This telephone receiver differs from its predecessors in dispensing with
an armature, the lateral vibration of the electro-magnet itself being
utilized. In previous systems in which an electro-magnet is used, the
sonorous vibrations are due either to the motion of an iron diaphragm
or armature placed close to the poles of the electro-magnet, or to the
expansion and contraction of the magnet itself. In Theiler's telephone
the electro-magnet may be of the usual U-shape, and may consist either
of soft iron or of hardened steel permanently magnetized, wound with a
suitable number of turns of insulated wire. This electro magnet is fixed
in such a manner that the vibration of either one or of both its limbs
is communicated to a diaphragm or diaphragms The patentees also employ
two or more electro-magnets in the same circuit, and utilize the
vibration of both magnets in the manner described. By attaching a light
disk or disks to the vibrating limbs, the diaphragm may be dispensed
with. Fig. 1 represents one of the telephone receivers provided with two
diaphragms or sounding boards, connected to the two limbs or cores of
the U-shaped electro-magnet by short tongues. These tongues are firmly
inserted in the diaphragms and fixed to the magnet, as shown. The poles
of the electro-magnet are brought very close together by being shaped as
shown, and the middle part of the magnet is firmly screwed to the case
of the instrument. The ends of the helix surrounding the magnet cores
may be attached as usual to two terminals, or soldered to a flexible
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
connected to the secondry circuit of the induction coil. This device
possesses the advantage that the electro-magnet can be powerfully
magnetized with very little battery power, no matter how long the line
may be, and that steel magnets are entirely dispensed with. It is not
necessary to have a separate battery for this purpose, as the microphone
battery may also be used for the telephone receiver. The shape of the
vibrating electro-magnets is immaterial, as they may be made of a
variety of forms.--_Eng. Mechanic_.
[Illustration: FIG. 1. FIG. 2]
* * * * *