[Illustration:  Fig. 8.  Hughes’ Microphone]

Carbon. Adaptability. The application of carbon to use in transmitters has taken many forms.  They may be classified as those having a single contact and those having a plurality of contacts; in all cases, the intimacy of contact is varied by the diaphragm excursions.  An example of the single-contact type is the Blake transmitter, long familiar in America.  An example of the multiple-contact type is the loose-carbon type universal now.  Other types popular at other times and in particular places use solid rods or blocks of carbon having many points of contact, though not in a powdered or granular form.  Fig. 9 shows an example of each of the general forms of transmitters.

The use of granular carbon as a transmitter material has extended greatly the radius of speech, and has been a principal contributing cause for the great spread of the telephone industry.

[Illustration:  Fig. 9.  General Types of Transmitters]

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.