Figure 5-8.--Sound-powered transmitter receiver unit.

compressions and rarefaction in the atmosphere in

Capacitor C1 is connected in parallel with the

which they travel. In sound-powered telephones, the

sound-powered units for tone compensation. When

sound waves created by a speaker's voice cause the

using the sound-powered handset, switch S1 must be

diaphragm in the transmitter unit to vibrate in unison

depressed for both transmitting and receiving.

with the sound wave vibrations. Figure 5-8, view A,

SOUND-POWERED UNITS.-- The sound-

shows the armature of a transmitter unit when there are

powered transmitter (microphone) and receiver units in

no sound waves striking the diaphragm. Note that the

a sound-powered handset are identical and

armature is centered between the pole pieces with the

interchangeable. As illustrated in figure 5-8, views A, B,

magnetic lines of force passing from the north to the

and C, a unit consists of two permanent magnets, two

south pole and that there are no lines of force passing

pole pieces, an armature, a driving rod, a diaphragm, and

lengthwise through the armature.

a coil. The armature is located between four pole tips,

Figure 5-8, views B and C, show sound waves

one pair at each end of the armature. The spacing

striking the diaphragm and causing the diaphragm to

between the pole tips at each end is such that an air space

vibrate. The vibrations are impressed upon the armature

remains after the armature is inserted between them.

by the drive rod. During the compression part of the

This air space has an intense magnetic field, supplied by

wave (fig. 5-8, view B) this action causes the armature

the two magnets that are held in contact with the pole

to bend and reduce the air gap at the upper south pole.

tips.

The reduction of the air gap decreases the reluctance

The armature is clamped rigidly at one end near one

between the upper south pole and the armature, while

of the pairs of poles and is connected to the other end to

increasing the reluctance between the armature and the

the diaphragm by the drive rod. Therefore, any

upper north pole. This action reduces the lines of force

movement of the diaphragm causes the free end of the

that travel between the two upper pole pieces. There is

armature to move toward one of the pole pieces. The

no large change in the reluctance at the lower poles;

armature passes through the exact center of a coil of wire

however, the armature has less reluctance than the lower

that is placed between the pole pieces in the magnetic

air gap and a large number of magnetic lines of force

field.

will follow the armature to the upper south pole.

When the sound wave rarefaction reaches the

Principles of Operation of the Transmitter

diaphragm (fig. 5-8, view C), it recoils, causing the

Unit.-- Sound waves are vibrations that cause

Figure 5-8.--Sound-powered transmitter receiver unit.

5-10