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.

    471/502 page 451
    of the wheel. When a signal is received, the armature is attracted and the wheel is brought against the paper, making a long or short mark. When the current ceases, the armature is raised by the antagonistic spring s, and the wheel no longer marks the paper. 44-1. The Telephone.—Graham Bell has produced an instrument called the telephone, by means ot which an in- sulated wire is made the medium of conveying speech. It oper- ates in this way : we have in the first place an iron membrane or plate placed symmetrically in front of one pole of a bar magnet, which is covered with insulated wire. One operator talks immediately in front of the iron membrane, which is consequently thrown into a complicated state of vibration. This complicated state of vibration represents the three elements of the speaker’s voice, namely, pitch, intensity, quality. But in each vibration of the iron membrane we have a portion of the iron alternately approaching toward;; or receding from the magnet, and the same result will be produced as when a piece of soft iron is made rapidl}' to approach or recede from a magnet covered with insulated wire, namely, that if the circuit of insulated wire be complete, secondary currents will be produced in the insulated wire, these being in one direction for the approach, and in the opposite for the recession, of the soft iron. When the iron membrane vibrates, therefore, in front of the magnet, it produces in the insulated wire, which is arranged so as to form a complete circuit, a series of second- ary currents, which in fact register all the peculiarities of the speaker's voice, so far as currents can be said to do so. -At the other end of the circuit, which may be many miles distant, we have another instrument precisely similar to that now described, with the difference that the operator now places his ear instead of his mouth to the iron membrane. .Now the currents passing along the insulated wire, which wire is wrapped round the magnet at this end, just as it was round the other magnet, produce magnetic changes in the magnetism of the magnet, and of the iron membrane in front of it. These magnetic changes cause a state of vibration of this second iron membrane precisely similar to that of the membrane at the other end. The consequence is that an operator at one end placing his car to the iron membrane will
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