We illustrate in the annexed engraving the microphone-telephone
constructed by Messrs. Mix & Genest, of Berlin, which, after extended
trials, has been adopted in preference to others by the imperial
postal department of Germany. There are now more than 5,000 of these
instruments in use, and we need scarcely mention that the invention
has been patented in many countries.
In some microphones a rattling noise is frequently occasioned, which
borne along with the sound of the human voice causes an audible
disturbance in the telephone. The chief cause of these disturbances
may be ascribed to the fact that the carbon rollers in their journals,
rest loose in the flutings of the beam, which is fastened to the sound
plate. Owing to the shocks given to the entire apparatus, and
independent of the oscillations of the sound plate, they are set in
motion and roll to and fro in their bearings.
In microphones in which the sound plates are arranged vertically (as
shown in Fig. 2), these disturbances assume such a character that
there is no possibility of understanding the speaker, for in this case
the horizontally directed oscillations of the sound plate, _m_, cause
themselves a backward and forward motion on the part of the carbon
rollers without increasing or decreasing at the same time the lying-on
pressure of the roller journals, and by doing so bring the places of
contact one on the other, and thus occasion a conducting resistance of
greater or less force. This circumstance serves as an explanation of
the reason why the sound plates in Ader's microphones are not arranged
vertically, although this way of arranging them offers many advantages
over a horizontal or slightly inclined arrangement of the sound
plates. Speaking is more convenient in the vertical arrangement, and
moreover the plates can be fitted on to instruments better in this
way.
All the drawbacks just enumerated and found in Ader's microphones are
avoided in the apparatus made by Messrs. Mix & Genest. A sort of
braking contrivance operates on the carbon rollers in such a way as to
prevent their journals from lying on the lower points in the flutings
of the beams. Thus, for instance, if in a microphone with a horizontal
sound plate, as illustrated in Fig. 3, the carbon rollers are pressed
upward by outward force, it is evident that only a very trifling
rolling and disturbing motion can occur, and only small pieces of
carbon can be knocked off, which would act injuriously as a secondary
contact. The same may be said of the journals of microphones with
vertical sound plates, as represented in Fig. 2, when the carbon
rollers are pressed in the direction of the arrow, _p_, that is to
say, against the sound plate. In this case the journals, _a_, are
fixed in the flutings of the beams, _b_, in a direction given them by
the power and gravity operating on them, which is clearly represented
in the accompanying design, Fig. 2.
[Illustration: FIG. 1.
FIG. 2.
FIG. 3.
THE MIX AND GENEST TELEPHONE]
In all such cases the regulating contrivance applied to brake the
carbon rollers in their motion has the result that only the
oscillations transmitted from the sound plate on to the contacts come
in operation, whereas disturbing mechanical shocks resulting from any
outward influences occasion very insignificant vibrations, which are
not perceptible in the telephone. The separate contacts thus form a
firm system with the sound plate, so that the former are influenced in
their motions and effects solely and alone by the shocks and
oscillations which operate direct on these sound plates. The roller
motion of the carbon is thus removed, and the distinctness of the
words spoken is greatly augmented.
The above Figs. 1 and 2 show the microphone in side view and in cross
section.
A metal ring, R (see Fig. 1), is fastened by means of the four screws,
_r_{1}_ _r_{2}_ _r_{3}_ _r_{4}_, on a wooden mouthpiece. In a recess of the
above ring is the diaphragm, M, which is provided on its outer edge
with an India rubber band and is held in position by the two clamps, _a_
and _a_{1}_. The diaphragm is cut out of finely fibered firwood and is
well lacquered to preserve it against dampness. On it there are two
carbon beams, _b_, and in the perforations of the latter are the
journals of the carbon rollers, _k_. The alterations in contact take
place in the touching points. The cross piece, _f_, that runs straight
across the carbon rollers serves as a braking contrivance, which is
regulated as may be necessary by the large projecting screws.
Fig. 3 shows the apparatus in cross section. T is the mouth piece, R
the metal ring, M the diaphragm, _f_ the breaking cross piece. On the
latter is a metal block fastened by means of two screws. On this metal
block is a soft elastic strip (d) of felt or similar material. The
letters _s_ and _s_ indicate the regulating screws for the braking
contrivance.
The excellent qualities of other microphones, in particular their
extreme sensibility for the very least impressions, are undeniable;
but it is just this sensibility that is the cause of the complaints
made by the public. In practical use this overgreat sensibility proves
to be a fault.
In the apparatus constructed by Messrs. Mix and Genest the well-known
deficiencies of other systems are avoided. The effect of the sound and
the distinctness of the human voice are clearer and far more
intelligible. One simple regulation of the microphone suffices for the
installation, for there is no danger of its getting out of order.
Owing to its peculiar construction, this new microphone is very firm
and solid, and for this very reason offers another advantage, namely,
the possibility of transmitting sound over very long distances. In the
competitive trials instituted by order of the imperial postal
department, apparatus of various systems and constructions were
subjected to tests, and the apparatus we are speaking of showed the
favorable results just mentioned. This microphone has overcome in
particular the difficulties connected with the using of combined lines
above and below ground, and with the aid of it the excellent
telephonic communication is carried on in Berlin, in which city the
telephone net is most extensive and complicated. At the same time this
microphone transmits the sound over long distances (up to 200 kilom.
even) in the most satisfactory manner. Another peculiar advantage of
this construction is that it exercises a very small inductive effect
on cables and free lines, and consequently the simultaneous speaking
on parallel lines causes but little disturbance.
After repeated trials made by the German imperial postal department
with the microphones constructed by Messrs. Mix and Genest, these
apparatus have been introduced in the place of the telephones and
Bell-Blake microphones hitherto used in the telephone service. At
present we understand there are about 8,000 of these apparatus in use.
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