Mr. J. Munro, whose name is well known not only as a very clear writer
upon electrical subjects, but as an original investigator, has recently,
with the assistance of Mr. Benjamin Warwick, been conducting a most
interesting experimental investigation of the action of the microphone
as a telephonic transmitter, with the result of proving that metals may
advantageously be employed in the place of carbon in a transmitting
instrument, a practical development of one of the very earliest of
Professor Hughes' microphones. The fact that metallic electrodes can
practically be employed in microphonic transmitters has been denied of
late with so much assurance and in such high quarters, that Mr. Munro's
successful applications of that portion of Professor Hughes' discovery
possess an especial interest, and must to a considerable extent affect
the aspect of litigation in future contests in which the discovery of
the microphone and the invention of the carbon transmitter are vital
points at issue.
In investigating the properties of metallic conductors employed in the
construction of microphones, Mr. Munro's first experiments were made
with wires. These, in some cases, were caused by the action of a
diaphragm, to rub the one on the other in such a manner as to make the
point of contact vary (under the influence of the vibrations of the
diaphragms) on one side or other of a position of normal potential, so
that by the movement of a wire attached to a vibrating tympan along a
fixed wire conveying a current from a battery, and thereby shunting the
current at various positions along the length of the fixed wire, the
strength of the current in the derived circuit, in which was included a
suitable receiver, was varied accordingly. In other experiments mercury
was employed, either as a sliding-drop, inclosing the fixed wire, or as
an oscillating column; but these experiments, though instructive and
interesting, did not for various reasons give encouraging results with a
view to the practical application of the principle.
They, however, led Mr. Munro to proceed with compound wire structures,
such as gratings resting upon or rubbing against one another, and one of
the first experiments in this direction proved very successful, and led
Mr. Munro to the construction of his gauze telephone, which is the most
characteristic and efficient of his practical apparatus.
This instrument consists essentially of two pieces of iron-wire gauze,
the one fixed in a vertical plane, and the other resting more or less
lightly against it, the pressure between them being regulated by an
adjustable spring or weight. These gauze plates are so connected in a
telephonic circuit as to constitute the electrodes of a microphone; for
touching one another lightly in several points, they allow the current
to be transmitted between them in inverse proportion to the resistance
offered to it in its passage from one to the other. Under the influence
of sonorous vibrations the one plate dances more or less on the other,
thus varying the resistance; and very perfect articulation is produced
in a telephonic receiver included in the circuit. The gauze transmitter
so constructed may be fixed within a wall-box with or without a
mouthpiece; but as the sound waves acting directly upon the gauze plates
set them into agitation through their sympathetic vibration or by direct
impact, no sort of diaphragm or equivalent device is necessary, and none
is employed.
[Illustration: FIG. 1.]
A convenient form of this apparatus is shown in Fig. 1, and to which the
name of "The Lyre Telephone" has been given from its resemblance to that
impossible musical instrument. In this apparatus, G¹ is a plate of iron
wire gauze stretched vertically between two horizontal wires attached to
a lyre-shaped framework of mahogany; against the plate rests the smaller
plate, G², the normal pressure between them being regulated by an
adjustable spring acting in opposition to a weighted lever, W. The two
plates are connected respectively with the attachment screws, X and
Y, by which the instrument is placed in a circuit with a battery and
telephonic circuit.
[Illustration: FIG. 2.]
A modification of this apparatus is shown in the diagram sketch, Fig.
2, which will probably be a more practical form. In this instrument the
electrodes consist of two circular disks of iron wire gauze of different
diameters, the larger disk, G¹, which is fixed, being pierced with holes
of smaller diameter than the smaller disk, G². In the diagram the two
disks are shown separated for the purpose of explanation, but in reality
they rest the one against the other; the smaller and movable disk,
G², is held up against G¹ with greater or less pressure by the spiral
spring, S, the tension of which can be adjusted by a screw or other
suitable device at N. This form of the apparatus is more suitable for
inclosure in a wall box with or without a mouthpiece, but it does not
require the employment of any kind of diaphragm or tympan. Mr. Munro
can employ with all his instruments an induction coil for installations
where the resistance of the line wire makes it desirable to do so; the
microphone and battery being included in the primary circuit and the
telephones in the secondary.
[Illustration: FIG. 3.]
Fig. 3 is an ingenious arrangement devised by Mr. Munro, in which the
adjusting spring or weight is substituted by a magnet which may be
either a permanent or an electro-magnet. The figure shows an arrangement
in which the fixed gauze, g¹, is perforated as in the apparatus
illustrated in Fig. 2, and the movable electrode, g, is bent or dished
so as to press upon g¹ around its edge. E is a magnet which by its
attractive influence upon g holds t up against g¹ with a pressure
dependent upon its magnetic intensity and upon its distance from the
gauze. By making E an electro-magnet, and including its coil in the
telephonic circuit, an instrument may be constructed in which the normal
pressure between the electrodes can be automatically adjusted to the
strength of the current, and in cases where an induction coil is
employed the magnet, E, may be the core of such a coil.
[Illustration: FIG. 4.]
Fig. 4 illustrates an apparatus devised by Mr. Munro, and to which the
name thermo-microphone might be given, as it is a microphone in which
thermo-electric currents are employed in the place of voltaic currents,
its special feature of interest lying in the fact that the heated
junction of the thermo-electric couple is identical with the microphone
contacts of the two electrodes. In this very elegant experiment a piece
of iron wire gauze, G, is supported in a horizontal position by a light
metallic support, B. To another support. A, is loosely hinged a frame,
which at its further extremity carries a little coil of German silver
wire, C, which by its weight rests upon the center of the gauze plate,
G; and in contact therewith, and to increase the pressure of contact, a
little bar weight is laid within the convolutions of the core. The
two electrodes, the gauze, and the coil are connected, as shown, to a
receiving telephone, T. Upon the application of heat, as from the flame
of a spirit lamp placed below, a thermo-electric current is set up
throughout the circuit; in this condition the apparatus becomes a very
perfect microphone, and when the pressure between the electrodes is
properly adjusted it is a very efficient telephonic transmitter,
transmitting articulate speech and musical sounds with remarkable
clearness and fidelity.
[Illustration: FIG. 5]
Mr. Munro is, with the aid of Mr. Warwick's manipulative skill,
extending this portion of his investigation further by experimenting
with gauzes and coils of various metals forming other couples in
the thermo-electric series, as well as with iron and other gauzes
electrotyped with bismuth and other metals, and we hope in due time to
lay the results of those experiments before our readers.
Mr. Munro has, moreover, observed that if two pieces of gauze of
identical material and in microphonic contact be heated, a peculiar
sighing sound is heard in a telephone connected with them and with a
battery, and he attributes this phenomenon to the electrical discharge
between the gauze plates being facilitated and increased by the
action of heat, but we are rather inclined to trace the effect to the
mechanical action of the one gauze moving over the other under the
influence of expansion and contraction of the metals by the variable
temperature of the flame and convection currents of heated air, such
movement producing the sounds just as would be produced if one of the
electrodes of an ordinary microphone were as delicately moved by the
hand or other agent.
[Illustration: FIG. 6]
Figs. 5 and 6 illustrate another and distinct form of metallic
microphone transmitter designed by Mr. Munro and Mr. Warwick, in which
a small chain, preferably of iron, forms the microphonic portion of the
apparatus. In Fig. 5, A is a plate of sonorous wood forming a diaphragm
or collector of the sonorous waves; to the back of this is attached a
short length of chain, C, the opposite ends of which are by the wires, X
and Y, included in the telephonic circuit. The points of junction of the
links with one another constitute the variable microphonic contacts, and
the normal pressure between them is adjusted by the spiral spring, S,
the tension of which may be varied by the cord and winding pin, B. Fig.
6 is the section of a transmitter constructed upon this principle, and
in which two chains, c and c', are employed attached at one end by a
wire, f, to a diaphragm mouthpiece, N, and at their opposite extremities
to the adjusting springs, s and s'; an induction coil, D, may be
employed if the resistance of the line render it advantageous.
[Illustration: FIG. 7]
Fig. 7 is a form of pencil microphone experimented with by Mr. Munro,
which differs from some of the Hughes' transmitters adopted by Crossley,
Gower, Ader, and many others only in the material of which it is
composed, Mr. Munro's being of cast iron, while the others to which we
have referred are of carbon rods such as are used in electric lighting.
In Fig. 7 a light cast-iron bar, i², of the form shown, is supported in
holes drilled in two blocks of cast iron, i i', and the pressure between
the bar and the blocks can be adjusted by a regulating spring, s. In
connection with this apparatus Mr. Munro has observed that rust has no
appreciable effect upon the efficiency of the instrument unless it be
to such an extent as to cause the two to adhere, or to be "rusted up"
together.
[Illustration: FIG. 8]
We now come to another class of metallic transmitters with which Mr.
Munro and his associate have been making experiments, and to which he
has given the name "Grain transmitter," since it consists of a box
having metallic sides, e e', to which terminal screws, t t', are
attached and filled in between with iron or brass filings, granules of
spongy iron, or indeed small metallic particles in any form; one of the
most efficient transmitters being a box such as is shown in Fig. 8,
filled with a quantity of ¼ in. screws.
[Illustration: FIG. 9]
The results of Mr. Munro's experiments have led him to the opinion
that the action of the microphone must be attributed to the action
of sonorous vibrations upon the air or gaseous medium separating the
so-called contact-points of the electrodes, and that across these
spaces, or films of gaseous matter, silent electrical discharges take
place, the strengths of which, being determined by the thickness of the
gaseous strata through which they pass, vary with the motion of the
electrodes; and as, according to this hypothesis, the distances of the
electrodes from one another is determined by the sound-waves, the sound
in that way controls the current.--_Engineering_.
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