of the French Engineers, which is very successful in transmitting
inarticulate sounds. In this case the vertical carbon is pear-shaped,
and its larger end rests in a hole made in the lower carbon; its upper
and pointed end goes into a small hole made in the upper carbon, but so
as hardly to touch it, and there is a screw to regulate the distance
between the two carbons. Under such conditions, the contacts are so
unstable that almost anything will put an end to them, and consequently
the variations in the intensity of the transmitted current are so
strong that the sounds produced by the telephone may be heard at the
distance of several yards.
[Illustration: FIG. 41.]
Fig. 41 represents another arrangement, devised by M. Ducretet. The
two carbon blocks are at D D′, the moveable carbon pencil is at C, the
telephone at T, and the binding screws at B B′. An enlarged figure of
the arrangement of the carbons is given on the left. The arm which
holds the upper carbon D is fastened to a rod, bearing a plate P′,
of which the surface is rough, and a little cage C′, made of wire
netting, can be placed upon the plate, so as to enable us to study the
movements of living insects.
When speech is to be transmitted with a force which can make the
telephone audible in a large room, the microphone must have a special
arrangement, and fig. 42 represents the one which Mr. Hughes considers
the most successful, to which he has given the name of _speaker_.
[Illustration: FIG. 42.]
In this new form, the moveable carbon which is required to produce
the variable contacts is at C, at the end of a horizontal bar B A,
properly balanced so as to move up and down on its central point. The
support on which the bar oscillates is fastened to the end of a spring
plate in order that it may vibrate more easily, and the lower carbon
is placed at D below the first. It consists of two pieces laid upon
each other, so as to increase the sensitiveness of the instrument, and
we represent the upper piece at E, which is raised so as to show that
when it is desired only one of these carbons need be used. For this
purpose the carbon E is fastened to a morsel of paper, which is fixed
to the little board and contributes to the articulation. A spring R, of
which the tension can be regulated by the screw _t_, serves to regulate
the pressure of the two carbons. Mr. Hughes recommends the use of
metallised charcoal prepared from deal.[14] The whole is then enclosed
in a semi-cylindrical case H I G, made of very thin pieces of deal, and
the system is fixed, together with another similar system, in a flat
box, M J L I, which, on the side M I, presents an opening before which
the speaker stands, taking care to keep his lower lip at a distance of
two centimètres from the bottom of the box. If the two telephones are
connected for strength, and if the battery employed consists of two
cells of bichromate of potash, it is possible to act so strongly on the
current, that, after traversing an induction coil only six centimètres
long, a telephone of Bell’s square model can be made to speak, so as
to be heard from all parts of a room; a speaking tube, about a yard
long, must indeed be applied to it. Mr. Hughes asserts that the sounds
produced by it are nearly as loud as those of the phonograph, and this
is confirmed by Mr. Thomson.
M. Boudet de Paris has lately invented a microphone speaker of the same
kind, with which it is possible to make a small telephone utter a loud
sound. An induction coil, influenced by a single Leclanché cell, must
be employed.
Suppose that a very small carbon rod with pointed ends is placed at
the bottom of a box, of about the size of a watch. One end of the rod
rests against a morsel of carbon, which is fastened to a very thin
steel diaphragm, placed before a mouthpiece which acts as a lid to
the box, and is screwed above it. Next suppose that a small piece of
paper, folded in two, in the shape of the letter V, is fixed above that
part of the carbon in contact with the carbon of the diaphragm. This
constitutes the instrument, and in order to work it, it must be held
in a vertical position before the mouth, at a distance of about three
centimètres, and it is necessary to speak in the ordinary tone. If the
telephone is placed in direct communication with this instrument, it
will send the voice to a distance. Without employing a Leclanché cell,
the voice may be heard at the distance of ten yards, if one of the
carbons used for the phonograph is placed before the mouthpiece of the
telephone.
In this system, the sensitiveness of the instrument is entirely due to
the slightness of the contact between the two carbons, and the slight
elasticity of the folded paper constitutes the contact. Perhaps the
paper itself has some influence; at any rate the most delicate spiral
spring is incapable of producing the same effect, and it is necessary
to suspend the instrument vertically, in order that the weight of the
moveable carbon may not affect it. It can be regulated by depressing or
elevating that part of the paper which rests on the carbon rod.
Although it is possible to work all telephones with this instrument,
some are more effective than others. The mouthpiece must be concave,
and the diaphragm must be close to its rim, and must be made of a
particular kind of tin. The ordinary diaphragm does not act well, and
M. Boudet de Paris has tried several, so as to obtain the maximum
effect.
It is certain that when the instruments are as well regulated as those
which the inventor has deposited with me, their results are really
surprising. It is even possible, by using several microphones at the
sending station, to obtain the reproduction of duets, and even of
trios, with remarkable effect.
With this kind of microphone speaker M. Boudet de Paris is able to
transmit speech into a snuff-box telephone, merely consisting of a
flat helix of wire, placed before a slightly magnetised steel plate,
and without insertion of a magnetic core. A single Leclanché cell was
enough. An experiment of the same nature was tried in England, but it
was found necessary to use six Leclanché cells.
[Illustration: FIG. 43]
[Illustration: FIG. 44.]
The microphone may also be made of morsels of carbon pressed into a
box between two metallic electrodes, or enclosed in a tube with two
electrodes represented by two elongated fragments of carbon. In the
latter case the carbons ought to be as cylindrical as possible, and
those made by M. Carré for the Jablochkoff candles are very suitable.
Fig. 43 represents an instrument of this kind which M. Gaiffe arranged
for me, and which, as we shall see, serves as a thermoscope (fig.
44). It is composed of a quill filled with morsels of carbon, and
those at the two ends are tipped with metal. One of these metal tips
ends in a large-headed screw which, by means of its supports A B, is
able to press more or less on the morsels of carbon in the tube, and
consequently to establish a more or less intimate contact between
them. When the instrument is properly regulated, speech can be
reproduced by speaking above the tube. It is therefore a microphone
as well as a thermoscope. Mr. Hughes has remarked one curious fact,
namely, that if the different letters of the alphabet are pronounced
separately before this sort of microphone, some of them are much
more distinctly heard than others, and it is precisely those which
correspond to the breathings of the voice.
A microphone of this kind may be made by substituting for the carbon
powders of more or less conductivity, or even metal filings. I have
shown in my paper on the action of substances of moderate conductivity,
that such power varies considerably with the pressure and the
temperature; and as the microphone is based on the differences of
conducting power which result from differences of pressure, we can
understand that these powders may be used as a means of telephonic
transmission. In a recent arrangement of this system Mr. Hughes has
made the powder adhere together with a sort of gum, and has thus made a
cylindrical pencil which, when connected with two electrodes which are
good conductors, can produce effects analogous to those we have just
described. As I have said, it is possible to use metal filings, but Mr.
Hughes prefers powdered charcoal.
Mr. Blyth states that a flat box, about 15 inches by 9, filled with
coke, and with two tin electrodes fixed to the two ends, is one of
the best arrangements for a microphone. He says that three of these
instruments, hung like pictures against the wall of a room, would
suffice, when influenced by a single Leclanché cell, to make all the
sounds produced in a telephone audible, and especially vocal airs. Mr.
Blyth even asserts that a microphone capable of transmitting speech can
be made with a simple piece of coke, connected with the circuit by its
two ends, but it must be coke: a retort carbon, with electrodes, will
not act.
It is a remarkable property of these kinds of microphones that they can
act without a battery, at least when they are so arranged as to form a
voltaic element for themselves, and this can be done by throwing water
on the carbons. Mr. Blyth, who was the first to speak of this system,
does not distinctly indicate its arrangement, and we may assume that
his instrument did not differ from the one we have already described,
to which water must have been added. In this way, indeed, I have been
able to transmit not only the ticking of a watch and the sounds of
a musical box, but speech itself, which was often more distinctly
expressed than in an ordinary microphone, since it was free from the
sputtering sound which is apt to accompany the latter. Mr. Blyth also
asserts that sounds may be transmitted without the addition of water,
but in this case he considers that the result is due to the moisture of
the breath. Certainly much moisture is not required to set a voltaic
couple in action, especially when a telephone is the instrument of
manifestation. The ordinary microphone may be used without a battery,
if the circuit in which it is inserted is in communication with the
earth by means of earthen cakes; the currents which then traverse the
circuit will suffice to make the tickings of a watch placed upon the
microphone perfectly audible. M. Cauderay, of Lausanne, in a paper sent
to the Académie des Sciences, July 8, 1878, informs us that he made
this experiment on a telegraphic wire which unites the Hôtel des Alpes
at Montreux with a _châlet_ on the hill--a distance of about 550 yards.
_The Microphone used as a Speaking Instrument._--The microphone can
not only transmit speech, but it can also under certain conditions
reproduce it, and consequently supply the place of the receiving
telephone. This seems difficult to understand, since a cause for the
vibratory motion produced in part of the circuit itself can only be
sought in the variations in intensity of the current, and the effects
of attraction and magnetisation have nothing to do with it. Can the
action be referred to the repulsions reciprocally exerted by the
contiguous elements of the same current? Or are we to consider it to
be of the same nature as that which causes the emission of sounds from
a wire when a broken current passes through it, so that an electric
current is itself a vibratory current, as Mr. Hughes believes? It is
difficult to reply to these questions in the present state of science;
we can only state the fact, which has been published by Messrs.
Hughes, Blyth, Robert Courtenay, and even by Mr. Edison himself.
I have been able to verify the fact myself under the experimental
conditions indicated by Mr. Hughes, but I was not so successful in
the attempt to repeat Mr. Blyth’s experiments. This gentleman stated
that in order to hear speech in a microphone it would be enough to
use the model made from fragments of carbon, as we have described,
to join to it a second microphone of the same kind, and to introduce
into the circuit a battery consisting of two Grove elements. If anyone
then speaks above the carbons of one of the microphones, what is said
should be distinctly heard by the person who puts his ear to the
other, and the importance of the sounds thus produced will correspond
with the intensity of the electric source employed. As I have said,
I was unable by following this method to hear any sound, still less
articulate speech; and if other experiments had not convinced me, I
should have doubted the correctness of the statement. But this negative
experiment does not in fact prove anything, since it is possible that
my conditions were wrong, and that the cinders which I employed were
not subject to the same conditions as Mr. Blyth’s fragments of coke.
[Illustration: FIG. 45.]
With respect to Mr. Hughes’s experiments, I have repeated them with
the microphone made by MM. Chardin and Berjot, using that by M. Gaiffe
as the sender, and I ascertained that with a battery of only four
Leclanché cells, a scratch made on the sender, and even the tremulous
motion and the airs played in a little musical box placed on the
sender, were reproduced--very faintly, it is true--in the second
microphone; in order to perceive them, it was enough to apply the ear
to the vertical board of the instrument. It is true that speech was not
reproduced, but of this Mr. Hughes had warned me; it was evident that
with this arrangement the instrument was not sufficiently sensitive.
A different arrangement of the microphone is required for the
transmission and the reproduction of speech by this system, and a
section of the one which Mr. Hughes found most successful is given in
fig. 45. It somewhat resembles Mr. Hughes’s microphone speaker, placed
in a vertical position, and the fixed carbon is fastened to the centre
of the stretched membrane of a string telephone. The ear or mouth tube
is at A, the membrane at D D, the carbon just mentioned at C: this
carbon is of metallised charcoal prepared from deal, and so also is the
double carbon E, which is in contact with it and is fastened to the
upper end of the little bar G I. The whole is enclosed in a small box,
and the pressure exerted on the contact of the two carbons is regulated
by a spring R and a screw H. The tube of the telephone serves as an
acoustic tube for the listener, and Mr. Hughes’s speaker, described
above, acts as sender. It is hardly necessary to say that the two
instruments are placed at each end of the circuit, that the carbons
are connected with the two poles of a battery of one or two cells of
bichromate of potash, or two Bunsen or six Leclanché cells, and the
two instruments are connected by the line wire. Under such conditions,
conversation may be exchanged, but the sounds are always much less
distinct than they are in a telephone.
I was able to ascertain this fact with a roughly made instrument
brought from England by Mr. Hughes. MM. Berjot, Chardin, and de
Méritens, who were also present at the experiments, were able with me
to hear speech perfectly, and I have since successfully repeated the
experiment alone, but it does not always succeed, and under its present
conditions the instrument has no importance in a scientific point of
view. It is evident that the instrument can dispense with any support,
and the little box then forms the handle of the instrument; in this
case the two binding screws are placed at the end of this handle, as in
a telephone. The microphone speaker with a disk, represented in fig.
5, which acts as sender to the singing condenser, can be used, when
properly regulated, as a receiving microphone. M. Berjot has obtained
good results from a little instrument of the same kind as that in fig.
45, but with a metal diaphragm, and the microphonic system consists
of a cylindrical piece of carbon resting on a small disk of the same
substance, which is galvanised and soldered to the diaphragm. The whole
is enclosed in a small round box, with its upper part cut in the form
of a mouthpiece.
It seems that all microphone senders with disks can reproduce speech
more or less perfectly; it is a question of adjusting and refining
the carbon points of contact. A weak battery, consisting of a single
Leclanché cell, is better for these instruments than a strong battery,
precisely because of the effects of oxidation and polarisation, which
are so energetically produced at these points of contact when the
battery is strong.
The effects of the microphone receiver explain the sounds, often very
intense, produced by the Jablochkoff candles when they are influenced
by electro-magnetic machines. These sounds always vibrate in unison
with those emitted by the machine itself, and they result, as I have
already shown, from the rapid magnetisations and demagnetisations which
are effected by the machine. These effects, observed by M. Marcel
Deprez, were particularly marked in M. de Méritens’ first machines.
_Other Arrangements of Microphones._--An arrangement such as we have
just described has been employed by M. Carette to form an extremely
powerful microphone speaker. The only difference is that the stretched
membrane is replaced by a thin metallic disk: he fastens one of the
carbons to the centre of this disk, and applies to it the other carbon,
which is pointed, and held by a _porte-carbon_ with a regulating
screw, so that the pressure which takes place between the two carbons
may be regulated at pleasure. By this arrangement speech may be heard
at a distance from the telephone. In other respects it resembles the
telephone sender represented in fig. 5.
M. de Méritens has executed the system represented, fig. 45, on a large
scale, forming the tube A B of a zinc funnel a yard in length, and in
this way he has been able to magnify the sounds, so that a conversation
held in a low voice, three or four yards from the instrument, has been
produced in a telephone with more sonorous distinctness. The instrument
was placed on the floor of the apartment, with the opening of the
funnel above, and the telephone was in the cellars of the house.
The form of the microphone has been varied in a thousand ways, to suit
the purposes to which it was to be applied. In the ‘English Mechanic
and World of Science,’ June 28, 1878, we see the drawings of several
arrangements, one of which is specially adapted for hearing the steps
of a fly. It is a box, with a sheet of straw paper stretched on its
upper part; two carbons, separated by a morsel of wood, and connected
with the two circuit wires, are fastened to it, and a carbon pencil,
placed crosswise between the two, is kept in this position by a groove
made in the latter. A very weak battery will be enough to set the
instrument at work, and when the fly walks over the sheet of paper it
produces vibrations strong enough to react energetically on an ordinary
telephone. The instrument must be covered with a glass globe. When a
watch is placed on the membrane, with its handle applied to the morsel
of wood which divides the two carbons, the noise of its ticking may
be heard through a whole room. Two carbon cubes placed side by side,
and only divided by a playing-card, may also be used instead of the
arrangement of carbons described above. A semicircular cavity, made
in the upper part of the two carbons, in which are placed some little
carbon balls, smaller than a pea and larger than a mustard seed, will
make it possible to obtain multiple contacts which are very mobile and
peculiarly fit for telephonic transmissions. This arrangement has been
made by Mr. T. Cuttriss.
Several other arrangements of microphones have been devised by
different makers and inventors, such as those of Messrs. Varey, Trouvé,
Vereker, de Combettes, Loiseau, Lippens, de Courtois, Pollard, Voisin,
Dumont, Jackson, Paterson, Taylor, &c., and they are more or less
satisfactory. The instruments of MM. Varey, Trouvé, Lippens, and de
Courtois are the most interesting, and we will describe them.
M. Varey’s microphone consists of a sounding box of deal, mounted in
a vertical position on a stand, and two microphones are arranged on
either side of it, with vertical carbons united for tension. A small
Gaiffe cell of chloride of silver, without liquid, is applied to the
standard of the instrument, and is enough to make it work perfectly.
This system is extremely sensitive.
M. Trouvé’s microphones, represented in figs. 46, 47, 48, are extremely
simple, so that he is able to sell them at a very moderate price. They
generally consist of a small vertical cylindrical box, as we see in
the figure, with disks of carbon at its two ends, which are united by a
carbon rod, or by a metallic tube tipped with carbon. This rod or tube
turns freely in two cavities made in the carbons, and the box, while
acting as a sounding box, becomes at the same time a prison for the
insects whose movements and noises are the objects of study.
[Illustration: FIG. 46.]
These boxes may be suspended on a cross-bar (fig. 47) by the two
communicating wires, so as to be completely insulated. In this case the
ticking of a watch placed upon the board, friction, and external shocks
are hardly heard, but on the other hand the sound vibrations of the air
alone are transmitted, and they acquire great distinctness. We have
often repeated these experiments, and have always found that the tones
of the voice were perfectly preserved.
The model represented fig. 48 is still more simple, and appears to
be the latest development of this kind of instrument. It consists
of a stand and a disk united by a central rod. The upper disk moves
round the central rod, and permits the vertical carbon to assume any
inclination which is desired. It is evident that the instrument will
become less sensitive when the carbon is more oblique.
[Illustration: FIG. 47.]
We must also mention a very successful microphone devised by M.
Lippens. It is a slightly made box, like that of M. Varey, and on its
opposite faces there are applied, on two frames left empty for the
purpose, two thin plates of hardened caoutchouc, in the centre of which
inside the box, two carbons are fastened, and on their outer surface a
half-sphere is hollowed.
[Illustration: FIG. 48.]
The interval between the two carbons hardly amounts to two millimètres,
and a carbon ball is inserted into the two cavities which form its
spherical case. This ball is supported by a spiral spring which can be
extended more or less by means of a wire wound on a windlass which is
fixed above the instrument, like the spring of an electric telegraph
instrument. By means of this spring, the pressure of the carbon ball
against the sides of the cavity which contains it can be regulated at
pleasure, and the sensitiveness of the instrument and its capacity
for transmitting speech can be adjusted. Under these conditions, the
vibrations of the caoutchouc plates directly affect the microphone,
and the currents of air have no influence on it, so that the effects
are more distinct. It is so sensitive that it is best for the speaker
to place himself at the distance of at least 50 centimètres from the
instrument. M. Lippens’ instrument is a pretty one, mounted on a wooden
stand, which is neatly turned.
In order to put an end to the sputtering usual in microphones, it
occurred to M. de Courtois to prevent any cessation of contact
between the carbons by keeping them close together, and to effect the
variations of resistance necessary for articulate sounds by making them
slide over each other, so as to insert a shorter or longer portion
of the carbon in the circuit. For this purpose a vibrating disk is
placed in a vertical position in a rigid frame, and a small conducting
rod, terminated by a pointed carbon, is applied to it, with this
carbon point resting on another flat piece of carbon placed below it.
Influenced by the vibrations of the disk, the carbon point moves to and
fro, effecting more or less extensive contacts with the lower carbon,
and thus producing variations of resistance which almost correspond to
the range of vibrations on the disk.
_Experiments made with the Microphone._--I must now mention the
interesting experiments which led Mr. Hughes to the invention of
the remarkable instrument of which we have spoken, as well as those
undertaken by other scientific men, either from a scientific or a
practical point of view.