Telephone

[Gk. tele, far and phone, a sound.] An instrument which enables persons to talk to each other at considerable distances by electric wires. In its use a thin sheet of metal is set in vibration by the tones of a person’s voice. These vibrations cause rapid alternations of strength in a current of electricity pa ing through a wire. At the opposite end of the circuit the varying currents act to set a second thin plate in vibra¬ tion, and sounds are conveyed to the listener’s ear like those of the speaker’s voice. In this way magnifies from 180 to 3,000 diameters; and the Yerkes of the Chicago University, with a 40-inch object-gla and 64 feet of focal length. One of 48-inch object-gla was shown at the Paris Ex¬ position of 1900. The Lick telescope separated the closest double stars known to us, and dis¬ covered the fifth satellite of Jupiter. Lord Ro e’s telescope at Parsons town is a reflector with 72- inch aperture. Common’s reflector at Ealing is 5 feet in diameter. If the brightne of a star seen with the eye alone is one, with a 2-inch telescope it is 100 times as bright; with a 4-inch telescope it is 400 times as bright; with an 8-inch telescope it is 1,600 times as bright; with a 32-inch telescope it is 25,600 times as bright; with a 36-inch teles¬ cope it is 32,400 times as bright. That is, stars can be seen with the 36-inch telescope which are 30,000 times fainter than the faintest stars visible to the naked eye.