the inertia of the adjacent diaphragm; there are, also, further energy losses in the connecting wire, in the coils at both ends
of the connections, in the two permanent magnets, and again in overcoming the inertia of the diaphragm at the distant end.
The useful energy then is that which appears as sound at the distant end and it can be only the original energy minus all
the energy losses.
(5) The only source of power furnished the instruments discussed above is that supplied by the person speaking,
assuming speech transmission. Such transmission thus is said to be accomplished by use of a sound-powered transmitter,
which is discussed further in Lesson 3 Chapter 6. As a matter of interest, such a transmitter actually is used in present-day
communications as a receiver. As explained in paragraph 12, the average power contained in speech at a normal
conversational level is about 10 microwatts. It is this power limitation, plus the lack of amplifying facilities, that limits
the distances over which such instrumentalities can provide satisfactory sound transmission.
(6) The transmission limitations of the sound-powered transmitter were overcome with the advent of the carbon
transmitter, the operating principles of which are described below.
19.
Carbon Transmitter.
a. Operating principle of carbon transmitter.
(1) The operating principle of
the carbon transmitter can be explained
with the help of the simplified circuit
shown in figure 9. The circuit consists
of battery B and variable resistance
R which represents the variable
resistance of the carbon granules.
Assume that the battery has an
emf (electromotive force) of 6 volts, and
that R may be varied from 0 to 1000
ohms, with a normal setting of 300
ohms. The normal or average value
of current I that flows is 6 volts divided
by 300 ohms, or 20 ma (milliamperes).
If the resistance, R, is reduced to
FIGURE 9. Equivalent Circuit of Carbon Transmitter
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