284 



THE POPULAR EDUCATOR. 



VOLTAIC ELECTRICITY. XVIII. 



THE TELEPHONE (continued) EDISON'S TRANSMITTER En- 



SON'S RECEIVER THE TELEPHONE EXCHANGE PROFESSOR 



HUGHES' MICROPHONE ITS CONSTRUCTION AND USES 

 MR. MONRO'S EXPERIMENTS THE ELECTRIC PROBE THE 

 AUDIOMETER THE SPHYGMOPHONE. 



THE indefatigable Edison has constructed two forms of telephone 

 the one serving as a transmitter and the other as a receiver. 

 The transmitter is otherwise known as the carbon telephone 

 (see Yig. 90), and must be used with a battery in circuit with it. 

 It consists of a diaphragm which is made to press upon a little 

 disc, or button, of lamp-black, 

 the electrical resistance of 

 which varies with the amount 

 of pressure to which it is sub- 

 jected. As these variations 

 of pressure are regulated by 

 the vibrations of the dia- 

 phragm when thrown into 

 movement by the action of 

 the sound-waves, variations 

 of the current are created 

 which will affect a suitable 

 telephonic receiver. This 

 form of transmitter is most 

 efficient when used with Bell's 

 instrument as receiver, the 

 sounds given being powerful 

 and very distinct. But the 

 form of receiver adopted by 

 Edison although now, for 

 several reasons, of no prac- 

 tical importance was most 

 ingenious in design, and of 

 peculiar interest in exhibiting 

 phenomena of a totally new 

 character. The action of the apparatus will be best under- 

 stood by describing an experiment which can be easily 

 performed with very simple materials. A piece of blotting-- 

 paper moistened with a solution of phosphate of soda is 

 laid upon a metal surface which is in communication with 

 one pole of a battery. A small strip of brass is fastened by 

 flexible wire to the other pole of the battery. The strip is then 

 pressed upon the paper, and dragged over it. So long as the 

 battery current flows, all friction between the paper and brass 

 strip seems to have been destroyed, and the two surfaces glide 

 over one another as if they were ice. But directly the current 

 is broken the friction immediately becomes apparent. 



In Edison's receiver which has been called the loud- 

 speaking or shouting telephone in compliment to its extra- 

 ordinary power this curious action is utilised in the following 

 manner. The blotting-paper is represented by a chalk cylinder 

 which is kept in rotation by a handle outside the instrument. 

 Pressing upon this cylinder is a strip of brass, the friction of 

 which against the moving surface of the chalk varies according 

 to the variation of the electric current ; and supposing a carbon 

 transmitter is in the circuit, such variations will be produced 

 by the action of the voice. These variations are translated 



Fig. 90. SECTION OF EDISON'S 



CARBON TRANSMITTER. 



