III.

Bell’s Magnetic Telephone

The basic unit of Bell’s invention consisted of a transmitter, a receiver, and a single connecting wire. The transmitter and receiver were identical; each contained a flexible metallic diaphragm and a horseshoe magnet with a wire coil. Sound waves striking the diaphragm caused it to vibrate in the field of the magnet. This vibration generated an electric current in the coil that varied in proportion to the vibrations of the diaphragm. The current travelled through a wire to the receiving station, where it produced changes in the strength of the magnetic field of the receiver, causing its diaphragm to vibrate and reproducing the original sound.

In the receiver of the modern telephone the magnet has been flattened into the form of a watch, and the magnetic field acting on the ferrotype iron diaphragm has been made more intense and uniform. The modern transmitter consists of a thin diaphragm mounted behind a perforated grill. At the centre of the diaphragm is a small dome forming an enclosure filled with carbon granules. Sound waves passing through the grill cause the dome to move in and out. When the diaphragm presses in, the granules become densely packed, allowing an increase in the flow of current through the transmitter.