Bellscher telephone
The Bellsche Fernsprecher is an electro-acoustic telephone developed by Alexander Graham Bell in 1877 .
functionality
Bell's telephone is based on the observation that a thin iron plate swinging in front of a magnet causes changes in the magnet strength in the magnet, which in turn generate induction currents in a roll of wire surrounding the magnet .
If these currents are passed through the wire roll of a second apparatus of the same construction, then the changes in the magnetic strength of the receiving apparatus will set the iron membrane of the same into exactly corresponding vibrations and thereby reproduce the original tone with the overtones that determine the timbre .
The illustration opposite shows a cross-section of the Bell telephone. A is a cylindrical bar magnet, which is provided at one pole end with an extension a made of soft iron. This pole approach is surrounded by an induction coil BB , the ends of which are attached to strong copper wires leading to the clamping screws DD . The hatched housing accommodates the magnet together with the coil and is closed by a cover provided with a round opening J , which also serves to clamp the thin sheet iron plate pp opposite the pole end of the magnet.
If you connect two such devices by a line and speak into the sound opening of one, its iron membrane starts to vibrate and generates induction currents in the wire coil, which propagate through the line to the second telephone and there through their action on the magnet the membrane set in matching vibrations; as a result, the same sounds arise here that acted on the membrane of the first apparatus.
Soon after Bell's invention became known, large numbers of modified constructions appeared, most of which were intended to give the telephone a greater volume effect by using horseshoe magnets instead of bar magnets.