easily understood, if we consider that the variations of resistance
in the circuit, resulting from the greater or less range in the
vibrations of the transmitting plate, are of constant value, and can
only manifest their effects distinctly in short circuits; consequently
the articulate sounds which result from them can only be really
appreciable in circuits of great resistance. According to Mr. Warren
de la Rue’s experiments (reported in the ‘Telegraphic Journal,’ March
1, 1878), the currents produced by the vibrations of the voice in an
ordinary telephone represent in intensity those of a Daniell cell
traversing 100 megohms of resistance (or 10,000,000 kilomètres); and
it is plain that the simple question of greater or less intensity in
the currents acting on the receiving telephone is not the only thing we
have to consider. With an energetic battery, it is evident, in fact,
that the differential currents will always be more intense than the
induced currents produced by the action of the instrument. I myself
am inclined to believe that induced currents owe the advantages they
possess to the succession of inverse currents and their brief duration.
These currents, of which M. Blaserna considers that the duration does
not exceed 1/200 of a second, are much more susceptible than voltaic
currents of the multiplied vibrations which are characteristic of
phonetic vibrations, and especially since the succession of inverse
currents which take place discharge the line, reverse the magnetic
effects, and contribute to make the action more distinct and rapid.
We cannot therefore be surprised that the induced currents of the
induction coil, which can be produced under excellent conditions at the
sending station, since the circuit of the voltaic current is then very
short, are able to furnish results, not only more effective than the
voltaic currents from which they take their origin, but even than the
induced currents resulting from the action of the Bell telephone, since
they are infinitely more energetic.
As for the effects produced by the currents of Bell telephones, which
are relatively great when we consider their size, they are easily
explained from the fact that they are produced under the influence of
the vibrations of the telephone plate, so that their variations of
intensity always maintain the same proportion, whatever may be the
resistance of the circuit, and consequently they are not effaced by the
distance which divides the two telephones.
_Experiments on the part taken by the different telephonic organs
in the transmission of speech._--In order to introduce all the
improvements of which a telephone is capable, it is important to be
quite decided as to the effects produced in the several parts of which
it is composed, and as to the part taken by the several organs which
are at work. To attain this object several men of science and engineers
have undertaken a series of experiments which have produced very
interesting results.
One of the points on which it was most important to throw light
was that of ascertaining whether the vibrating plate used in their
telephone receivers by Messrs. Bell and Gray is the only cause of the
complex vibrations which reproduce speech, or if the different parts
of the electro-magnetic system of the instrument all conduce to this
effect. The experiments made by Mr. Page in 1837 on the sounds produced
by the resonant electro-magnetic rods, and the researches pursued in
1846 by Messrs. de la Rive, Wertheim, Matteucci, &c., on this curious
phenomenon, allow us to state the question, which is certainly more
complex than it at first appears.
In order to start from a fixed point, it must first be ascertained
whether a telephone can transmit speech without a vibrating plate.
Experiments made by Mr. Edison[9] in November 1877, with telephones
provided with copper diaphragms, which produced sounds, make the
hypothesis credible; and it received greater weight from the
experiments made by Mr. Preece and Mr. Blyth. The fact was placed
beyond a doubt by Mr. Spottiswoode (see the ‘Telegraphic Journal’
of March 1, 1878), who assures us that the vibrating plate of
the telephone may be entirely suppressed without preventing the
transmission of speech, provided that the polar extremity of the
magnet be placed quite close to the ear; and it was after this that
I presented to the Académie des Sciences my paper on the theory of
the telephone, which led to an interesting discussion of which I
shall speak presently. At first the authenticity of these results was
denied, and then an attempt was made to explain the sounds heard by Mr.
Spottiswoode as a mechanical transmission of the vibrations, effected
after the manner of string telephones; but the numerous experiments
which have subsequently been made by Messrs. Warwick, Rossetti, Hughes,
Millar, Lloyd, Buchin, Canestrelli, Wiesendanger, Varley, and many
others, show that this is not the case, and that a telephone without a
diaphragm can transmit speech electrically.
Colonel Navez himself, who had first denied the fact, now admits that a
telephone without a diaphragm can emit sounds, and even, under certain
exceptional conditions, can reproduce the human voice; but he still
believes that it is impossible to distinguish articulate words.
This uncertainty as to the results obtained by the different physicists
who have studied the matter shows that at any rate the sounds thus
reproduced are not clearly defined, and that in physical phenomena,
only appreciable to our senses, the appreciation of an effect so
undefined must depend on the perfection of our organs. We shall
presently see that this very slight effect can be largely increased
by the arrangement adopted by Messrs. Bell and Gray, and we shall
also see that, by a certain mode of magnifying the vibrations, it
has been decisively proved that a telephone without a diaphragm can
readily reproduce speech. I proceed to give the description of such a
telephone, which was shown by Mr. Millar at the meeting of the British
Association at Dublin in August 1878.
This instrument consists of a small bar magnet, three inches in length
and 5/16 of an inch in width and thickness, and a copper helix (No. 30)
of about six mètres in length is wound round the bar. It is fixed in a
box of rather thick pasteboard, fitted above and below with two zinc
plates, which render it very portable. With a telephonic battery sender
and a single Leclanché cell, speech can be perfectly transmitted; the
whistling of an air, a song, and even the act of respiration become
audible. It seems also that the instrument can act without a magnet,
merely with a piece of iron surrounded by the helix; but the sounds are
then much fainter.
Signor Ignace Canestrelli obtained the same results by making one of
the carbon telephonic senders react on a telephone without a diaphragm,
by means of an induction coil influenced by two Bunsen cells. He writes
as follows on the subject:
‘With this arrangement I was able to hear the sound of any musical
instrument on a telephone without a diaphragm: singing, speaking, and
whistling were perfectly audible. Whistling could be heard, even when
the telephone without a diaphragm was placed at some distance from
the ear. In some cases, depending on the pitch of the voice, on the
distance of the sending station, and on the joint pressure exerted by
the carbons, I could even distinguish words.
‘I finally discharged the currents of the transmitter into the coils
of insulated copper wire with which the two poles of a magnet were
provided. This magnet was placed on a musical box, made of very thin
slips of wood, and on placing the ear at the opening of the box I
obtained the same results as with the ordinary telephones without a
diaphragm.’
M. Buchin, after repeating experiments of the same kind as the above,
intimates that it is easy to hear the sounds produced by a telephone
without a diaphragm, by introducing into the ear the end of an iron
rod, of which the other end is applied to the active pole of the bar
magnet of the telephone. (See ‘Le Journal d’Electricité,’ October 5,
1878.)
I repeat finally the account of some experiments made by Mr. Hughes and
M. Paul Roy which are interesting from our present point of view.
1. If an armature of soft iron is applied to the poles of an
electro-magnet, with its two branches firmly fixed on a board, and if
pieces of paper are inserted between this armature and the magnetic
poles, so as to obviate the effects of condensed magnetism; if,
finally, this electro-magnet is connected with a speaking microphone,
of the form given in fig. 39, it is possible to hear the words spoken
in the microphone on the board which supports the electro-magnet.
2. If two electro-magnets are placed in communication with a
microphone, with their poles of contrary signs opposite to each other,
and if their poles are separated by pieces of paper, speech will be
distinctly reproduced, without employing armature or diaphragm. These
experiments are, however, delicate, and demand a practised ear.
3. If, instead of causing the current produced by a microphone to pass
through the helix of a receiving telephone, it is sent directly into
the bar magnet of this telephone in the direction of its axis--that
is, from one pole to another--the words pronounced in the microphone
may be distinctly heard. This experiment by M. Paul Roy indicates,
if it is exact, that the electric pulsations which traverse a magnet
longitudinally will modify its magnetic intensity. The experiment,
however, demands verification.
Another point was obscure. It was important to know whether the
diaphragm of a telephone really vibrates, or at least if its vibrations
could involve its displacement, such as occurs in an electric vibrator,
or in wind instruments which vibrate with a current of air. M.
Antoine Bréguet has made some interesting experiments on the subject,
which show that such a movement cannot take place, since speech was
reproduced with great distinctness from telephones with vibrating
plates of various degrees of thickness, and he carried the experiment
so far as to employ plates fifteen centimètres in thickness.[10]
When pieces of wood, caoutchouc, and other substances were laid upon
these thick plates, the results were the same. In this case it cannot
be supposed that the plates were moved to and fro. I have moreover
ascertained, by placing a layer of water or of mercury on these plates,
and even on thin diaphragms, that no sensible movement took place, at
least when the induced currents produced by the action of speaking
were used as the electric source. No ripples could be seen on the
surface of the liquid, even when luminous reflectors were employed
to detect them. And indeed it can hardly be admitted that a current
not more intense than that of a Daniell element, which has traversed
10,000,000 kilomètres of telegraphic wire--a current which can only
show deviation on a Thomson galvanometer--should be powerful enough to
make an iron plate as tightly stretched as that of a telephone vibrate
by attraction, even if we grant that the current was produced by laying
a finger on the diaphragm.
Very nice photographic experiments do, however, show that vibrations
are produced on the diaphragm of the receiving telephone; they are
indeed excessively slight, but Mr. Blake asserts that they are enough
to cause a very light index, resting on the diaphragm, to make slight
inflections on a line which it describes on a register. Yet this small
vibration of the diaphragm does not show that it is due to the effect
of attraction, for it may result from the act of magnetisation itself
in the centre of the diaphragm.[11] An interesting experiment by Mr.
Hughes, repeated under different conditions by Mr. Millar, confirms
this opinion.
If the magnet of a receiving telephone consists of two magnetised bars,
perfectly equal, separated from each other by a magnetic insulator, and
they are so placed in the coil as to bring alternately the poles of
the same and of contrary signs opposite to the diaphragm, it is known
that the telephone will reproduce speech better in the latter case than
in the former. Now, if the effects were due to attraction, this would
not be the case; for the actions are in disagreement when the poles of
contrary signs are subjected to the same electric influence, while they
are in agreement when these poles are of like signs.
On the other hand, it is known that if several iron plates are
put together in order to form the diaphragm of the receiver, the
transmission of sounds is much stronger than with a simple diaphragm;
and yet the attraction, if it has anything to do with it, could only be
exerted on one of the diaphragms.
It further appears that it is not merely the magnetic core which emits
sounds, but that they are also produced with some distinctness by the
helices. Signor Rossetti had already ascertained this fact, and had
even remarked that they could be animated by a slight oscillatory
movement along the bar magnet, when they were not fixed upon it.
Several observers, among others M. Paul Roy, Herr Wiesendanger, and
Signor Canestrelli, have since mentioned similar facts, which are
really interesting.
‘If,’ writes M. Paul Roy, ‘a coil of fine wire, which is at the
extremity of the bar magnet of a Bell telephone, receives the pulsatory
currents transmitted by a carbon telephone, it is only necessary to
bring the coil close to the ear in order to hear the sounds.
‘The sounds received in this way are very faint, but become much
stronger if a piece of iron is introduced into the circuit coil. A
magnet acts with still greater force, even when it consists of a simple
magnetised needle. Finally, the sound assumes its maximum intensity
when an iron disk is inserted between the ear and the coil.
‘By placing the end of the coil to the ear, and sending a current
through it from the bar magnet, it is ascertained that the sound is
at its minimum when the neutral line of the magnet is enclosed by the
coil, and that it increases until attaining its maximum, when the
magnet is moved until one of its poles corresponds to the coil.
‘This fact of the reproduction of sounds by a helix is universal. Every
induction coil and every electro-magnet are capable of reproducing
sound when the currents of the sender are of sufficient intensity.’
Signor Canestrelli writes as follows: ‘With the combination of a
carbon telephone and one without diaphragm or magnet--that is, with
only a simple coil--I was able to hear whistling through the coil,
placed close to the ear. This coil was of very fine copper wire, and
the currents were produced through an induction coil by two Bunsen
elements. The contacts of the telephone were in carbon, and it was
inserted in the primary circuit.
‘I fastened the coil to the middle of a tightly stretched membrane
which served as the base of a short metal cylinder. When a magnet was
placed near this part of the coil, the sounds were intensified, and
when I fixed the magnet in this position, I could hear what was said.
‘I afterwards substituted for the magnet a second coil, fastened to
a wooden bar, and on causing the induced currents to pass into both
coils at once I was able to hear articulate speech, although not
without difficulty.
‘Under these latter conditions I found it possible to construct a
telephone without a magnet, but it required a strong current, and it
was necessary to speak into the sender in a special manner, so as to
produce strong and concentrated sounds.’
Another very interesting experiment by M. A. Bréguet shows that all
the constituent parts of the telephone--the handle, the copper rims,
and the case, as well as the diaphragm and the electro-magnet--can
transmit sounds. M. Bréguet ascertained this fact by the use of string
telephones, which he attached to different parts of the telephone on
which the experiment was made. In this way he was not only able to
establish a correspondence between the person who worked the electric
telephone and the one who was listening through the string telephone,
but he also made several string telephones act, which were attached to
different parts of the electric telephone.
These two series of experiments show that sounds may be obtained
from different parts of the telephone without any very appreciable
vibratory movements. But Signor Luvini wished for a further assurance
of the fact, by ascertaining whether the magnetisation of any magnetic
substance, followed by its demagnetisation, would involve a variation
in the form and dimensions of this substance. He consequently caused
a large tubular electro-magnet to be made, which he filled with a
quantity of water, so that, when its two ends were corked, the liquid
should rise in a capillary tube fitted to one of the corks. In this
way the slightest variations in the capacity of the hollow part of
the electro-magnet were revealed by the ascent or descent of the
liquid column. He next sent an electric current of varying intensity
through the electro-magnet, but he was never able to detect any change
in the level of the water in the tube; although by this arrangement
he could measure a change of volume of 1/30 of a cubic millimètre.
It appears from this experiment that the vibrations produced in a
magnetic substance under the influence of successive magnetisations and
demagnetisations, are wholly molecular. Yet other experiments made by
M. Canestrelli seem to show that these vibrations are so far sensible
as to produce sounds which can be detected by the microphone. He writes
as follows on the subject:
‘When the broken currents of an induction coil are discharged into
a coil placed on a sounding-box, it is possible to hear at a little
distance the sounds produced by the induced currents thus generated.
On approaching the magnet to the opening of the coil, these sounds are
intensified, and the vibrations of the magnet become sensible to the
touch; this vibration might even be made visible by suspending the
magnet inserted into the coil to a metallic wire, which is fitted to
a membrane stretched on a drum, and the latter will then reproduce
sounds. When the same magnet is suspended to a microphone, it is
possible, with the aid of a telephone, to ascertain the same effects,
which are then increased.’
We shall presently consider how these different deductions are to
be interpreted, so as to render the true theory of the telephone
intelligible; but, before doing so, we will mention some other
experiments which are not without interest.
We have seen that the experiments of Messrs. Edison, Blyth, and Preece,
show that sounds may be reproduced by a telephone with a diaphragm
made of some unmagnetic substance, and they also show, which is still
more curious, that these sounds may be transmitted under the influence
of induced currents produced by these diaphragms when they are placed
in vibration before the magnet. Messrs. Edison and Blyth had already
adduced this fact, which was received with incredulity, but it has
been confirmed by Mr. Warwick in an article published in the ‘English
Mechanic.’ He writes that in order to act upon the magnet, so as to
produce induced currents, something possessed of greater energy than
gas must first be made to vibrate. It is not, however, necessary that
this substance should be magnetic, for diamagnetic substances act
perfectly.[12] Mr. Preece sought for the cause in the induced currents
developed in any conducting body when a magnet is moved before it,
currents which give rise to the phenomenon discovered by Arago and
known by the name of magnetism by rotation. Yet these facts do not
appear to us to be sufficiently well established to make the theory
worthy of serious consideration, and it is possible that the effects
observed resulted from simple mechanical transmissions.
To conclude the account of these experiments, we will add that
Mr. W. F. Barrett thinks it somewhat difficult to define the mode
of vibration of the diaphragm, since, while a certain amount of
compression exerted on the iron destroys the sounds resulting from the
peculiar effects of magnetisation, a still stronger compression causes
them to reappear. It is certain that the question is full of obscurity,
and demands great research: it is enough to have shown that the theory
hitherto held is insufficient.
On the other hand, Colonel Navez considers that the intensity of sound
reproduced in a telephone depends not only on the range of vibrations,
but also on the vibrating surface and the effect it produces on the
stratum of air which transmits the sound. (See paper by Colonel Navez
in the ‘Bulletin de l’Académie de Belgique,’ July 7, 1878.)
_Experiments on the Effects which result from Mechanical Shocks
communicated to different parts of a Telephone._--If a piece of iron is
applied to the screw which holds the magnet of the ordinary telephone,
it is observed that the transmitted sounds are more distinct, owing to
the force supplied to the active pole of the magnet; but at the moment
when the piece of iron is applied to the screw a distinct noise is
heard, which seems to be due to the mechanical vibrations caused in
the magnet at the moment of the shock. M. des Portes, a lieutenant in
the French navy, has lately made some interesting experiments on this
class of phenomena. He has observed that if, in a telephonic circuit
of 90 yards completed by the earth, the sending telephone is reduced
to a simple magnet, provided with the coil which constitutes its
electro-magnetic organ, and if this magnet is suspended vertically by a
silken thread, with the coil above it, a blow struck upon the magnet,
either by a copper rod or a piece of wood, will cause distinct sounds
to be produced in the receiving telephone--sounds which will increase
in intensity when the blow is struck close to the coil, and which will
become still stronger, but less clear, if a vibrating plate of soft
iron is placed in contact with the upper pole of the magnet.
When the striking instrument is made of iron, the sounds in question
are more strongly marked than if it is of wood, and when the magnet has
a vibrating disk applied to its active pole, a vibration of the disk
takes place at the moment when the shock is heard.
If the striking body is a magnet, the sounds produced resemble those
obtained when it is of iron, if the effect is produced between poles
of the same nature; but if the poles are of contrary natures, a second
noise is heard after each blow, which is produced by drawing away the
magnet, and which appears to be a blow struck with much less force.
The sound is of course increased if the magnet is provided with its
vibrating disk.
If words are uttered on the vibrating disk of the sending telephone,
when it is applied to the pole of the magnet, various sounds are
heard on the receiving telephone, somewhat similar to those produced
by vibrating one of the strings of a violin, and the sound made in
withdrawing the disk from contact with the magnet is distinctly heard
in the receiver.
The person who applies his ear to the vibrating disk of the sender when
it is arranged as above, may hear the voice of anyone who speaks into
the receiver, but cannot distinguish the words, owing, no doubt, to the
condensed magnetism at the point of contact between the magnet and the
vibrating disk, which slackens the magnetic variations and makes it
more difficult for them to take place.
A coil is not necessary in order to perceive the blows struck upon
the magnet with a rod of soft iron. It is enough to wind three turns
of naked conducting wire, which acts as line wire, round one end of
the magnet, and the sounds perceived cease, as in other experiments,
when the circuit is broken, plainly showing that they are not due to
mechanical transmission. It is a still more curious fact that if the
magnet is placed in the circuit, so as to form an integral part of it,
and if the two ends of the conducting wire are wound round the ends of
the magnet, the blows struck upon the latter with the soft iron rod
are perceived in the telephone as soon as one pole of the magnet is
provided with a vibrating disk.