Microphone is an instrument for magnifying faint sounds, invented by Hughes in 1878. It depends upon the fact that if two conductors of electricity are in loose contact, any alteration of pressure at this contact will alter the resistance and therefore increase or diminish the electric current. Suppose a carbon pencil loosely touches two carbon blocks which are in circuit with a battery and a telephone, the carbons being fixed to a sounding board: any sound, however faint, is due to vibrations of the air: these vibrations are communicated by the board to the carbon blocks, so that the pressure between them and the pencil is constantly changing. The variation of the current so caused sets up vibrations in the telephone, where the original sound is heard in increased intensity. By this means even the foot-fall of a fly may be rendered audible.