In an article by Count du Moncel, published in SCIENTIFIC AMERICAN
SUPPLEMENT, No 274, page 4364, the author, after describing Dr.
Herz's telephonic systems, deferred to another occasion the
description of a still newer system of the same inventor, because at
that time it had not been protected by patent. In the current number
of _La Lumière Electrique_, Count Moncel returns to the subject to
explain the principles of these new apparatus of Dr. Herz, and says:
I will first recall the fact that Dr. Herz's first system was based
upon the ingenious use (then new) of derivations. The microphone
transmitter was placed on a derivation from the current going to the
earth, taken in on leaving the pile, and the different contacts of the
microphone were themselves connected directly and individually with
the different elements of the pile. The telephone receiver was located
at the other end of the line, and when this receiver was a condenser
its armatures were, as a consequence of this arrangement, continuously
and preventively polarized, thus making it capable of reproducing
conversation.
[Illustration: DR. HERZ'S TELEPHONIC SYSTEMS.]
This arrangement evidently presented its advantages; but it likewise
possessed its inconveniences, one of the most important of these being
the necessity of employing rather strong piles and consequently of
exposing the line to those effects of charge which react in so
troublesome a manner in electrical transmissions when they occur on
somewhat lengthy lines. Now the fact should be recalled that Dr.
Herz's principal object was the application of the telephone to long
lines, and he has been applying himself to this problem ever since. He
at first thought of employing reversed currents, as in telegraphy; but
how was such a result to be attained with systems based upon the use
of sonorously-vibrating transmitters? He might have been able to solve
the problem with the secondary currents of an induction bobbin, as
Messrs. Gray, Edison, and others had done; but then he would no longer
have been benefited by those amplifications which are furnished by the
variations of pressure-derivations in microphones, and this led him to
endeavor to increase the effects of the induced currents themselves by
prolonging their duration, or rather by combining them in such a way
that they should succeed each other, two by two, in the same
direction; and this is the way he solved the problem in the beginning.
The fact should also be recalled that Dr. Herz had, from his first
experiments, recognized the efficiency of those microphonic contacts
that are obtained by the superposition of carbon disks or other
semi-conducting substances. He has employed these under different
arrangements and with very diverse groupings, but, as a general thing,
it has been the horizontal arrangement which has given him the best
effects.
Let us suppose, then, that four systems of contacts of this nature are
arranged at the four corners of an ebonite plate, C C (Figs. 1 and 2),
at A, A¹, B, B¹, and that they are connected with each other, as shown
in the cuts--that is to say, the upper disks, _e_, _f_, _g_, _h_,
parallel with the sides of the plate, and the lower disks, A, A¹, B,
B¹, diagonally. Let us admit, further, that the plate pivots about an
axis, R; that the disks are traversed by small pins fixed in the
plate; and that small leaden disks rest upon the upper disks. Finally,
let us imagine that the plate is connected at one end, through a rod
T, with a telephone diaphragm. Now it will be readily understood that
the vibrations produced by the diaphragm will cause the oscillation of
the plate, C C, and that there will result therefrom, on the part of
the disks, two effects that will succeed one another. The first will
be, for the ascending vibrations, an increase of pressure effected
between the disks of the left side, by reason of their force of
inertia being increased by that of the lead disks; and the second will
be, for the disks to the right, and, for the same reason, a reduction
of pressure which will take place through resilience, at the moment of
change in direction of the vibrating motions.
If the current from a pile, P, traverses all these disks, through the
connections that we have just mentioned, and passes through the
primary helix (through the wire, I) of an induction coil H H' (Fig.
2), located beneath the apparatus, and if the secondary current from
this bobbin corresponds, through the wire I, with a telephone line in
which there is interposed a telephone or a speaking condenser, there
will be set up an inverse induced current, which, being reversed as a
consequence of the crosswise connections of the disks, will continue
the action of the first or increase its duration, and, consequently,
its force, through the telephone receiver.
The results of this system are very good; but Dr. Herz has endeavored
to simplify it still further, and with this object in view has
experimented on several arrangements. For example, to obtain inversion
a contact was simply placed on each side of the vibrating plate.
Although the movements of this latter are not, as we know, of the
nature of ordinary sonorous vibrations, it was thought that they might
prove to be in opposite directions on the two sides of the plate, and
that one of the contacts might be compressed while the other was free.
So notwithstanding the advantages of this arrangement, it was thought
necessary to place the plate vertically in order to give the same
regulation to the two contacts which it is essential should be
identical. But it became difficult to regulate by weight; and even to
succeed in regulating at all, it became necessary to employ two
parallel diaphragms, vibrating in unison, and each carrying its
contact, but in opposite directions. Afterwards, the horizontal
arrangement was again adopted; but, by a clever combination, the two
principles applied by Dr. Herz--derivation and inversion--were united.
The current is then led to a double contact, where it divides. This
contact is arranged under the plate in such a way that its two points
of variable resistance act in opposite directions to each other, or,
in some apparatus, so that one of the points has no variation, while
the other is in action. The result that occurs may be easily imagined.
The system has been experimented with under different forms; in one
case the derivation is simple, that is, a single one of the currents
being sent into the line, while in another case it is double, each of
the branches being provided with a bobbin and communicating with the
receiver. In the latter case the result is remarkably good, but the
apparatus is not free from a certain amount of complication, and
demands, moreover, particular care in its construction, experience
having shown that the induction coils must not be equal, but that they
must present resistances combined according to the circuit doing duty.
It should be added that researches have been continued as to the
bodies proper to be employed as microphonic contact, with the result
of bringing out the important fact that the number of substances that
can be put to this use is almost unlimited. The contacts of the Herz
apparatus are now being made of conducting bodies (metals for example)
reduced to powder and conglomerated by chemical means with a sort of
non-conductive cement. The proportion of the elements depends upon the
conductivity of the materials employed, and it alone determines the
microphonic value of the compound, the nature of the elements
apparently having scarcely any influence.
Nor has the speaking condenser been neglected. As regards this,
efforts have seemingly been made toward finding a convenient
arrangement and a regular mode of construction, the good working of
these apparatus being absolutely dependent upon the care with which
they are set up.
In Dr. Herz's opinion, the telephone is not to remain a single
apparatus, varied only as to form, but, on the contrary, must be
actually modified according to the purposes for which it is designed.
He thinks that a telephone operating at great distances must differ
from a city apparatus, and that an instrument for transmitting song
can not be absolutely the same as one for conversational purposes. So
he has endeavored to create types that shall prove appropriate for
these different applications.
* * * * *
DECISION OF THE CONGRESS OF ELECTRICIANS ON THE UNITIES OF
ELECTRIC MEASURES.
For these measures there are adopted the fundamental
unities--centimeter, gramme, second, and this system is briefly
designated by the letters C., G., S. The practical units, the _ohm_ and
the _volt_, will retain their present definitions; the ohm is a
resistance equal to 10^{9} absolute unities (C., G., S.), and the volt
is an electromotive force equal to 10^{8} absolute unities (C., G., S.).
The practical unit of resistance (ohm) will be represented by a column
of mercury of 1 square mm. in section at the temperature of 0°C. An
international commission will be charged with ascertaining for
practice, by means of new experiments, the height of this column of
mercury representing the ohm. The name _ampère_ will be given to the
current produced by the electromotor force of 1 volt in a circuit
whose resistance is 1 ohm. _Coulomb_ is the quantity of electricity
defined by the condition that in the current of an ampère the section
of the conductor is traversed by a coulomb per second. _Farad_ is the
capacity defined by the condition that a coulomb in a condenser, whose
capacity is a farad, establishes a difference of potential of a volt
between the armatures.
* * * * *