In general it may be said that the packing difficulty
In general it may be said that the packing difficulty has been almost
entirely eliminated, not by the employment of remedial devices, such
as those often proposed for stirring up the carbon, but by preventing
the trouble by the design and manufacture of the instruments in such
forms that they will not be subject to the evil.
Carrying Capacity. Obviously, the power of a transmitter is
dependent on the amount of current that it may carry, as well as on
the amount of variation that it may make in the resistance of the path
through it. Granular carbon transmitters are capable of carrying much
heavier current than the old Blake or other single or multiple
electrode types. If forced to carry too much current, however, the
same frying or sizzling sound is noticeable as in the earlier types.
This is due to the heating of the electrodes and to small arcs that
occur between the electrodes and the granules.
One way to increase the current-carrying capacity of a transmitter is
to increase the area of its electrodes, but a limit is soon reached in
this direction owing to the increased inertia of the moving electrode,
which necessarily comes with its larger size.
The carrying capacity of transmitters may also be increased by
providing special means for carrying away the heat generated in the
variable-resistance medium. Several schemes have been proposed for
this. One is to employ unusually heavy metal for the electrode
chamber, and this practice is best exemplified in the White solid-back
instrument. It has also been proposed by others to water-jacket the
electrode chamber, and also to keep it cool by placing it in close
proximity to the relatively cool joints of a thermopile. Neither of
these two latter schemes seems to be warranted in ordinary commercial
practice.
Sensitiveness. In all the transmitters so far discussed damping
springs of one form or another have been employed to reduce the
sensitiveness of the instrument. For ordinary commercial use too great
a degree of sensitiveness is a fault, as has already been pointed out.
There are, however, certain adaptations of the telephone transmitter
which make a maximum degree of sensitiveness desirable. One of these
adaptations is found in the telephone equipments for assisting
partially deaf people to hear. In these the transmitter is carried on
some portion of the body of the deaf person, the receiver is strapped
or otherwise held at his ear, and a battery for furnishing the current
is carried in his pocket. It is not feasible, for this sort of use,
that the sound which this transmitter is to reproduce shall always
occur immediately in front of the transmitter. It more often occurs at
a distance of several feet. For this reason the transmitter is made as
sensitive as possible, and yet is so constructed that it will not be
caused to produce too loud or unduly harsh sounds in response to a
loud sound taking place immediately in front of it. Another adaptation
of such highly sensitive transmitters is found in the special
intercommunicating telephone systems for use between the various
departments or desks in business offices. In these it is desirable
that the transmitter shall be able to respond adequately to sounds
occurring anywhere in a small-sized room, for instance.
Acousticon Transmitter. In Fig. 46 is shown a transmitter adapted
for such use. This has been termed by its makers the _acousticon
transmitter_. Like all the transmitters previously discussed, this is
of the variable-resistance type, but it differs from them all in that
it has no damping springs; in that carbon balls are substituted for
carbon granules; and in that the diaphragm itself serves as the front
electrode.
This transmitter consists of a cup _1_, into which is set a
cylindrical block _2_, in one face of which are a number of
hemispherical recesses. The diaphragm _3_ is made of thin carbon and
is so placed in the transmitter as to cover the openings of the
recesses in the carbon block, and lie close enough to the carbon
block, without engaging it, to prevent the carbon particles from
falling out. The diaphragm thus serves as the front electrode and the
carbon block as the rear electrode. The recesses in the carbon block
are about two-thirds filled with small carbon balls, which are about
the size of fine sand. The front piece _4_ of the transmitter is of
sheet metal and serves to hold the diaphragm in place. To admit the
sound waves it is provided with a circular opening opposite to and
about the size of the rear electrode block. On this front piece are
mounted the two terminals of the transmitter, connected respectively
to the two electrodes, terminal _5_ being insulated from the front
piece and connected by a thin metal strip with the diaphragm, while
terminal _6_ is mounted directly on the front piece and connected
through the cup _1_ with the carbon block _2_, or back electrode of
the transmitter.
[Illustration: Fig 46. Acousticon Transmitter]