b. Application of operating principle to carbon transmitter. A, figure 11, illustrates the operation of the carbon
transmitter, the principle of which is based on the simplified circuit in figure 11. The basic circuit components are battery,
B, a cup of carbon granules, C, metal diaphragm, D, and an induction coil. The negative terminal of the battery is
connected to a small carbon disk which is fastened rigidly to the diaphragm. This disk rests against one side of the cup of
carbon granules; the other side of the cup is connected to one end of the primary of the induction coil. The circuit is
completed by the return of the primary to the positive terminal of the battery.
(1) When no sound waves strike the diaphragm it remains stationary, the resistance of the carbon granules
remains constant, and, as a result, a steady direct current flows through the circuit (A, fig. 11). The value of this current
depends on the combined resistance of the carbon granules and the dc resistance of the primary of the induction coil.
Since an induction coil is, in effect, a transformer, no emf is induced in the secondary when steady direct current flows in
the primary. Therefore, when no sound energy is transferred to the diaphragm (that is, when the diaphragm does not
move), no current flows in the secondary of the induction coil. The normal resistance of an actual new transmitter unit is
approximately 35 ohms; the dc resistance of the primary of the induction coil varies with the type of coil used. These
coils will be discussed more completely in a later chapter.
(2) When sound waves strike the diaphragm, it vibrates in accordance with the variations of intensity
and frequency of the waves (A, fig. 11). This vibration causes a varying pressure to be exerted on the carbon
granules, which changes their state of compression. As the compression increases, the resistance of the granules
decreases, causing the current in the circuit to increase. As the compression decreases, the resistance of the
granules increases, causing the current to decrease. Because the amplitude and frequency of the current vary directly as
the amount and rate of change of the compression of the carbon granules, they vary as the amount and rate of change
of the pressure exerted on the diaphragm, and, therefore, vary as the intensity and frequency of the sound waves which
strike the diaphragm. The varying current is a pulsating direct current. (Figure 12 shows such a current, resulting from a
simple wave. The ac component of a current resulting from speech is, of course, a complex wave, but again it is
superimposed on a direct current to form a pulsating direct current.) Because the emf induced in the