Telephony, Telephones Wall telephone with Edison chalk receiver, early 20th century

Telephony, Telephones Wall telephone with Edison chalk receiver, early 20th century. 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. ©SSPL/Science Museum