FIG. 62.—Portable pack set. The receiving outfit is contained in the left hand case; also the key and interrupter. The tubular condenser, spark gap, and induction coil may be seen in the right hand case.
FIG. 63.—Complete receiving set, consisting of two "Perikon" detectors, potentiometer, loose coupler, variable condenser, etc.
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, galena, 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 telephone receiver is shunted across the terminals of the detector.
FIG. 64.—Showing construction of a "watch case" telephone receiver.
A telephone is shown in diagram in Fig. 64. It consists 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 in the proper direction to form a magnetic field flowing in the same direction as that of the permanent magnet will strengthen the latter and assist it in attracting the diaphragm and causing it to further approach the magnet. That portion of the current flowing in the opposite direction detracts from the magnetic pull and allows the diaphragm to recede from the magnet. The diaphragm thus takes up a vibrating motion corresponding to the electrical waves supplied to the coil and it imparts motion to the surrounding air, the result being sound.
FIG. 65.—Pickard adjustable telephone receivers for wireless purposes.