408 ELEMENTARY LESSONS ON [CHAP. xn. 



placed in the circuit. The resistance of carbon is greatly 

 diminished by pressure ; hence the varying pressures due 

 to the vibrations cause the button to offer a varying re- 

 sistance to any current flowing (from a battery) in the 

 circuit, and vary its strength accordingly. This varying 

 current may be received as before in an electro-magnetic 

 receiver of the type described above, and there set up 

 corresponding vibrations. Edison has also invented a 

 Telephone Receiver of singular power, which depends 

 upon a curious fact discovered by himself, namely, that if 

 a platinum point presses against a rotating cylinder of 

 moist chalk, the friction is reduced when a current 

 passes between the two. And if the point be attached 

 to an elastic disc, the latter is thrown into vibrations 

 corresponding to the fluctuating currents coining from 

 the speaker's transmitting instrument. 



Telephone Receivers have also been invented by Varley and 

 Dolbear, in which the attraction between the oppositely-elec- 

 trified armatures of a condenser is utilised in the production at 

 sounds. The transmitter is placed in circuit with the primary 

 wire of a small induction-coil ; the secondary wire of this coil 

 is united through the line to the receiving condenser. In 

 Dolbear's Telephone the receiver consists merely of two thin 

 metal discs, separated by a very thin air-space, and respectively 

 united to the two ends of the secondary coil. As the varying, 

 currents flow into and out of this condenser the two discs 

 attract one another more or less strongly, and thereby vibra- 

 tions are set up which correspond to the vibrations of the original 

 sound. 



437. Hughes' Microphone. Hughes, in 1878, dis- 

 covered that a loose contact between two conductors, 

 forming part of a circuit in which a small battery and a 

 telephone are included, may serve to transmit sounds, 

 because the smallest vibrations will affect the amount 

 of the resistance at the point of loose -contact. The 

 Microphone (Fig. 167) embodies this principle. In 

 the form shown in the figure, a small thin pencil of 

 carbon is supported loosely between two little blocks of 



