Hand and Head Receivers

For illustrating the theory of operation, the old-style telephone receiver will be used because of its simplicity.

The permanent magnet of the receiver of Fig. 22 provides a constant magnetic field which passes from the north pole, through the magnetic (soft-iron) pole piece, across the air gap, through the soft-iron diaphragm, across the air gap, through the pole piece, and to the south magnetic pole. The coils through which the speech currents flow are placed on the soft-iron pole pieces and are connected in series so that they aid.

Figure 22. Cross section of a Western Electric telephone receiver. This is used with the transmitter on page 95. This receiver, like the transmitter there described, will gradually be replaced by those of later design, but many of this older type will remain in service for years.

The necessity for the constant pull on the diaphragm is made clear by Fig. 23. Suppose that one cycle of alternating current passes through the coil in (a). As the current increases from zero to a positive maximum, the diaphragm is pulled in to the dotted position. The adjacent air particles on the right of the diaphragm D will flow in, thus causing a rarefaction. Now as the current dies out to zero, the diaphragm will return to a position of the zero displacement, and in so doing the air particles on the right will be compressed and a condensation will be produced. When the current builds up in the negative direction the diaphragm will be again drawn in, producing another rarefaction; and, when it again dies out to zero, the diaphragm will return to the position of rest and will produce another condensation. When one cycle of alternating current flows through the speech coils, two complete cycles of sound waves are set up. Thus, if a constant pull is not exerted on a diaphragm, the reproduced sound waves will be twice the frequency of the speech currents.

Figure 23. Illustrating how a double-frequency tone is produced by a receiver not having a constant pull on the diaphragm.

If the windings are placed on a permanent magnet as in (6), the diaphragm is bowed in when no current flows, as shown by the full line D. When the current increases from zero to a positive maximum, the diaphragm is pulled in further to the dotted position, producing one half of a rarefaction. When the current dies out to zero the diaphragm returns momentarily to the position shown by the full line producing one half of a condensation. As the current builds up in the opposite direction the flux due to this current neutralizes part of the flux from the permanent magnet and the diaphragm moves to the outward dotted position, thus causing the other half of the condensation. As the current dies out to zero, the diaphragm returns momentarily to the full-line position, causing the other half of the rarefaction. With a constant pull on the diaphragm, therefore, one cycle of current causes one cycle of sound wave, and thus the frequency of the sound is the same as that of the exciting current.