Wall telephone with Edison chalk receiver, early 20th century.
© Science Museum / Science & Society Picture Library
This wall telephone uses the chalk receiver invented by Thomas Alva Edison (1847-1931) in 1878. Edison redesigned the telephone shortly after its invention by Alexander Graham Bell (1847-1922) in 1876. He got around Bell's patents by inventing a carbon button transmitter, which became widely used, and a chalk receiver called the motograph, which did not. In this device, the user turned the handle which caused a diaphragm in the receiver to be tensioned by the friction created between a platinum strip and a revolving drum coated with damp chalk. Varying electrical currents from the transmitter controlled the friction and the tension, making the diaphragm produce sound.