Despite the simplicity of their parts, and the slight value of the
materials employed, the existing micro-telephonic apparatus keep at
relatively high prices, and the use of them is often rejected, to the
benefit of speaking tubes, when the distance between stations is not
too great. We propose to describe a new style of apparatus that are in
no wise inferior to those in general use, and the price of which is
relatively low.
The microphone transmitter may have several forms. The most elementary
of these consists of two pieces of carbon, from one to one and a
quarter inches in length by one-half inch in width, between which are
fixed two _nails_, about two inches in length, whose extremities,
filed to a point, enter small conical apertures in the carbons. Fig. 1
gives an idea of the arrangement.
[Illustration: FIG. 1.]
Fig. 2 represents a model which is a little more complicated, but
which gives remarkable results. The largest nail is here two inches in
length, and the shortest three-quarter inch.
[Illustration: FIG. 2.]
The receivers may be Bell telephones of the simplest form found in the
market (Fig. 3); but for these there may be substituted a bar of soft
iron, cast iron, or steel, one of the extremities of which is provided
with a bobbin upon, which is wound insulated copper wire 0.02 inch in
diameter. The apparatus is mounted like an ordinary Bell telephone. A
horseshoe electro may also be used, and the poles be made to act (Fig.
4). The current sent by the transmitter suffices to produce a magnetic
field in which the variations in intensity produced by the microphone
succeed perfectly in reproducing speech and music. With four Leclanche
elements, the sounds are perceived very clearly. The elements used may
be bichromate of potash ones, those of Lelande and Chaperon, etc.
[Illustration: FIG. 3.--RECEIVER.]
[Illustration: FIG. 4.]
To this apparatus there may be added a second bobbin of coarser wire
into which is passed a current from a local pile. This produces a much
intenser magnetic field, and, consequently, louder sounds. This
modification, however, is really useful only for long distances.
Any arrangement imaginable may be given the transmitter and receiver;
but, aside from the fact that the ones just indicated are the
simplest, they give results that are at least equal, if not superior,
to all others.
We shall insist here only upon the arrangement of the microphone,
which is new (at least in practice), and upon the uselessness of
having well magnetized steel bars and wires of extreme fineness in the
receiver.
[Illustration: FIG. 5.]
We have stated that the nail microphones are the simplest. The nails
may be replaced by copper or any other metal, or they may be well
nickelized; but common nails answer very well, and do not oxidize
much. An apparatus of this kind (Fig. 5) that has been for more than a
year in a laboratory filled with acid vapors is yet working very well.
These apparatus possess the further advantage of being very strong,
and of undergoing violent shocks without breaking or even getting out
of order. They may be used either with or without induction coils. We
have not yet measured their range, but can cite the following fact:
One of these apparatus, quite crudely mounted, was put into a circuit
with a resistance of 300 ohms. With a single already exhausted
bichromate element, giving scarcely 2 volts, musical sounds and speech
reached the receiver without being notably weakened. Such resistance
represents a length of eighteen miles of ordinary telegraph wire.
After this, 700 ohms were overcome with 3.4 volts. This result was
obtained by direct transmission, and without an induction coil, and it
is probable that it might be much exceeded without sensibly increasing
the electromotive force of the current.--_Le Genie Civil._
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