Bell's hand transmitter was not only a telephone complete in itself, but was a self-contained generator of the alternating-current type. It was operated by the voice creating sound waves that in turn generated electric waves through the movements of the diaphragm. These electrical waves were similar in form to the sound waves and were transmitted to the receiver and there changed back to sound waves. When in operation the flow of the current, and every variation in its strength, was dependent on the varying motions of a diaphragm moving in a magnetic field; that is, on the speed of an armature of a miniature dynamo driven by the spoken word. In other words, in the hand telephone respondent vibratory motion of a soft iron induction armature in a magnetic field was the essential element in the successful transmission of speech.
In the Blake and other forms of variable resistance transmitters, whether single or multi-contact, there is no electro-magnet and no armature. A battery, usually of the sal-ammoniac type, supplies a constant current, the flow of which is regulated by increasing or decreasing the pressure of the diaphragm against the carbon button, the changes in pressure being governed by the impact of the changing sound waves on the diaphragm. Thus a carbon transmitter is not so sensitive, nor does it possess that delicate responsiveness so noticeable in a magneto transmitter. It matters little what may be the nature or character of the diaphragm in a variable resistance transmitter, so long as it is sensitive enough to reciprocally respond to the sound waves produced by the vocal cords. But only a soft iron inductive diaphragm will serve in the magneto type of transmitter.
In the White or solid-back transmitter, now so familiar a part of Bell equipment, the single-contact feature of the Blake transmitter is succeeded by a multi-contact arrangement composed of two carbon electrodes made of the hardest of pure carbon separated by carbon granules. The selected granules insure a multitude of contacts, and talking qualities that are unexcelled.
In all these variable-contact transmitters the current is always knocking at the carbon gateway and seeping through. When the telephone is not in use, the carbon offers just sufficient resistance to prevent the current from forcing the gate wide open. When a person is talking, the vibrations of the diaphragm decrease the resistance of the carbon and enable the current to flow through the partially or wholly opened gateway.
