There are many who feel that more experimental data is required before
a conclusive and satisfactory theory can be set up. There is need of
one, for a proper theory often points the way for effective advance in
practice.

Carbon and magneto transmitters differ wholly in their methods of
action. The magneto transmitter _produces_ current; the carbon
transmitter _controls_ current. The former is an alternating-current
generator; the latter is a rheostat. The magneto transmitter produces
alternating current without input of any electricity at all; the
carbon transmitter merely controls a direct current, supplied by an
external source, and varies its amount without changing its direction.

The carbon transmitter, however, may be associated with other devices
in a circuit in such a way as to _transform_ direct currents into
alternating ones, or it may be used merely to change constant direct
currents into _undulating_ ones, which _never_ reverse direction, as
alternating currents _always_ do. These distinctions are important.

[Illustration: Fig. 10. Battery in Line Circuit]

_Limitations._ A carbon transmitter being merely a resistance-varying
device, its usefulness depends on how much its resistance can vary in
response to motions of air molecules. A granular-carbon transmitter
may vary between resistances of 5 to 50 ohms while transmitting a
particular tone, having the lower resistance when its diaphragm is
driven inward. Conceive this transmitter to be in a line as shown in
Fig. 10, the line, distant receiver, and battery together having a
resistance of 1,000 ohms. The minimum resistance then is 1,005 ohms