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.