Lessons in elementary physics / by Balfour Stewart.

  • Balfour Stewart
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: Lessons in elementary physics / by Balfour Stewart. Source: Wellcome Collection.

    472/502 page 452
    hear the very sounds given out by a speaker at the other end of the circuit. In Fig. 149 we have the various parts of such a telephone. Here A denotes the mouth-piece, B the iron membrane, N the north pole of the magnet, whose other pole is s. At c is the coil of insulated wire connected at E with the earth, and at L with (he telegraphic line which goes to, and is similarly connected with, a similar instrument at the other end, the whole forming one circuit. 442. The Microphone.—We are indebted for this instru- ment to D. E. Hughes. It was found by him that if, in the circuit of an electric current, several pieces of carbon loosely Fig. i.|g. in contact with each other be placed, any sonorous vibration, by affecting the closeness and nature ol the contact between these various pieces ot carbon, will aflect the resistance ot the circuit, and hence also the strength ol the current. Hg. ii;o shows one form of microphone. Ihe pieces ol carbon A and B fixed to a wooden block .m have cups drilled in them so as to liold loosely a spindle ol carbon c. The battery n is placed in circuit with the carbons and the telephone T. I he ilisturbance proiluceil in the resistances at the points of contact of the carbons by the ticking ol a watch placeil on the base- hoard of tlu' mici'ophone causes variations in the current strength, and lienee in tlu' attraction of the telephone dia- phragm, which will therefore be made to vibrate so strongly as to ,c;iuse a loud noise.
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