes it better than a microphone for places
. Transactions . variations of current cause thediaphragm or diaphragms to vibrate in unison with the sound-vibrationsreceived by the transmitter ; when used as a transmitter the sound-wavevibrations cause the diaphragm or diaphragms in their rapid advances toand retrogressions from the magnet or core to induce currents capable ofacting on another receiver like the variations caused by a microphone,though not generally to so great an extent. Generally, the reproducedspeech is not quite so loud, although the simplicity of the magnetic tele-phone only makes it better than a microphone for places where no skilledelectrician is employed or his services are not available to look after it. As a microphone setfor underground use, thewriter prefers the Steadcarbon - ball transmitter(Figs. 1 and 2); it con-sists of a carbon blockprovided with live holes,in which lie (one in each)small carbon balls, in frontof which a carbon plateis placed. The induction-coil, etc., are of the usualconstruction. FIG. 1. Fig. Stead Carbon-Ball Transmitter. D EBONITE KWIRE NETTING The Collier-Marr, double-pole Bell, or Ader receivers, are all suitablefor use with or without the microphone. A special point about the 100 TELEPHONIC COMMUNICATIONS IN COAL-MINES. transmitter and the receivers referred to is that there are no springs,regulating-screws, or other small parts likely to become deranged. The above form the apparatus for a station, and the lines or con-nexions will now be described. The lines must run between each twostations, so as to form a circuit. Several stations may be worked with thesame pair of mains or lines, or one line only, and an earth, a wire-rope, orthe rails of an engine-plane, may form the return, but generally two wiresor a so-called metallic circuit is best: only one wire need be insulated, bybeing either covered wire or a bare wire fastened to insulators ofearthenware or glass. When several stations are worked from one pair ofmains, every station is called whenever
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