latter was chiefly employed in perfecting the method as a telegraphic
system, Prof. Bell had set before himself the more difficult problem of
transmitting speech. This he has actually accomplished, as we have so
often been reminded during the past year.

Thoroughly conversant with the acoustic researches of Helmholtz, and
keeping in mind the complex form of the air vibrations produced by the
human voice, he attempted to make these vibrations produce corresponding
pulsations in an electric current in the manner analogous to the
electric interrupter.

Observing that membranes when properly stretched can vibrate to any kind
of a sound, he sought to utilize them for this purpose. So did Reiss;
but Reiss inserted the vibrating membrane into the circuit, and it was
quite evident that such a plan would not answer, therefore the current
must not be broken; but could an electric current be interfered with
without breaking the connections?

The well-known re-actions of magnets upon electrical currents, first
noted by Oersted, and fully developed by Faraday, gave the clew to the
solution. A piece of iron should be made to vibrate by means of sound
vibrations, so as to affect an electro-magnet and induce corresponding
electrical pulsations.

FIRST FORM OF SPEAKING-TELEPHONE.

A membrane of gold-beater's skin was tightly stretched over the end of a
speaking-tube or funnel; on the middle of this membrane a piece of iron,
N S, Fig. 13, was glued. In front of this piece of iron an
electro-magnet M is so situated that its poles are opposite to it, but
not quite touching it. One of the terminal wires of the electro-magnet
goes to the battery B; the other goes to the receiving instrument R,
which consists of a tubular electro-magnet, the coil being enclosed in a
short tube of soft iron; the wire thence goes to the plate E', which is
sunk in the earth. On the top of R, at P, is a rather loose, thin disk
of iron, which acts as an armature to the electro-magnet below it.

[Illustration: FIG. 13.]

Supposing that all the parts are thus properly connected, the current of
electricity from the battery makes both M and R magnetic; the
electro-magnet M will inductively make the piece of iron N S, a magnet,
with its poles unlike those of the inducing electro-magnet; and the two
will mutually attract each other. If now this piece of iron N S be made
to move toward M, a current of electricity will be induced in the coils,
which will travers