 and all related signal processing equipment. All types of - and 4-wire circuits are still designed around that 500 set. b. The receiver (Figure 1.5). The receiver is the "mouth" of the telephone in that it speaks into the ear of the person using the telephone. It also contains a diaphragm whose movement is caused by the strengthening and weakening of the field cre- ated by the magnet within the receiver. The receiver converts the varying electrical current representing the transmitted speech signal to variations in air pressure perceived as sound by the human ear. An electromagnetic receiver consists of coils of many turns of fine wire wound on permanently mag- netized soft iron cores that drive an armature. The armature is a diaphragm made of a soft iron material. 16 The Telecom Handbook CHAPTER 1 Figure 1.5 The Receiver ELECTROMAGNETIC HORSESHOE PLASTIC CAP PERMANENT MAGNET THIN IRON DIAPHRAGM When someone speaks a word into a transmitter, the cur- rent flow in the circuit is alternately increased and decreased as the moving electrode moves in and out of the carbon cham- ber. A requirement for an electromagnetic receiver is a permanent magnet to provide a constant bias field for the varying electromagnetic field to work against. Otherwise, both positive and negative currents would push the armature in the same direction. The varying electrical current repre- senting speech flows through coils and produces a varying electromagnetic field. It alternately aids and opposes the per- manent magnetic field; thus, it alternately increases and decreases the total magnetic field acting on the diaphragm. This causes the diaphragm to vibrate in step with the varying current and moves the air to reproduce the original speech that caused the current changes. Other types of receivers op- erate similarly, except that the armature is a separate part and The Telecom Handbook @ 17 THE TELEPHONE is connected to a conical non-magnetic diaphragm. The rock- ing action of the armature causes the aluminum diaphragm to vibrate to repro