While in telephonic arrangements, based upon the principle of magnetic
induction, a relatively considerable expenditure of force is required in
order to set the tightly stretched membrane in vibration, in the so-called
carbon telephones only a very feeble impulse is required to produce the
differences in the current necessary for the transmission of sounds. In
order to produce relatively strong currents, even in case of sound-action
of a minimum strength, Franz Kröttlinger, of Vienna, has made an
interesting experiment to use thermo electric currents for the transmission
of sound to a distance. The apparatus which he has constructed is
exceedingly simple. A current of hot air flowing from below upward is
deflected more or less from its direction by the human voice. By its action
an adjacent thermo-battery is excited, whose current passes through the
spiral of an ordinary telephone, which serves as the receiving instrument.
As a source of heat the inventor uses a common stearine candle, the flame
of which is kept at one and the same level by means of a spring similar to
those used in carriage lamps. On one side of the candle is a sheet metal
voice funnel fixed upon a support, its mouth being covered with a movable
sliding disk, fitted with a suitable number of small apertures. On the
other side a similar support holds a funnel-shaped thermo-battery. The
single bars of metal forming this battery are very thin, and of such a
shape that they may cool as quickly as possible. Both the speaking-funnel
and the battery can be made to approach, at will, to the stream of warm air
rising up from the flame. The entire apparatus is inclosed in a tin case
in such a manner that only the aperture of the voice-funnel and the polar
clamps for securing the conducting wires appear on the outside. The inside
of the case is suitably stayed to prevent vibration. On speaking into the
mouth-piece of the funnel, the sound-waves occasion undulations in the
column of hot air which are communicated to the thermo-battery, and in this
manner corresponding differences are produced in the currents in the wires
leading to the receiving instrument.--_Oesterreichische-Ungarische Post._
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