During the positive half-cycle of such current, shown in B, as the current varies from 0 to maximum and back to 0, the
strength of the magnetic field varies from its original value to maximum and back to its original value. The pull on the
diaphragm at the same time varies from its normal value to maximum and back to its normal value. During the negative
half-cycle in C, as the current varies from 0 to maximum in the opposite direction and back to 0, the strength of the
magnetic field varies from its original value to minimum (because of the reversed direction of current) and back to its
original value. The pull on the diaphragm at the same time varies from its normal value to minimum and back to its
normal value. These actions in sequence cause a vibration of the diaphragm. The vibration is actually a sinusodial
displacement of the diaphragm about a normal, or neutral, position, shown in D. A series of vibrations results in the
generation of a series of sound waves of corresponding frequency and waveform. Figure 16 shows comparative graphs of
the sound-wave input at the transmitter, the current in the transmitter, the magnetic pull on the diaphragm, and the sound-
wave output at the receiver.
(2) Figure 17 illustrates the reason for using a permanent magnet in the telephone receiver. The permanent
magnet is replaced by an electromagnet, with a coil wound on a soft-iron core. When no current flows in the coil, there is
no magnetic field; therefore the diaphragm remains in its neutral position, as in A. When a sinusodial current flows in the
coil during the positive half-cycle, a magnetic field of similar variations is produced, as in B, and this field attracts the
diaphragm so that its motion corresponds to the variation of the field. During the negative half-cycle, in C, the polarity of
the magnetic field is reversed, but the displacement of the diaphragm is exactly as before, since only attraction (not
repulsion) can be exerted on it; consequently, the diaphragm moves inward for both half-cycles of current, instead of
alternately inward and outward as described in (1) above. The sound wave produced by this action would have two
condensations and two rarefactions for each cycle of current. The sound, therefore, would have a fundamental frequency
twice as great as that of the current, as well as a distorted waveform. Since these results would cause the sound to be
considerably different from the original sound introduced at the transmitter, this system, which does not contain a
permanent magnet, would be useless for telephone transmission.
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