Superiority. The superiority of carbon over other resistance-varying materials for transmitters is well recognized, but the reason for it is not well known. Various theories have been proposed to explain why, for example, the resistance of a mass of carbon granules varies with the vibrations or compressions to which they are subjected.
Four principal theories respectively allege:
First, that change in pressure actually changes the specific resistance of carbon.
Second, that upon the surface of carbon bodies exists some gas in some form of attachment or combination, variations of pressure causing variations of resistance merely by reducing the thickness of this intervening gas.
Third, that the change of resistance is caused by variations in the length of electrical arcs between the particles.
Fourth, that change of pressure changes the area of contact, as is true of solids generally.
One may take his choice. A solid carbon block or rod is not found to decrease its resistance by being subjected to pressure. The gas theory lacks experimental proof also. The existence of arcs between the granules never has been seen or otherwise observed under normal working conditions of a transmitter; when arcs surely are experimentally established between the granules the usefulness of the transmitter ceases. The final theory, that change of pressure changes area of surface contact, does not explain why other conductors than carbon are not good materials for transmitters. This, it may be noticed, is just what the theories set out to make clear.
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.