Electro-motograph

—An apparatus invented by Edison and consisting of a rotating cylinder of chalk moistened with caustic soda against which is pre ed by a spring an arm carrying a metallic brush, and connected with a diaphragm producing constant tension: when an electric current pa es through the point of contact at the surface of the cylinder, electrolytic action causes a change in the friction. This varying pre ure is applied in a type of loud speaking telephone in which the pitch of the sound emitted varies with the friction.