Edison, in 1877, was the first to invent a working telephone, which, instead of generating the current, merely controlled the strength of it, as the sluice of a mill-dam regulates the flow of water in the lead.  Du Moncel had observed that powder of carbon altered in electrical resistance under pressure, and Edison found that lamp-black was so sensitive as to change in resistance under the impact of the sonorous waves.  His transmitter consisted of a button or wafer of lamp-black behind a diaphragm, and connected in the circuit.  On speaking to the diaphragm the sonorous waves pressed it against the button, and so varied the strength of the current in a sympathetic manner.  The receiver of Edison was equally ingenious, and consisted of a cylinder of prepared chalk kept in rotation and a brass stylus rubbing on it.  When the undulatory current passed from the stylus to the chalk, the stylus slipped on the surface, and, being connected to a diaphragm, made it vibrate and repeat the original sounds.  This “electro-motograph” receiver was, however, given up, and a combination of the Edison transmitter and the Bell receiver came into use.

At the end of 1877 Professor D. E. Hughes, a distinguished Welshman, inventor of the printing telegraph, discovered that any loose contact between two conductors had the property of transmitting sounds by varying the strength of an electric current passing through it.  Two pieces of metal—­for instance, two nails or ends of wire—­when brought into a loose or crazy contact under a slight pressure, and traversed by a current, will transmit speech.  Two pieces of hard carbon are still better than metals, and if properly adjusted will make the tread of a fly quite audible in a telephone connected with them.  Such is the famous “microphone,” by which a faint sound can be magnified to the ear.

Figure 57 represents what is known as the “pencil” microphone, in which M is a pointed rod of hard carbon, delicately poised between two brackets of carbon, which are connected in circuit with a battery B and a Bell telephone T. The joints of rod and bracket are so sensitive that the current flowing across them is affected in strength by the slightest vibration, even the walking of an insect.  If, therefore, we speak near this microphone, the sonorous waves, causing the pencil to vibrate, will so vary the current in accordance with them as to reproduce the sounds of the voice in the telephone.

The true nature of the microphone is not yet known, but it is evident that the air or ether between the surfaces in contact plays an important part in varying the resistance, and, therefore, the current.  In fact, a small “voltaic arc,” not luminous, but dark, seems to be formed between the points, and the vibrations probably alter its length, and, consequently, its resistance.  The fact that a microphone is reversible and can act as a receiver, though a poor one, tends to confirm this theory.  Moreover, it is not unlikely that the slipping of the stylus in the electromotograph is due to a similar cause.  Be this as it may, there can be no doubt that carbon powder and the lamp-black of the Edison button are essentially a cluster of microphones.