conductor communicating with the other parts of the telephone
apparatus. When a vibratory current is sent through the helix of the
electro-magnet, the extremities are rapidly attracted and repelled, and
this vibratory motion of the magnet cores being communicated to the
diaphragms or sounding boards, the latter are set in vibration of
varying amplitude produced by a current of varying strength, as in all
other telephones. Instead of making the electro-magnet of one continuous
piece of iron, as represented in Fig. 1, the patentees find it
more practicable to make it of the form shown in Fig. 2, where the
electro-magnet represented consists of two limbs or cores, a sole piece,
and pole extensions, the whole being screwed together, and practically
constituting one continuous piece of iron carrying the two coils. In
Fig. 2 only one of the limbs or cores of the electro-magnet is attached
to the diaphragm, the other limb being held fixed by a screw. Sometimes
the patentees hinge one of the magnet cores, or both, in the sole piece,
in which case the diaphragms or sounding boards can be made much thicker
than when the cores are rigidly fixed to the sole piece, because
the magnetic attraction of the poles has then only to overcome the
resistance of the diaphragm. Instead of using a diaphragm, they
sometimes fix a stem to one of the cores of the electro-magnet, and
mount thereon a light disk of vulcanite, wood, ivory, gutta-percha, or
any other substance which it is capable of vibrating. When using this
telephone receiver, the disk is pressed to the ear in such a manner
that its surface covers the aperture of the ear. When these telephone
receivers are used on a line of some considerable length, the patentees
prefer to magnetize the electro-magnet by a constant current from
a local battery, and to effect the variation of this constant
magnetization inductively and not directly. The electro-magnet is,
then, not inserted in the line at all, but in the primary circuit of
an induction coil, and connected with a local battery. The line is