Wireless Telegraphy and Telephony

52 WIRELESS TELEGRAPHY from the distant transmitting station. These currents are exceedingly feeble, too feeble in fact to operate any form of electrical apparatus except a telephone receiver, which is one of the most sensitive instruments in existence. There are probably more different forms of detector than any other piece of radiotelegraph apparatus. Those in most common use to-day are the mineral detectors. A small crystal of certain minerals, iron pyrites, silicon, ga- Q/APHRACM INTERIOR V/EW OF TELEPHONE RECEIVER Fig. 64.—Showing construction of a “watch case” telephone receiver. lena, etc., is placed between two contact points which are adjustable so that the pressure may be regulated and the most sensitive portion of the mineral selected. A tele- phone receiver is shunted across the terminals of the detector. A telephone is shown in diagram in Fig. 64. It con- sists of a U shaped permanent magnet of bar steel, so mounted as to exert a polarizing,influence upon a pair of little electromagnets, before the poles of which an iron diaphragm is mounted. For convenience these elements are assembled within a small cylindrical casing usually of hard rubber. The permanent magnet exerts a continual pull upon the diaphragm tending to distort it, concave inwards. When alternating currents are sent through the receiver coils, that part of the alternation which is flowing