on the screw which, in ordinary telephones, fastens the magnet to
the instrument, and I have ascertained that, whenever the circuit was
complete, the blows struck with an ivory knife were repeated by the
telephone: they were, it is true, very faint when the vibrating disk
was removed, but very marked when the disk was in its place. On the
other hand, no sound was perceived when the circuit was broken. These
sounds were louder when the blows were struck upon the screw than when
they were struck on the pole of the magnet itself above the coil: for
this reason, that in the first case the magnet could vibrate freely,
while in the second the vibrations were stifled by the fixed position
of the bar magnet.
These effects may be to some extent explained by saying that the
vibrations caused in the magnet by the shock produce undulatory
displacements of the magnetised particles in the whole length of the
bar, and that induced currents would, according to Lenz’s law, result
in the helix from these displacements--currents of which the force
would increase when the power of the magnet was further excited by the
reaction of the diaphragm, which acts as an armature, and also by that
of the striking instrument when it also is magnetic. Yet it is more
difficult to explain M. des Portes’ later experiments, and the effect
may be produced by something more than the ordinary induced currents.
These are not the only experiments which show the effects produced
under the influence of molecular disturbance of various kinds. Mr.
Thompson, of Bristol, has observed that if a piece of iron and a tin
rod placed perpendicularly on the iron are introduced into the circuit
of an ordinary telephone, it is enough to strike the tin rod in order
to produce a loud sound in the telephone. He has also shown that if
the two ends of a bar magnet are enclosed by two induction coils which
are placed in connection with the circuit of a telephone, and if the
flame of a spirit lamp is moved below the magnet in the space dividing
the two coils, a distinct sound is heard as soon as the flame exerts
its influence on the bar magnet. This effect is undoubtedly due to the
weakening of the magnetic force of the bar which is produced by the
action of heat. I have myself observed that a scratching sound on one
of the wires which connect the telephones is heard in both of them, at
whatever point in the circuit the scratch is made. The sounds produced
are indeed very faint, but they can be distinctly heard, and they
become more intense when the scratch is made on the binding-screws of
the telephone wires. These sounds cannot result from the mechanical
transmission of vibrations, since they are imperceptible when the
circuit is broken. From these experiments it appears that some sounds
which have been observed in telephones tried on telegraph stations may
arise from the friction of the wires on their supports--a friction
which produces those very intense sounds which are sometimes heard on
telegraphic wires.
_Theory of the Telephone._--It appears from the several experiments
of which we have spoken that the explanation generally given of the
effects produced in the telephone is very imperfect, and that the
transmission of speech, instead of resulting from the repetition by the
membrane of the receiving telephone (influenced by electro-magnetism)
of vibrations caused by the voice on the membrane of the transmitting
membrane, is due to molecular vibrations produced in the whole
electro-magnetic system, and especially on the magnetic core contained
in the helix. These vibrations must be of the same nature as those
which have been observed in resonant electro-magnetic rods by MM. Page,
de la Rive, Wertheim, Matteucci, &c., and these have been employed in
telephones by Reiss, by Cecil and Leonard Wray, and by Vanderweyde.
According to this hypothesis, the principal office of the vibrating
plate consists in its reaction, in order to produce the induced
currents when the voice has placed it in vibration, and by this
reaction on the polar extremity of the bar magnet it strengthens the
magnetic effects caused in the centre of the bar when it vibrates under
the electro-magnetic influence, or at least when it is affected by the
magnet. Since the range of these vibrations for a single note is great
in proportion to the flexibility of the note, and since, on the other
hand, the variations in the magnetic condition of the plate are rapid
in proportion to the smallness of its mass, the advantage of employing,
as Mr. Edison has done, very thin and relatively small plates is
readily understood. In the case of transmission, the wider range of
vibration increases the intensity of the induced currents transmitted.
In the case of reception the variations in the magnetising force which
produces the sounds are rendered clearer and more distinct, both in
the armature membrane and in the bar magnet: something is gained,
therefore, in each case. This hypothesis by no means excludes the
phonetic effects of the mechanical and physical vibrations which may
be produced in the armature plate under the influence of magnetisation
and demagnetisation to which it is subjected, and these join their
influence to that of the magnetic core.
What is the nature of the vibrations sent into the receiving telephone?
This question is still obscure, and those who have studied it are far
from being in agreement: as early as 1846 it was the subject of an
interesting discussion between MM. Wertheim and de la Rive, and the new
discoveries render it still more complex. M. Wertheim considers that
these vibrations are at once longitudinal and transverse, and arise
from attractions exchanged between the spirals of the magnetising helix
and the magnetic particles of the core. M. de la Rive holds that in
the case we are considering the vibrations are simply longitudinal,
and result from molecular contractions and expansions produced by the
different combinations assumed by the magnetic molecules under the
influence of magnetisation and demagnetisation. This appears to us to
be the most natural explanation, and it seems to be confirmed by the
experiment made by M. Guillemin in 1846. M. Guillemin ascertained that
if a flexible iron rod, surrounded by a magnetising helix, is kept in
position by a vice at one end, and bent back by a weight at the other,
it can be made to return instantly to its normal position by sending a
current through the magnetising helix. This recovery can in such a case
be due to nothing but the contraction caused by the magnetic molecules,
which, under the influence of their magnetisation, tend to produce
intermolecular attractions, and to modify the elastic conditions of the
metal. It is known that when iron is thus magnetised it becomes as hard
as steel, and a file makes no impression on its surface.
It is at any rate impossible to dispute that sounds are produced in
the magnetic core, as well as in the armature, under the influence
of intermittent electric action. These sounds may be musical or
articulate; for as soon as the sender has produced the electric action
required, there is no reason why vibrations which are effected in a
transverse or longitudinal direction should transmit the one more than
the other. These vibrations may, as we have seen, be termed microscopic.
Signor Luvini, who shares our opinion of the foregoing theory, does
not, however, think it wholly satisfactory, unless account is taken
of the reaction caused by the bar magnet on the helix which surrounds
it. ‘There cannot,’ he says, ‘be _action_ without _reaction_, and
consequently the molecular action produced in the magnet ought to cause
corresponding variations in the helix, and these two effects ought to
contribute to the production of sounds.’ He supports this remark by a
reference to Professor Rossetti’s experiment, of which we have spoken
above.
We believe, however, that this double reaction of which Signor Luvini
speaks is not indispensable, for we have seen that insulated helices
can produce sounds; it is true that the spirals, reacting on each
other, may be the cause of this.
The difficulty of explaining the production of sounds in an
electro-magnetic organ destitute of armature caused the authenticity
of the experiments we have described to be at first denied, and
Colonel Navez started a controversy with us which is not likely to be
soon terminated; yet one result of this controversy is that Colonel
Navez was obliged to admit _that the sound of the human voice may be
reproduced by a telephonic receiver without a disk_. But he still
believes that this reproduction is so faint that it is not possible
to recognise articulate words, and he maintains that the transverse
vibrations of the disk, which are due to effects of attraction, are
the only ones to reproduce articulate speech with such intensity as to
be of any use.
It is certain that the articulation of speech requires a somewhat
intense vibration which cannot easily be produced in a telephone
without a diaphragm; for it must be remembered that in an instrument
so arranged, the magnetic effects are reduced in a considerable ratio,
which is that of the magnetic force developed in the magnet, multiplied
by itself, and that so faint an action as that effected in a telephone
becomes almost null when, in consequence of the suppression of the
armature, it is only represented by the square root of the force which
produced it. It is therefore possible that the sounds which are hardly
perceptible in a telephone without a diaphragm become audible when
the cause which provokes them is multiplied by itself, and when there
are in addition the vibrations produced in the heart of the armature
itself, influenced by the magnetisations and demagnetisations to which
it is subjected.
In order to show that the action of the diaphragm is less indispensable
than Colonel Navez seems to imagine, and that its vibrations are not
due to electro-magnetic attractions, it will be enough to refer to Mr.
Hughes’s experiments, which we have mentioned above. It is certain
that if this were the effect produced, we should hear better when the
two bar magnets present their poles of the same nature before the
diaphragm, than when they present the poles of contrary natures, since
the whole action would then converge in the same direction. Again, the
more marked effects obtained with multiple diaphragms in juxtaposition
completely exclude this hypothesis. It is, however, possible that
in electro-magnetic telephones the iron diaphragm, in virtue of the
rapid variations of its magnetic condition, may contribute to render
the sounds clearer and more distinct; it may react in the way the
tongue does; but we believe that the greater or less distinctness of
the articulate sounds must be chiefly due to the range of vibrations.
Thus Mr. Hughes has shown that the carbons of metallised wood employed
in his microphonic speakers were to be preferred to retort carbons
for the transmission of speech, for the very reason that they had
less conductivity, so that the differences of resistance which result
from differences of pressure are more marked, and consequently it is
easier to seize the different degrees of vocal sounds which constitute
articulate speech.
It must be clearly understood that what we have just said only applies
to the Bell telephone, that is, to a telephone in which the electric
currents have such a faint intensity that it cannot be supposed there
is any external attractive effect. When these currents are so energetic
as to produce such an effect, a transverse electro-magnetic vibration
certainly takes place, which is added to the molecular vibration, and
helps to increase the sounds produced. But it is no less true that this
transverse vibration by attraction or by movement of the diaphragm
is not necessary for the reproduction of sounds, whether musical or
articulate.
We are not now concerned with the discussion of magnetic effects;
there has been an advance in science since Colonel Navez started the
controversy, and we must ask how his theory of the movements of the
telephone diaphragm by attraction will explain the reproduction of
speech by a receiving microphone destitute of any electro-magnetic
organ, and I can assert that my experiments show that there can be no
mechanical transmission of vibrations, since no sound is heard when
the circuit is broken or deprived of its battery. Colonel Navez must
therefore accept the molecular vibrations. This certainly gives us a
new field for study; but it is because European men of science persist
in remaining bound by incomplete theories that we have allowed the
Americans who despise them to reap the glory of the great discoveries
by which we have lately been astonished.
The experiments quoted above show that sounds may be reproduced not
only by simple helices without an electro-magnetic organ, but also by
the plates of a condenser, in spite of the pressure exerted upon them;
and when we add to this the effects I have just pointed out, it may
be supposed that vibrations of sound must result from every reaction
between two bodies which has the effect of producing abruptly and at
close intervals modifications in the condition of their electric or
magnetic equilibrium. It is known that the presence of ponderable
matter is necessary for the production of electric effects, and it
is possible that the molecular vibrations of which I have spoken may
be the result of molecular movements, due to the variations of the
electric force which holds the molecules in a special condition of
reciprocal equilibrium.
In conclusion, the theory of the telephone and microphone, considered
as reproductive organs of speech, is still far from being perfectly
clear, and it would be imprudent to be too positive on questions of
such recent origin.
The theory of the electric transmission of sounds in electro-magnetic
telephones is somewhat complex. It has been seen that they can be
obtained from diaphragms of non-magnetic substance, and even from
simple mechanical vibrations produced by shocks. Are we to ascribe
them in the first case to the inductive reaction of the magnet on the
vibrating plate, and in the second case to the movements of magnetic
particles before the spirals of the helix? The matter is still very
obscure; yet it is conceivable that the modifications of the inducing
action of the magnet on the vibrating diaphragm may involve variations
in the magnetic intensity, just as we can admit an effect of the same
kind due to the approach and withdrawal of the magnetic particles of
the spirals of the helix; M. Trève, however, believes that there is in
the latter case a special action, which he has already had occasion
to study under other circumstances, and he sees in the current thus
caused the effect of the transformation of the mechanical labour
produced amidst the magnetic molecules. The question is complicated by
the fact that these effects are often produced by purely mechanical
transmissions.
There is another point to consider, on which Colonel Navez has made
some interesting remarks; that is, whether the effects in the receiver
are stronger with permanent than with temporary magnets. In the
first model of the telephone, exhibited by Mr. Bell at Philadelphia,
the receiver was, as I have said, made of a tubular electro-magnet,
furnished with a vibrating disk at its cylindrical pole; but this
arrangement was abandoned by Mr. Bell, with the object, as he states
in his paper, of rendering his instrument both a receiver and a
sender.[13] Yet Colonel Navez maintains that the magnet plays an
important part, and is even indispensable under the present conditions
of its form. ‘It is possible,’ he says, ‘under certain circumstances,
and by making the instrument in a special way, to make a Bell receiver
speak without a permanent magnet, yet with an instrument of the usual
construction the sound ceases when the magnet is withdrawn and replaced
by a cylinder of soft iron. In order to restore the voice of the
telephone, it is enough to approach the pole of a permanent magnet to
the cylinder of soft iron. It follows from these experiments that a
Bell telephone cannot act properly unless the disk is subjected to an
initial magnetic tension obtained by means of a permanent magnet. It is
easy to deduce this assertion from a consideration of the theory.’
The assertion may be true in the case of Bell telephones, which
are worked by extremely weak currents, but when these currents are
relatively strong, all electro-magnets will reproduce speech perfectly,
and we have seen that M. Ader made a telephone with the ordinary
electro-magnet which acted perfectly.
The action of the currents sent through the helix of a telephone can be
easily explained. Whatever may be the magnetic conditions of the bar,
the induced currents of different intensity which act upon it produce
modifications in its magnetic state, and hence the molecular vibrations
follow from contraction and expansion. These vibrations are likewise
produced in the armature under the influence of the magnetisations and
demagnetisations which are produced by the magnetic action of the core,
and they contribute to the vibrations of the core itself, while at the
same time the modifications in the magnetic condition of the system are
increased by the reaction of the two magnetic parts upon each other.
When the bar is made of soft iron, the induced currents act by creating
magnetisations of greater or less energy, followed by demagnetisations
which are the more prompt since inverse currents always succeed to
those which have been active, and this causes the alternations of
magnetisation and demagnetisation to be more distinct and rapid. When
the bar is magnetised, the action is differential, and may be exerted
in either direction, according as the induced currents corresponding to
the vibrations which are effected pass through the receiving coil in
the same or opposite direction as the magnetic current of the bar. If
these currents are in the same direction, the action is strengthening,
and the modifications are effected as if a magnetisation had taken
place. If these currents are of opposite direction, the inverse effect
is produced; but, whatever the effects may be, the molecular vibrations
maintain the same reciprocal relations and the same height in the scale
of musical sounds. If the question is considered from the mathematical
point of view, we find the presence of a constant, corresponding
with the intensity of the current, which does not exist in mechanical
vibrations, and which may possibly be the cause of the peculiar tone
of speech reproduced by the telephone, a tone which has been compared
to the voice of Punch. M. Dubois Raymond has published an interesting
paper on this theory, which appeared in ‘Les Mondes,’ February 21,
1878, but we do not reproduce it here, since his remarks are too
scientific for the readers for whom this work is intended. We will only
add that Mr. C. W. Cunningham asserts that the vibrations produced in
a telephone cannot be manifested under precisely the same conditions
as those which affect the tympanum of the ear, because the latter has
a peculiar funnel-shaped form, which excludes every fundamental note,
specially adapted to it, and this is not the case with the bars and
magnetic plates which possess fundamental notes capable of greatly
altering the half-tones of the voice. He considers the alteration
of the voice observed in the telephone must be ascribed to these
fundamental notes.
_M. Wiesendanger’s Thermophone._--M. Wiesendanger, in an article
inserted in the ‘English Mechanic and World of Science,’ September
13, 1878, ascribes the reproduction of speech in certain telephones
to vibratory movements resulting from molecular expansions and
contractions produced by variations of temperature, and these
variations would follow from the currents of varying intensity which
are transmitted through the telephonic circuits. He was conscious of
one objection to this theory, namely, that the movements of expansion
and contraction due to heat are slowly produced, and consequently are
not capable of substantial action, rapid enough to produce vibrations;
but he considers that molecular effects need not take place under the
same conditions as those which are displayed in the case of material
substances.
M. Wiesendanger believes that this hypothesis will explain the
reproduction of speech in the receiving microphones of Mr. Hughes,
and that it may even be applied to the theory of the electro-magnetic
telephone, if we consider that a magnetising helix, as well as a
magnetic core, round which an electric current circulates, is more
or less heated, according to the intensity of the current which
traverses it, especially when the wire of the helix and the core
are bad conductors of electricity and of magnetism. Pursuing this
idea, M. Wiesendanger has sought to construct telephones in which
calorific effects are more fully developed, and with this object he
used very fine wire of German silver and platinum to make the coils.
He ascertained that these coils could produce sounds themselves, and,
to increase their intensity, he put them between disks of iron, or on
tin tubes, placed on resonant surfaces close to the disks. In this way
he says that he was able to make a good receiving telephone without
employing magnets. He afterwards arranged the instrument in different
ways, of which the two following are the most noteworthy.
In the first, the electro-magnetic system was simply formed by a
magnetic disk with a helix wound round it, of which the wire was in
connection with the circuit of a microphone, and which was fastened to
the centre of the parchment membrane of an ordinary string telephone;
the disk consisted of two iron plates separated by a carbon disk of
smaller diameter, and the whole was so compressed as to form a solid
mass.
In the second, the helix was wound on a tin tube, six inches long and
five-eighths of an inch in diameter, which was soldered by merely a
point to the centre of the diaphragm of an ordinary telephone.
The inventor asserts that the tube and diaphragm only act as
resonators, and that the sounds produced by this instrument are nearly
the same as those obtained from the ordinary string telephone: the
tunes of a musical box were heard, and the reproduction of speech
was perfect, both in intensity and in distinctness of sound; it even
appeared that telephonic sounds were audible with the tin tube alone,
surrounded by the helix. M. Wiesendanger says that ‘these different
receiving telephones show clearly that the diaphragm and magnet are
not essential, but merely accessory, parts of a telephone.’
VARIOUS EXPERIMENTS MADE WITH THE TELEPHONE.
We must now consider a series of experiments which demonstrate the
wonderful properties of the telephone, and which may also give some
indication of the importance of the influences by which it is liable to
be affected.
_Experiments by M. d’Arsonval._--We have seen that the telephone is an
extremely sensitive instrument, but its sensitiveness could scarcely be
appreciated by ordinary means. In order to gauge it, M. d’Arsonval has
compared it to the nerve of a frog, which has hitherto been regarded
as the most perfect of all galvanoscopes, and it appears from his
experiments that the sensitiveness of the telephone is two hundred
times greater than that of the frog’s nerve. M. d’Arsonval has given
the following account of his researches in the records of the Académie
des Sciences, April 1, 1878:
‘I prepared a frog in Galvani’s manner. I took Siemens’ instrument of
induction, used in physiology under the name of the chariot instrument.
I excited with the ordinary pincers the sciatic nerve, and I withdrew
the induced coil until the nerve no longer responded to the electric
excitement. I then substituted the telephone for the nerve, and the
induced current, which had ceased to excite the latter, made the
instrument vibrate strongly. I withdrew the induced coil, and the
telephone continued to vibrate.