Mr. Hughes went on to consider whether sound vibrations, transmitted
to a conductor traversed by a current, would not also modify this
conductivity by provoking the contraction and expansion of the
conducting molecules, which would be equivalent to the shortening
or lengthening of the conductor thus affected. If such a property
existed, it would make it possible to transmit sounds to a distance,
since variations in the conductivity would result from variations
corresponding to the intensity of the current acting on the telephone.
The experiment which he made on a stretched metal wire did not,
however, fulfil his expectation, and it was only when the wire vibrated
so strongly as to break, that he heard a sound at the moment of its
fracture. When he again joined the two ends of the wire, another sound
was produced, and he soon perceived that imperfect contact between the
two broken ends of wire would enable him to obtain a sound. Mr. Hughes
was then convinced that the effects he wished to produce could only be
obtained with a divided conductor, and by means of imperfect contacts.
He then sought to discover the degree of pressure which it was most
expedient to exert between the two adjacent ends of the wire, and
for this purpose he effected the pressure by means of weights. He
ascertained that when the pressure did not exceed the weight of an
ounce on the square inch at the point of connection, the sounds
were reproduced with distinctness, but somewhat imperfectly. He next
modified the conditions of the experiment, and satisfied himself that
it was unnecessary to join the wires end to end in order to obtain this
result. They might be placed side by side on a board, or even separated
(with a conductor placed crosswise between them), provided that the
conductors were of iron, and that they were kept in metallic connection
by a slight and constant pressure. The experiment was made with three
Paris points, and arranged as it appears in fig. 49, and it has since
been repeated under very favourable conditions by Mr. Willoughby Smith
with three of the so-called rat-tail files, which made it possible to
transmit even the faint sound of the act of respiration.[15]
[Illustration: FIG. 49.]
He afterwards tried different combinations of the same nature, which
offered several solutions of continuity, and a steel chain produced
fairly good results, but slight inflections, like those caused by
the _timbre_ of the voice, were not reproduced, and he tried other
arrangements. He first sought to apply metallic powders to the points
of contact; powdered zinc and tin, known in commerce under the name
of white bronze, greatly increased the effects obtained; but they
were unstable, on account of the oxidation of the contacts; and it
was in seeking to solve this difficulty, as well as to discover the
most simple means of obtaining a slight and constant pressure on the
contacts, that Mr. Hughes was led to the arrangement, previously
described, of carbons impregnated with mercury, and he thus obtained
the maximum effect.[16]
Mr. Hughes considers that the successful effects of the microphone
depend on the number and perfection of the contacts, and this is
doubtless the reason why some arrangements of the carbon pencil in the
instrument described above were more favourable than others.
In order to reconcile these experiments with his preconceived ideas,
Mr. Hughes thought that, since the differences of resistance proceeding
from the vibrations of the conductor were only produced when it
was broken, the molecular movements were arrested by the lateral
resistances which were equal and opposite, but that if one of these
resistances were destroyed, the molecular movement could be freely
developed. He considers that an imperfect contact is equivalent to
the suppression of one of these resistances, and as soon as this
movement can take place, the molecular expansions and contractions
which result from the vibrations must correspond to the increase
or diminution of resistance in the circuit. We need not pursue Mr.
Hughes’s theory further, since it would take too long to develope it,
and we must continue our examination of the different properties of the
microphone.[17]
Carbon, as we have said, is not the only substance which can be
employed to form the sensitive organ of this system of transmission.
Mr. Hughes has tried other substances, including those which are good
conductors, such as metals. Iron afforded rather good results, and the
effect produced by surfaces of platinum when it was greatly subdivided
was equal, if not superior, to that furnished by the mercurised carbon.
Yet since the difficulty of making instruments with this metal is
greater, he prefers the carbon, which resembles it in being incapable
of oxidation.
We have already said that the microphone may be used as a thermoscope,
in which case it must have the special arrangement represented in fig.
43. Under these conditions, heat, reacting on the conductivity of these
contacts, may cause such variations in the resistance of the circuit
that the current of three Daniell cells will be annulled by approaching
the hand to the tube. In order to estimate the relative intensity of
the different sources of heat, it will be enough to introduce into
the circuit of the two electrodes A and B, fig. 43, a battery P, of
one or two Daniell cells, and a moderately sensitive galvanometer G.
For this purpose one of 120 turns will suffice. When the deviation
decreases, it shows that the source of heat is superior to the
surrounding atmosphere; and conversely, that it is inferior when the
deviation increases. Mr. Hughes says that the effects resulting from
the intervention of sunshine and shadow are shown on the instrument by
considerable variations in the deviations of the galvanometer. Indeed
it is so sensitive to the slightest variations of temperature that it
is impossible to maintain it in repose.
I have repeated Mr. Hughes’s experiments with a single Leclanché cell,
and for this purpose I employed a quill, filled with five fragments
of carbon, taken from the cylindrical carbons of small diameter which
are made by M. Carré for the electric light. I have obtained the
results which are mentioned by Mr. Hughes, but I ought to say that the
experiment is a delicate one. When the pressure of the fragments of
carbon against each other is too great, the current traverses them with
too much force to allow the calorific effects to vary the deviation
of the galvanometer, and when the pressure is too slight, the current
will not pass through them. A medium degree of pressure must therefore
be effected to ensure the success of the experiment, and when it is
obtained, it is observed that on the approach of the hand to the
tube, a deviation of 90° will, after a few seconds, diminish, so that
it seems to correspond with the approach or withdrawal of the hand.
But breathing produces the most marked effects, and I am disposed to
believe that the greater or less deviations produced by the emission
of articulate sounds when the different letters of the alphabet are
pronounced separately, are due to more or less direct emissions of
heated gas from the chest. It is certain that the letters which require
the most strongly marked sounds, such as A, F, H, I, K, L, M, N, O, P,
R, S, W, Y, Z, produce the greatest deviations of the galvanometric
needle.
In my paper on the conductivity of such bodies as are moderately good
conductors, I had already pointed out this effect of heat upon divided
substances, and I also showed that after a retrograde movement, which
is always produced at once, a movement takes place in an inverse
direction to the index of the galvanometer when heat has been applied
for some instants, and this deviation is much greater than the one
which is first indicated.
In a paper published in the ‘American Scientific Journal,’ June 28,
1878, Mr. Edison gives some interesting details on the application of
his system of a telephonic sender to measuring pressures, expansions,
and other forces capable of varying the resistance of the carbon disk
by means of greater or less compression. Since his experiments on
this subject date from December 1877, he again claims priority in
the invention of using the microphone as a thermoscope; but we must
observe that according to Mr. Hughes’s arrangement of his instrument,
the effect produced by heat is precisely the reverse of the effect
described by Mr. Edison. In fact, in the arrangement adopted by the
latter, heat acts by increasing the conductivity acquired by the
carbon under the increased pressure produced by the expansion of a
body sensitive to heat: in Mr. Hughes’s system, the effect produced
by heat is precisely the contrary, since it then acts only on the
contacts, and not by means of pressure. Therefore the resistance
of the microphone-thermoscope is increased under the influence of
heat, instead of being diminished. This contrary effect is due to
the division of some substance which is only a moderate conductor,
and I have shown that under such conditions these bodies, when only
slightly heated, always diminish the intensity of the current which
they transmit. I believe that Mr. Edison’s arrangement is the best for
the thermoscopic instrument, and makes it possible to measure much less
intense sources of heat. Indeed he asserts that by its aid the heat of
the luminous rays of the stars, moon, and sun may be measured, and also
the variations of moisture in the air, and barometric pressure.
This instrument, which we give fig. 50, with its several details, and
with the rheostatic arrangement employed for measuring, consists of
a metallic piece A fixed on a small board C, and on one of its sides
there is the system of platinum disks and carbons shown in fig. 28. A
rigid piece G, furnished with a socket, serves as the external support
of the system, and into this socket is introduced the tapering end
of some substance which is readily affected by heat, moisture, or
barometric pressure. The other extremity is supported by another socket
I, fitted to a screw-nut H, which may be more or less tightened by a
regulating screw. If this system is introduced into a galvanometric
circuit _a, b, c, i, g_, provided with all the instruments of the
electric scale of measure, the variations in length of the substance
inserted are translated by greater or less deviations of the
galvanometric needle, which follow from the differences of pressure
resulting from the lengthening or shortening of the surface capable of
expansion which is inserted in the circuit.
[Illustration: FIG. 50.]
The experiments on the microphone made in London at the meeting of the
Society of Telegraphic Engineers on May 25, 1878, were wonderfully
successful, and they were the subject of an interesting article in
the ‘Engineer’ of May 31, which asserts that the whole assembly heard
the microphone speak, and that its voice was very like that of the
phonograph. When the meeting was informed that these words had been
uttered at some distance from the microphone, the Duke of Argyll,
who was present, while admiring the important discovery, could not
help exclaiming that this invention might have terrible consequences,
since, for instance, if one of Professor Hughes’s instruments were
placed in the room in Downing Street, in which Her Majesty’s ministers
hold their cabinet council, their secrets might be heard in the room
in which the present meeting took place. He added that if one of these
little instruments were in the pocket of Count Schouvaloff, or of Lord
Salisbury, we should at once be in possession of the secrets for which
all Europe was anxiously waiting. If these instruments were able to
repeat all the conversations held in the room in which they stood, they
might be really dangerous, and the Duke thought that Professor Hughes,
who had invented such a splendid yet perilous instrument, ought next to
seek an antidote for his discovery. Dr. Lyon Playfair, again, thought
that the microphone ought to be applied to the aërophone, so that by
placing these instruments in the two Houses of Parliament, the speeches
of great orators might be heard by the whole population within five or
six square miles.
The experiments lately made with the microphone at Halifax show that
the Duke of Argyll’s predictions were fully justified. It seems that
a microphone was placed on a pulpit-desk in a church in Halifax, and
connected by a wire about two miles long with a telephone placed close
to the bed of a sick person, who was able to hear the prayers, the
chanting, and the sermon. This fact was communicated to me by Mr.
Hughes, who heard it from a trustworthy source, and it is said that
seven patients have subscribed for the expense of an arrangement by
which they may hear the church services at Halifax without fatigue.
The microphone has also lately been applied to the transmission of a
whole opera, as we learn from the following account in the ‘Journal
Télégraphique,’ Berne, July 25, 1878:--
‘A curious micro-telephonic experiment took place on June 19 at
Bellinzona, Switzerland. A travelling company of Italian singers was to
perform Donizetti’s opera, “Don Pasquale,” at the theatre of that town.
M. Patocchi, a telegraphic engineer, took the opportunity of making
experiments on the combined effects of Hughes’s carbon microphone as
the sending instrument, and Bell’s telephone as the receiver. With
this object he placed a Hughes microphone in a box on the first tier,
close to the stage, and connected it by two wires, from one to half a
millimètre in thickness, to four Bell receivers, which were placed in a
billiard-room above the vestibule of the theatre, and inaccessible to
sounds within the theatre itself. A small battery of two cells, of the
ordinary type used in the Swiss telegraphic service, was inserted in
the circuit, close to the Hughes microphone.
‘The result was completely successful. The telephones exactly
reproduced, with wonderful purity and distinctness, the instrumental
music of the orchestra, as well as the voices of the singers. Several
people declared that they did not lose a note of either, that the
words were heard perfectly; the airs were reproduced in a natural key,
with every variation, whether _piano_ or _forte_, and several amateurs
assured M. Patocchi that by listening to the telephone they were able
to estimate the musical beauty, the quality of the singers’ voices,
and the general effect of the piece, as completely as if they had been
among the audience within the theatre.
‘The result was the same when resistances equivalent to 10 kilomètres
were introduced into the circuit, without increasing the number of
cells in the battery. We believe that this is the first experiment of
the kind which has been made in Europe, at least in a theatre, and
with a complete opera; and those who are acquainted with the delicacy
and grace of the airs in “Don Pasquale” will be able to appreciate the
sensitiveness of the combined instruments invented by Hughes and Bell,
which do not suffer the most delicate touches of this music to be lost.’
Although experiments with the microphone are of such recent date, they
have been very various, and among other curious experiments we learn
from the English newspapers that the attempt has been made to construct
an instrument on the same principle as the telephone, which shall be
sensitive to the variations of light. It is known that some substances,
and particularly selenium, are electrically affected by light, that is,
that their conductivity varies considerably with the greater or less
amount of light which is shed upon them. If, therefore, a circuit in
which a substance of this nature is inserted, is abruptly subjected to
a somewhat intense light, the increase of resistance which results from
it ought to produce a powerful sound in a telephone inserted in the
circuit. This fact has been verified by experiment, and Mr. Willoughby
Smith infers from it, as we have already suggested, that the effects
produced in the microphone are due to variations of resistance in the
circuit, which are produced by more or less close contacts between
imperfect conductors.
In order to obtain this effect under the most favourable conditions,
Mr. Siemens employs two electrodes, consisting of network of very
fine platinum wire, fitting into each other like two forks, of which
the prongs are interlaced. These electrodes are inserted between two
glass plates, and a drop of selenium, dropped in the centre of the two
pieces of network, connects them on a circular surface large enough
to establish sufficient conductivity in the circuit. It is on this
flattened drop that the ray of light must be projected.
APPLICATIONS OF THE MICROPHONE.
The applications of the microphone increase in number every day, and
in addition to those of which we have just spoken, there are others of
really scientific and even of practical interest. Among the number is
the use which can be made of it as a system of relays for telegraphy,
in science for the study of vibrations imperceptible to our senses, in
medicine and surgery, and even in manufactures.
_Its application to Scientific Research._--We have seen that several
physicists, including Messrs. Spottiswoode, Warwick, Rossetti,
Canestrelli, Wiesendanger, Lloyd, Millar, Buchin, and Blyth, have been
able to hear what is said in a telephone which has no iron diaphragm,
but it was so difficult to establish the fact that it has been often
disputed. More certain evidence was desirable, and the microphone is an
opportune agent for affording it.
The ‘Telegraphic Journal’ of September 1, 1878, observes that M.
du Moncel, in order to claim the victory in his controversy with
Colonel Navez, had still to show that the sounds which appeared
to be inarticulate in telephones without a diaphragm might become
intelligible if they were intensified. This has been done for him by
the use of Mr. Hughes’s microphone, and the following experiments were
made for the purpose.
1. If a magnetising coil, surrounding a bar of soft iron, is inserted
in the circuit of a microphone, with a battery of three cells, the
ticking of a watch and other sounds of the same kind may be heard
on approaching the ear to the electro-magnet which has been thus
constituted. It is true that these sounds are very faint when they are
not amplified, but if the electro-magnet is fastened to a board, and
a second microphone is fixed to the same board, the sounds produced by
the electro-magnet are magnified, and become distinctly audible in the
telephone which is placed in connection with this second microphone.
2. These sounds may be further amplified by resting one of the
extremities of the core of the electro-magnet on one of the poles of
a permanent magnet, which is fixed upon the board. Articulate speech
may then be heard in the telephone which is placed in connection with
the microphone resting on the board, and the point at issue between
MM. Navez and Du Moncel is completely decided in this way: for the
auxiliary microphone can only propagate and amplify the vibration of
articulate sounds, which are communicated by the bar magnet of the coil
to the board on which the two instruments are placed. In this way it
would be possible to render articulate sounds perceptible to M. Navez,
when transmitted by the bar magnet of a telephone without a diaphragm.
3. When a second bar magnet rests on the free pole of the
electro-magnet, so as to present to it a pole of the same nature as the
one with which it is already in communication--in a word, if a bar is
placed between the two poles of a horseshoe electro-magnet, the effects
are still more marked, and hence it may be assumed that the bar reacts
as an armature, by concentrating the lines of magnetic force in the
vicinity of the helix.
4. When the two poles of a horseshoe magnet are inserted together
inside a coil, their effects are equally energetic, although by this
arrangement one of the poles might be expected to neutralise the
effect of the other: but the most important effects have been obtained
by placing an armature of soft iron across the poles of the magnet
which has been already inserted in the coil. Under these conditions
articulate sounds are distinctly heard.
These experiments were confirmed by Mr. F. Varley, in a letter
published in the ‘Telegraphic Journal’ of September 15, 1878, and among
the fresh experiments mentioned by him, we will quote those which he
made with an iron tube inserted in a helix, in which the two opposite
poles of two bar magnets are introduced. These poles are only separated
from each other by the interval of an inch, so that the centre of the
iron tube may be strongly magnetised.
Mr. Varley says that this last arrangement reproduces the articulate
sounds which issue from a sending microphone, and this experiment is
more decisive than that of Professor Hughes, in which case it might
be supposed that the bar magnet, resting on the polar end of an
electro-magnetic bar, was only a modification of the disk in the Bell
telephone, set in vibration by the alternate currents passing through
the helix, and that these vibrations were communicated to the board,
and became sensible when enlarged by the microphone. But such an
objection cannot be alleged in the case of the arrangement described
above, for since the sound is produced between the current passing
into the helix and the magnetic current of the bar, it can only be
the result of a vibration produced by a disturbance of the reciprocal
relations subsisting between these two elements. Mr. Varley adds
that these experiments confirm M. du Moncel’s researches, which have
thrown considerable light upon the causes which are at work in the
action of the speaking telephone, and with which we have hitherto been
imperfectly acquainted.
_Its application to Telephonic Relays._--In February 1878, I first
began to consider the mode of forming telephonic relays, but I was
checked by the discovery that there was no vibration in the receiving
telephone, and I made the following communication on the subject to
the Académie des Sciences on February 25:--‘If the vibrations of the
disk in the receiving telephone were the same as those of the sending
telephone, it is easy to see that if a telephone with a local battery,
acting both as sender and receiver, were substituted for the receiving
telephone, it might, by the intervention of the induction coil, act
as a relay, and might therefore not only amplify the sound, but also
transmit it to any distance. It is, however, doubtful whether the
vibrations of the two corresponding disks are of the same nature, and
if the sound be due to molecular contractions and expansions, the
solution of the problem becomes much more difficult. Here is therefore
a field for experiments.’ These experiments have been successfully made
by Mr. Hughes, who acquainted me with them early in June 1878, and they
led to the discovery of a most interesting system of microphonic relays.
On a wooden board of moderate size, such as a drawing board, he placed
a microphone with a carbon brought to a fine point at each end, and
fixed in a vertical position. One or more telephones were placed in the
circuit, with their membranes facing the board, and a continuous sound
was heard, sometimes resembling a musical note, sometimes the singing
of boiling water in an oven; and the sound, which could be heard at
a distance, went on indefinitely, as long as the electric force was
exerted. Mr. Hughes explains this phenomenon in the following way.