when the induced coil was at a distance fifteen times greater than the
minimum at which the excitement of the nerve took place; consequently,
if the same law of inverse squares applies to induction and to
distance, it is evident that the sensitiveness of the telephone is two
hundred times greater than that of the nerve.
‘The sensitiveness of the telephone is indeed exquisite. We see how
much it exceeds that of the galvanoscopic frog’s leg, and I have
thought of employing it as a galvanoscope. It is very difficult to
study the muscular and nervous currents with a galvanometer of 30,000
turns, because the instrument is deficient in instantaneous action,
and the needle, on account of its inertia, cannot display the rapid
succession of electric variations, such as are effected, for example,
in a muscle thrown into electric convulsion. The telephone is free
from this inconvenience, and it responds by vibration to each electric
change, however rapid it may be. The instrument is therefore well
adapted for the study of electric tetanus in the muscle. It is certain
that the muscular current will excite the telephone, since this current
excites the nerve, which is less sensitive than the telephone. But for
this purpose some special arrangement of the instrument is required.
‘It is true that the telephone can only reveal the variations of an
electric current, however faint they may be; but I have been able, by
the use of a very simple expedient, to reveal by its means the presence
of a continuous current, also of extreme faintness. I send the current
in question into the telephone, and, to obtain its variations, I
break this current mechanically with a tuning-fork. If no current is
traversing the telephone, it remains silent. If, on the other hand,
the faintest current exists, the telephone vibrates in unison with the
tuning-fork.’
Professor Eick, of Wurzburg, has also used the telephone for
physiological researches, but in a direction precisely opposite to
that explored by M. d’Arsonval. He ascertained that when the nerves of
a frog were placed in connection with a telephone, they were forcibly
contracted when anyone was speaking into the instrument, and the
force of the contractions chiefly depended on the words pronounced.
For instance, the vowels _a_, _e_, _i_ produced hardly any effect,
while _o_ and especially _u_ caused a very strong contraction. The
words _Liege still_, pronounced in a loud voice, only produced a faint
movement, while the word _Tucker_, even when spoken in a low voice,
strongly agitated the frog. These experiments, reminding us of those
by Galvani, were necessarily based on the effects produced by the
induced currents developed in the telephone, and they show that if this
instrument is a more sensitive galvanoscope than the nerve of a frog,
the latter is more susceptible than the most perfect galvanometer.
_Experiments by M. Demoget._--In order that he might compare the
intensity of the sounds transmitted by the telephone with the intensity
of original sounds, M. Demoget placed two telephones in an open
space. He held the first to his ear, while his assistant withdrew
to a distance, constantly repeating the same syllable with the same
intensity of tone in the second instrument. He first heard the sound
transmitted by the telephone, and then the sound which reached him
directly, so that comparison was easy, and he obtained the following
results.
At a distance of 93 yards the original and the transmitted sounds
were received with equal intensity, while the vibrating disk was
about 5 centimètres from the ear. At this moment, therefore, the
relative intensity was as 25 to 81,000,000. In other words, the sound
transmitted by the telephone was only 1/3000000 of the emitted sound.
‘But,’ said M. Demoget, ‘since the stations at which we worked could
not be regarded as two points freely vibrating in space, the ratio may
be reduced by one half on account of the influence of the earth, and
the sound transmitted by the telephone may be supposed to be 1,500,000
times weaker than that emitted by the voice.
‘Again, since we know that the intensity of the two sounds is in
proportion to the square of the range of vibrations, it may be
concluded that the vibrations of the two telephone disks were in
direct proportion to the distance, that is, as 5 to 9,000, or that the
vibrations of the sending telephone were eighteen hundred times greater
than those of the receiving telephone. These latter may therefore be
compared to molecular vibrations, since the range of those of the
sending telephone was extremely small.
‘Without in any degree detracting from the merit of Bell’s remarkable
invention,’ continues M. Demoget, ‘it follows from what I have said
above that the telephone, considered as a sending instrument, leaves
much to be desired, since it only transmits the 18/100 part of the
original power; and if it has produced such unexpected results, this
is due to the perfection of the organ of hearing, rather than to the
perfection of the instrument itself.’
M. Demoget considers this loss of power which takes place in the
telephone to be chiefly owing to the eight transformations in
succession to which sound is subjected before reaching the ear, setting
aside the loss due to the electric resistance of the line, which might
in itself suffice to absorb the whole force.
In order to estimate the force of the induced currents which act upon
a telephone, M. Demoget has attempted to compare them with currents of
which the intensity is known, and which produce vibrations of like
nature and force: for this purpose he has made use of two telephones,
A and B, communicating through a line 22 yards in length. He placed a
small file in slight contact with the vibrating disk of the telephone
A, and caused friction between the file and a metallic plate: the sound
thus produced was necessarily transmitted by the telephone B, with
an intensity which could be estimated. He then substituted a battery
for the telephone A, and the file was introduced into the circuit by
connecting it with one of the poles. The current could only be closed
by the friction of the file with the plate, which had a spring, and was
in communication with the other end of the circuit. In this way broken
currents were obtained, which caused vibration in the telephone B, and
produced a sound of which the intensity varied with the strength of
the battery current. In this way M. Demoget endeavoured to find the
electric intensity capable of producing a sound similar to that of the
telephone A, and he ascertained that it corresponded in intensity to
that produced in a small thermo-electric battery formed of an iron and
a copper wire, two millimètres in diameter, flattened at the end, and
soldered to the tin: the faint current produced by this battery only
caused a short wire galvanometer to deviate two degrees.
This estimate does not appear to us to unite so many conditions of
accuracy as to enable us to deduce from it the degree of sensitiveness
possessed by a telephone, a sensitiveness which the experiments of
Messrs. Warren de la Rue, Brough, and Peirce show to be much greater.
Mr. Warren de la Rue, as we have seen, used Thomson’s galvanometer,
and compared the deviation produced on the scale of this galvanometer
with that caused by a Daniell cell traversing a circle completed by a
rheostat: he ascertained that the currents discharged by an ordinary
Bell telephone are equivalent to those of a Daniell cell traversing
100 megohms of resistance, that is, 6,200,000 miles of telegraphic
wire. Mr. Brough, the Director of Indian Telegraphs, considers that the
strongest current which at any given moment causes a Bell telephone
to work does not exceed 1/1000000 of the unit of current, that is,
one Weber, and the current transmitted to the stations on the Indian
telegraphic line is 400,000 times as strong. Finally, Professor
Peirce, of Boston, compares the effects of the telephonic current
with those which would be produced by an electric source of which the
electro-motive force should be 1/200000 part of a volt, or one Daniell
cell. Mr. Peirce justly remarks that it is difficult to estimate the
real value of these kinds of currents at any precise sum, since it
essentially varies according to the intensity of the sounds produced
on the transmitting telephone; but it may be affirmed that it is less
than the 1/1000000 part of the current usually employed to work the
instruments on telegraphic lines.
Signor Galileo Ferraris, who has recently published an interesting
treatise on this question in the ‘Atti della Reale Accademia delle
Scienze di Torino’ (June 13, 1878), states that the intensity of the
currents produced by the ordinary Bell telephone varies with the pitch
of the sound emitted.
_Experiments by M. Hellesen, of Copenhagen._--In order to estimate the
reciprocal effects of different parts of a telephone, M. Hellesen has
made telephones of the same size with three different arrangements
which act inversely to each other. The first was of the ordinary form,
the second like that of Bell’s first system, that is, with a membrane
supporting a small iron armature on its centre, instead of a vibrating
disk, and the third telephone consisted of a hollow cylindrical magnet,
with the vibrating disk fixed to one of its poles, and the disk was
adapted to move before a flat, snail-shaped spiral, of which the number
of spirals equalled those of the two other helices. In this last
arrangement, the induced currents resulting from the vibrations of the
voice might be assimilated to those which follow from the approximation
and withdrawal of the two parallel spirals, one of which should be
traversed by a current. It is this last arrangement which Mr. Bell has
adopted as producing the best effects, and it is rare in the history of
discoveries that an inventor hits at once on the best arrangement of
his instrument.
_Experiments by M. Zetsche._--There are always a few perverse minds,
impelled by a spirit of contradiction to deny evidence, and thus they
attempt to depreciate a discovery of which the glory irritates them.
The telephone and the phonograph have been the objects of such unworthy
criticism. It has been said that electric action had nothing to do with
the effects produced in the telephone, and that it only acted under the
influence of mechanical vibrations transmitted by the conducting wire,
just as in a string telephone. It was in vain to demonstrate to these
obstinate minds that no sound is produced when the circuit is broken,
and in order to convince them M. Zetsche has made some experiments to
show, from the mode in which sound is propagated, that it is absurd
to ascribe the sound produced in a telephone to mechanical vibration.
He wrote to this effect in an article inserted in the ‘Journal
Télégraphique,’ Berne, January 25, 1878:
‘The correspondence by telephone between Leipzig and Dresden affords
another proof that the sounds which reproduce words at the receiving
station are due to electric currents, and not to mechanical vibrations.
The velocity with which sound is transmitted by vibrations on the wire,
in the case of longitudinal undulations, may be estimated at three
miles one furlong a second, so that the sound ought to traverse the
distance from Leipzig to Dresden in 25 seconds. The same time ought
to elapse before receiving the answer. Consequently there should be
an interval of more than three-quarters of a minute allowed for each
exchange of communication, which is by no means the case.’
_Experiments which may be made by anyone._--We will conclude this
chapter, devoted to the account of the different experiments made
with the telephone, by the mention of a singular experiment, which,
although easily performed, has only been suggested a few months ago by
a Pennsylvanian newspaper. It consists in the transmission of speech by
a telephone simply laid on some part of the human body adjacent to the
chest. It has been asserted that any part of the body will produce this
effect, but according to my experience, I could only succeed when the
telephone was firmly applied to my chest. Under such conditions, and
even through my clothes, I could make myself heard when speaking in a
very loud voice, from which it appears that the whole of the human body
takes part in the vibrations produced by the voice. In this case, the
vibrations are mechanically transmitted to the diaphragm of the sending
telephone, not by the air, but by the body itself acting on the outside
of the telephone.
THE MICROPHONE.
The microphone is in fact only the sender of a battery telephone,
but with such distinctive characteristics that it may be regarded
as an original invention which is entitled to a special name. The
invention has lately given rise to an unfortunate controversy between
its inventor, Mr. Hughes, and Mr. Edison, the inventor of the carbon
telephone and the phonograph--a controversy which has been embittered
by the newspapers, and for which there were no grounds. For although
the scientific principle of the microphone may appear to be the same
as that of Mr. Edison’s carbon sender, its arrangement is totally
different, its mode of action is not the same, and the effect required
of it is of quite another kind. Less than this is needed to constitute
a new invention. Besides, a thorough examination of the very principle
of the instrument must make us wonder at Mr. Edison’s claim to
priority. He cannot in fact regard as his exclusive possession the
discovery of the property possessed by some substances of moderate
conductivity of having this power modified by pressure. In 1856, and
often subsequently, as for example in 1864, 1872, 1874, and 1875, I
made numerous experiments on this point, which are described in the
first volume of the second edition of my ‘Exposé des applications de
l’Electricité,’ and also in several papers presented to the Académie
des Sciences, and inserted in their _Comptes rendus_. M. Clarac again,
in 1865, employed a tube made of plumbago, and provided with a moveable
electrode, to produce variable resistances in a telegraphic circuit.
Besides, in Mr. Edison’s telephonic sender, the carbon disk, as we
have seen, must be subjected to a certain initial pressure, in order
that the current may not be broken by the vibrations of the plate on
which it rests, and consequently the modifications of resistance in
the circuit which produce articulate sounds are only caused by greater
or less increase and diminution of pressure, that is, by differential
actions. We shall presently see that this is not the case with the
microphone. In the first place, the carbon contact is effected in
the latter instrument on other carbons and not with platinum disks,
and these contacts are multiple. In the second place, the pressure
exerted on all the points of contact is excessively slight, so that the
resistances can be varied in an infinitely greater ratio than in Mr.
Edison’s system; and for this very reason it is possible to magnify
the sounds. In the third place, a microphone can be made of other
substances besides carbon. Finally, no vibrating disk is needed to make
the microphone act; the simple medium of air is enough, so that it is
possible to work the instrument from some little distance.
We do not therefore see the grounds for Mr. Edison’s assertions,
and especially for the way in which he has spoken of Messrs. Hughes
and Preece, who are well known in science and are in all respects
honourable men. I repeat my regret that Mr. Edison should have made
this ill-judged attack on them, since it must injure himself, and
is unworthy of an inventor of such distinction. If we look at the
question from another point of view, we must ask Mr. Edison why, if
he invented the microphone, he did not make us acquainted with its
properties and results. These results are indeed startling, since the
microphone has in so short a time attracted general attention; and it
is evident that the clear-sighted genius of this celebrated American
inventor would have made the most of the discovery if it were really
his. The only justification for Mr. Edison’s claim consists in his
ignorance of the purely scientific discoveries made in Europe, so that
he supposed the invention of the microphone to be wholly involved in
the principle which he regards as his peculiar discovery.
In Mr. Hughes’s instrument which we are now considering, the sounds,
instead of reaching the receiving stations much diminished, which is
the case with ordinary telephones, and even with that of Mr. Edison,
are often remarkably increased, and it is for this reason that Mr.
Hughes has given to this telephonic system the name of Microphone,
since it can be employed to discover very faint sounds. Yet we must add
that this increase really takes place only when the sounds result from
mechanical vibrations transmitted by solid substances to the sending
instrument. The sounds propagated through the air are undoubtedly a
little more intense than in the ordinary system, but they lose some
of their force, and therefore it cannot be said that in this case the
microphone has the same effect upon sounds as the microscope has on
objects on which light is thrown. It is true that with this system it
is possible to speak at a distance from the instrument, and I have
even been able to transmit conversation in a loud voice, when standing
at a distance of nine yards from the microphone. When close to the
instrument, I was also perfectly able to make myself heard at the
receiving station while speaking in a low voice, and even to send the
sounds to a distance of ten or fifteen centimètres from the mouthpiece
of the receiving telephone by raising the voice a little; but the
increase of sound is not really very evident unless it is produced by a
mechanical action transmitted to the standard of the instrument.
Thus the steps of a fly walking on the stand are clearly heard, and
give the sensation of a horse’s tread; and even a fly’s scream,
especially at the moment of death, is said by Mr. Hughes to be audible.
The rustling of a feather or of a piece of stuff on the board of the
instrument, sounds completely inaudible in ordinary circumstances, are
distinctly heard in the microphone. It is the same with the ticking of
a watch placed upon the stand, which may be heard at ten or fifteen
centimètres from the receiver. A small musical box placed upon the
instrument gives out so much sound, in consequence of its vibratory
movements, that it is impossible to distinguish the notes, and in order
to do so it is necessary to place the box close to the instrument,
without allowing it to come in contact with any of its constituent
parts. It therefore appears that the instrument is affected by the
vibrations of air, and the transmitted sounds are fainter than those
heard close to the box. On the other hand, the vibrations produced
by the pendulum of a clock, when placed in communication with the
standard of the instrument by means of a metallic rod, are heard
perfectly, and may even be distinguished when the connection is made by
the intervention of a copper wire. A current of air projected on the
system gives the sensation of a trickle of water heard in the distance.
Finally, the rumbling of a carriage outside the house is transformed
into a very intense crackling noise, which may combine with the ticking
of a watch, and will often overpower it.
_Different Systems of Microphones._--The microphone has been made in
several ways, but the one represented in fig. 39 is the arrangement
which renders it the most sensitive. In this system, two small carbon
cubes, A, B, are placed one above the other on a vertical wooden
prism; two holes are pierced in the cubes to serve as sockets for a
spindle-shaped carbon pencil, that is, with the points fined off at
the two ends, and about four centimètres long: if of a large size, the
inertia will be too great. One end of this pencil is in the cavity of
the lower carbon, and the other must move freely in the upper cavity
which maintains it in a position approaching to that of instable
equilibrium, that is, in a vertical position. Mr. Hughes states that
the carbons become more effective if they are steeped in a bath of
mercury at red heat, but they will act well without undergoing this
process. The two carbon cubes are also provided with metallic contacts
which admit of their being placed in connection with the circuit of
an ordinary telephone in which a Leclanché battery has been placed,
or one, two, or three Daniell cells, with an additional resistance
introduced into the circuit.
[Illustration: FIG. 39.]
In order to use this instrument, it is placed on a table, with the
board which serves to support it, taking care to deaden any extraneous
vibrations by interposing between this board and the table several
folds of stuff so arranged as to form a cushion, or, which is better,
a belt of wadding, or two caoutchouc tubes: what is said by a person
standing before this system is immediately reproduced in the telephone,
and if a watch is placed on the stand, or a box with a fly enclosed in
it, all its movements are heard. The instrument is so sensitive that
words said in a low voice are most easily heard, and it is possible, as
I have already said, to hear the speaker when he is standing nine yards
from the microphone. Yet some precautions are necessary in order to
obtain good results with this system, and besides the cushions placed
beneath the instrument to guard it from the extraneous vibrations which
might ensue from any movements communicated to the table, it is also
necessary to regulate the position of the carbon pencil. It must always
rest on some point of the rim of the upper cavity; but as the contact
may be more or less satisfactory, experience alone will show when it
is in the best position, and it is a good plan to make use of a watch
to ascertain this. The ear is then applied to the telephone, and the
pencil is placed in different positions until the maximum effect is
obtained. To avoid the necessity of regulating the instrument in this
way, which must be done repeatedly by this arrangement, MM. Chardin
and Berjot, who are ingenious in the construction of telephones on this
pattern, have added to it a small spring-plate, of which the pressure
can be regulated, and which rests against the carbon pencil itself.
This system works well.
[Illustration: FIG. 40.]
M. Gaiffe, by constructing it like a scientific instrument, has given
the instrument a more elegant form. Fig. 40 represents one of his
two models. In this case, the cubes or carbon dice are supported by
metallic holders, and the upper one E is made to move up and down a
copper column G, so as to be placed in the right position by tightening
the screw V. In this way the carbon pencil can be made to incline
more or less, and its pressure on the upper carbon can be altered at
pleasure. When the pencil is in a vertical position, the instrument
transmits articulate sounds with difficulty, on account of the
instability of the points of contact, and rustling sounds are heard.
When the inclination of the pencil is too great, the sounds are purer
and more distinct, but the instrument is less sensitive. The exact
degree of inclination should be ascertained, which is easily done by
experiment. In another model M. Gaiffe substitutes for the carbon
pencil a very thin square plate of the same material, bevelled on its
lower and upper surfaces, and revolving in a groove cut in the lower
carbon. This plate must be only slightly inclined in order to touch
the upper carbon, and under these conditions it transmits speech more
loudly and distinctly.