Microphone

Microphone. This instrument, invented in 1878 by Professor Hughes, does for faint sounds what the microscope does for matter too small for sight; the fall of a bit of tissue-paper or the tread of a fly being rendered audible at many miles' distance. One of the most sensitive substances for microphonic action is willow-charcoal, plunged in a state of white heat into mercury. The theory is that in a homogeneous conductor of electricity the compressions and dilatations of the molecules balance each other, and no variation of current ensues; while, with a state of fine grained non-homogeneity of the conductor, variations of pressure in the conductor produce variations in its conducting power, and thus induce variations in the strength of the electric current traversing it; and these variations of current, when the current passes through a second similar conductor, induce corresponding variations in its molecular stresses, which may act upon the surrounding air and give rise to sonorous waves; or the variations in the current may be detected by the Telephone (q.v.). One form of microphone consists of a piece of mercury-tempered carbon, an inch long, placed vertically between two carbon-blocks hollowed to receive its ends; wires connect the blocks with the battery and with the receiver by which the sounds are to be heard. 'A piece of willow-charcoal,' says the inventor, 'the size of a pin's head is sufficient to reproduce articulate speech.' Two nails laid parallel, with wire connections, and a third nail laid across them, make a simple form of microphone. A few cells of any form of battery may be used. Many useful applications of the microphone have been made or suggested.