the arms, a musical note can be perceived when the ear is closely
applied to the arm of the person experimented upon. The sound seems to
proceed from the muscles of the fore-arm and from the biceps muscle.
Mr. Elisha Gray[4] has also produced audible effects by the passage of
electricity through the human body.
‘An extremely loud musical note is occasioned by the spark of a
Ruhmkorff’s coil when the primary circuit is made and broken with
sufficient rapidity; when two rheotomes of different pitch are caused
simultaneously to open and close the primary circuit, a double tone
proceeds from the spark.
‘A curious discovery, which may be of interest to you, has been made
by Professor Blake. He constructed a telephone in which a rod of
soft iron, about six feet in length, was used instead of a permanent
magnet. A friend sang a continuous musical tone into the mouthpiece of
a telephone, like that shown in fig. 17, which was connected with the
soft iron instrument alluded to above. It was found that the loudness
of the sound produced in this telephone varied with the direction in
which the iron rod was held, and that the maximum effect was produced
when the rod was in the position of the dipping-needle. This curious
discovery of Professor Blake has been verified by myself.
‘When a telephone is placed in circuit with a telegraph line, the
telephone is found seemingly to emit sounds on its own account. The
most extraordinary noises are often produced, the causes of which
are at present very obscure. One class of sounds is produced by the
inductive influence of neighbouring wires and by leakage from them, the
signals of the Morse alphabet passing over neighbouring wires being
audible in the telephone, and another class can be traced to earth
currents upon the wire, a curious modification of this sound revealing
the presence of defective joints in the wire.
‘Professor Blake informs me that he has been able to use the railroad
track for conversational purposes in place of a telegraph-wire, and
he further states that when only one telephone was connected with the
track the sounds of Morse operating were distinctly audible in the
telephone, although the nearest telegraph-wires were at least forty
feet distant; and Professor Peirce has observed the most curious sounds
produced from a telephone in connection with a telegraph-wire during
the aurora borealis.’
Mr. Bell went on to describe instances in which airs sung or played
upon a musical instrument are transmitted by a telephone, when
it is not known whence they come; but the strongest proof of the
extraordinary sensibility of this instrument consists in its becoming
possible by its means to transmit speech through bodies which might
be supposed to be non-conductors. Thus communication with the earth
through the human body can be made in spite of the intervention of
shoes and stockings; and it may even be effected if, instead of
standing on the ground, the person stands on a brick wall. Only hewn
stone and wood are a sufficient hindrance to communication, and if the
foot touches the adjoining ground, or even a blade of grass, it is
enough to produce electric manifestations.
Mr. Bell says in conclusion:
‘The question will naturally arise, Through what length of wire can the
telephone be used? In reply to this, I may say that the maximum amount
of resistance through which the undulatory current will pass, and yet
retain sufficient force to produce an audible sound at the distant end,
has yet to be determined; no difficulty has, however, been experienced
in laboratory experiments in conversing through a resistance of 60,000
ohms, which has been the maximum at my disposal. On one occasion, not
having a rheostat at hand, I may mention having passed the current
through the bodies of sixteen persons, who stood hand in hand. The
longest length of real telegraph line through which I have attempted
to converse has been about 250 miles. On this occasion no difficulty
was experienced so long as parallel lines were not in operation. Sunday
was chosen as the day on which it was probable other circuits would
be at rest. Conversation was carried on between myself in New York,
and Mr. Thomas A. Watson in Boston, until the opening of business upon
the other wires. When this happened the vocal sounds were very much
diminished, but still audible. It seemed, indeed, like talking through
a storm. Conversation, though possible, could be carried on with
difficulty, owing to the distracting nature of the interfering currents.
‘I am informed by my friend Mr. Preece that conversation has been
successfully carried on through a submarine cable, sixty miles in
length, extending from Dartmouth to the Island of Guernsey, by means of
hand telephones.’
[Illustration: FIG. 20.]
_Mr. Elisha Gray’s Share in the Invention of the Telephone._--We
have seen that if Mr. Bell was the first to construct the speaking
telephone in a practical form, Mr. Gray had at the same time conceived
the idea of an instrument also capable of reproducing speech, and the
description given of it in his _caveat_ was so precise that if it had
been made from his design, it would have acted perfectly. This was, in
fact, afterwards proved by him. In order that our readers may judge
from their own knowledge of the share which should be ascribed to Mr.
Elisha Gray in the invention of the telephone, we reproduce in fig. 20
the drawing which accompanied the _caveat_ in question.
The sender, as we see, is composed of a sort of tube, closed at its
lower end by a membrane to which a platinum wire is fixed; this wire
dips into a liquid of moderate conducting power, and an electrode made
of platinum, in communication with a battery, is fixed at the bottom
of the vessel containing the liquid. The receiver is composed of an
electro-magnet, of which the armature is fixed to the centre of a
membrane, stretched on a kind of resonator or ear-trumpet which is held
to the ear, and the two instruments are united by the line wire as we
see in the plate.
Under these conditions, the undulatory currents necessary for the
reproduction of speech were obtained in a mode analogous to that
pointed out by Mr. Bell in his specification, that is, by the
variations of resistance in the liquid layer interposed between the
platinum wires of the transmitter; and their action, exerted on an
electro-magnet, of which the armature was fixed on the diaphragm of the
resonator, was produced under more favourable conditions than in Mr.
Bell’s specification (see fig. 13), since that gentleman regards this
arrangement (represented in fig. 14) as an important improvement on his
first conception.
The whole importance of the invention rests on the intervention of
undulatory currents, which, as we have seen, are indispensable for the
reproduction of speech, and it concerns us to know whether it was Mr.
Bell or Mr. Gray who first declared their importance; for in both the
specifications deposited on February 14, 1876, the use of undulatory
currents was declared to be indispensable. Mr. Gray asserts that he had
recognised their importance for the transmission of combined sounds
as early as 1874; but Mr. Bell believes that the undulatory currents
mentioned by Mr. Gray at that time were only currents analogous to
those he had designated under the name of pulsatory currents, which
we have represented in fig. 8. We have seen that since these currents
only represent the abrupt elevations and depressions of intensity, they
are unfit for the reproduction of articulate sounds, which, on the
contrary, demand that the variations of intensity should result from
successive efforts, in exact correspondence with all the inflections
of the sonorous vibrations effected by the voice. Mr. Bell’s claim to
priority on this question has been recognised by the American Patent
Office, since he has been placed in possession of the patent. However
this may be, Mr. Gray’s telephonic system was complete, and we see in
it, as we have already said, the origin of the battery telephones,
which have recently produced such important results. Let us now
consider the relation which this system bears to Mr. Bell’s.
The Bell system, as we have seen, although making use of a battery
in the first instance, only obtained the diminution and increase of
electric force necessary for the articulation of words by means of
induction currents produced by the movements of an armature of soft
iron, currents of which the intensity was consequently due to the
range and inflections of these movements. The battery only intervened
in order to communicate magnetic force to the inducer. This use of
induced currents in telephonic transmissions was already of great
importance, since various experiments subsequently made have proved
their superiority to voltaic currents for this purpose. But experience
soon convinced Mr. Bell that a powerful inductive apparatus worked by a
battery was not only unnecessary for the action of this apparatus, but
that a permanent magnet, very small and weak, would provide sufficient
currents. This discovery, in which, as we have seen, Mr. Peirce had
some share, was of great importance, since it became possible to
reduce the size of the instrument considerably, so as to make it
portable and adapted for sending and receiving; and it was shown that
the telephone was the most sensitive of all instruments in revealing
the action of currents. If, therefore, Mr. Bell was not the first to
employ the successful mode of transmitting articulate words, it must
be said that he sought, like Mr. Gray, to solve the problem by means
of undulatory currents, and that he obtained these currents by the
effect of induction, a system which, as soon as it was perfected, led
to the important results with which we are all acquainted. If he had
only given to the astonished world an instrument capable of reproducing
speech telegraphically, his fame would be great; for this problem had
hitherto been regarded as insoluble.
Mr. Gray’s claims to the invention of the telephone are given in the
following summary from a very interesting work, entitled ‘Experimental
Researches on Electro-harmonic Telegraphy and Telephony:’
‘1. I was the first to discover the means of transmitting compound
sounds and variable inflections through a closed circuit by means of
two or more electric waves.
‘2. I assert that I was the first to discover and utilise the mode
of reproducing vibrations by the use of a magnet receiver constantly
supplied with electric action.
‘3. I also assert that I was the first to construct an instrument
consisting of a magnet with a circular diaphragm of magnetic substance,
supported by its edge at a little distance from the poles of a magnet,
and capable of being applied to the transmission and reception of
articulate sounds.’
It is a curious fact, worth recording here, that Mr. Yates, of Dublin,
in 1865, when trying to improve Reiss’s telephone, realised to a
certain extent Mr. Gray’s conception of the liquid transmitter; for he
introduced into the platinum contacts of Mr. Reiss’s instrument a drop
of water which adapted it for the reproduction of articulate sounds.
However, no notice was then taken of this result.
EXAMINATION INTO THE FUNDAMENTAL PRINCIPLES ON WHICH BELL’S TELEPHONE
IS BASED.
Although the preceding account would suffice to make the principle
of Bell’s telephone intelligible to persons acquainted with electric
science, this would not be the case with the majority of our readers,
and we therefore think it necessary to enter into some details as to
the source of the electric currents which are employed in telephonic
transmissions. These details seem to us the more necessary, since
many persons still believe that Bell’s telephones are not electric,
because they do not require a battery, and they are often confounded
with string telephones, so that the difference of price between Bell’s
instruments and those hawked in the streets seems astonishing.
Without defining what is meant by an electric current, which would
be too elementary, we may say that electric currents can be produced
by different causes, and that, in addition to those which are due to
batteries, strong currents are also produced by the force exerted by
magnets on a conducting circuit properly arranged. Such currents are
called induction currents, and are used in Bell’s telephone. In order
to understand how they are developed under these conditions, it will
be enough to examine what takes place when the pole of a magnet is
brought near to, and withdrawn from, a closed circuit. To do this, let
us suppose a copper wire attached to a galvanometer in the form of a
circle, and that one pole of a permanent magnet is directed towards the
centre of the circle. Now observe what happens:
1. At the moment when the magnet approaches an electric current arises,
causing the galvanometer to deviate to one side. This deviation will
be great in proportion to the extent of the movement, and the tension
of the current will be great in proportion to the abruptness of the
movement. The current will however be only instantaneous.
2. At the moment when the magnet is withdrawn, a fresh current of the
same nature will arise, but it will appear in an opposite direction
from the former. It will be what is called a direct current, because it
is in the same direction as the magnetic current of the magnet which
produces it, while the other current is called _inverse_.
3. If, instead of advancing or withdrawing the magnet by means of a
single movement, it is advanced in jerks, a succession of currents
in the same direction is produced, of which the existence can be
ascertained by the galvanometer when there is a sufficient interval
between the movements, but when the intervals are very slight the
currents are interfused; and since inverse effects take place when the
magnet is moved in a contrary direction, the needle of the galvanometer
follows the movements of the magnet, and to a certain extent
stereotypes them.
4. If, instead of reacting on a simple closed circuit, the magnet
exerts its force on a considerable number of circumvolutions of this
circuit, that is, on a bobbin of coiled wire, the effects will be
considerably increased, and they will be still greater if there be a
magnetic core within the bobbin, since the inducing action will then
be more effectually exerted throughout the bobbin. As the magnetic
core, when it is magnetised and demagnetised under the influence of its
approach to or withdrawal from the inducing magnet, is subject to the
reaction from all the fluctuations which occur in the movements of the
magnet, the induced currents which ensue are perfectly defined.
5. If, instead of a movable magnet, we suppose it to be fixed in the
centre of the coil, the induced currents of which we have spoken may
then be determined by modifying its force. In order to do so, it is
enough that an iron armature should react upon its poles. When this
armature is brought close to one of the poles, or to both at once, it
acquires force, and produces an inverse current, that is, a current
in the direction which would have corresponded to an approach of the
magnet to the closed circuit. On its withdrawal the inverse effect is
produced; but in both cases the induced currents correspond with the
extent and direction of the movements accomplished by the armature,
and consequently they may reproduce its movements by their effects.
If this armature is an iron plate, which vibrates under the influence
of any sound in this disposition of the electro-magnetic system, the
alternate movements of the plate will be transformed into the induced
currents, and these will be stronger or weaker, more or less definite,
according to the range and complexity of the vibrations: they will,
however, be undulatory, since they will always result from successive
and continuous movements, and will consequently be in the conditions
which, as we have seen, are required for the transmission of speech.
As for the action produced upon the receiver, that is, on the
instrument for reproducing speech, it is somewhat complex, and we
shall have occasion to speak of it presently; but we can get a general
impression of it, if we consider that the effects produced by the
induced currents of variable intensity, which traverse the coil of the
electro-magnetic system, must determine, by the magnetisations and
demagnetisations which ensue, the vibrations of the armature disk;
these vibrations, more or less amplified and defined, exactly represent
those of the disk before which the speaker stands, and can only be
obtained from them. The effects are, however, in reality more complex,
although they are produced under analogous conditions, and we shall
have more to say about them when we come to speak of the experiments
made with the telephone. It must meanwhile be observed that, for the
reproduction of speech, it is not necessary that the magnetic core
should be of soft iron, since the vibratory effects may follow from
differential as well as from direct magnetisation.
ORDINARY ARRANGEMENT OF THE BELL TELEPHONE.
The arrangement most generally adopted for the telephone is the one
represented in fig. 21. It consists of a kind of circular wooden
box, fitted to the extremity of a handle M, which is also of wood,
and contains the magnetic bar N S. This bar is fixed by means of a
screw _t_, and is so arranged as to be moved forward and backward by
tightening or loosening the screw, a condition necessary in order to
regulate the instrument. At the free extremity of the bar the magnetic
coil B is fixed; this must, according to MM. Pollard and Garnier, be
made of wire No. 42, so as to present a considerable number of spirals.
The ends of this coil generally terminate at the lower end of the
handle in two copper rods _f_, _f_, which traverse its length, and are
fastened to two binding-screws I, I′, where the line wires C, C are
fixed. In the instruments made by M. Bréguet there are, however, no
binding-screws, but a little twist, made of two flexible wires covered
with gutta-percha and silk, is fastened to the two rods. A wooden cap
is screwed to the end of the handle, and the twist passes through a
hole made in this cap, so that there is no inconvenience in working the
instrument. By laying hold of the ends of the wire twist with pliers it
is possible to join them to the circuit. This instrument is represented
in fig. 22.
[Illustration: FIG. 21.]
By another arrangement, the wires of the coil end immediately in
the binding-screws which are placed below the wooden box, but this
arrangement is inconvenient.
Above the pole of the magnetic bar is placed the iron vibrating plate
L L, which is coated either with black or yellow varnish, with tin
or blue oxide, but which must always be very thin. This plate is in
the form of a disk, and by its rim, resting on a caoutchouc ring, it
is firmly fixed to the circular edges of the wooden box, which is for
this purpose made in two pieces. These pieces are adjusted to each
other, either by screws or by spirals cut in half the thickness of the
wood. This disk ought to be as near as possible to the polar end of
the magnet, yet not so near as to produce contact between the two by
the vibrations of the voice. Finally, the mouthpiece R R′ (fig. 21),
which is in form of a wide funnel, terminates the upper part of the
box, and should be so arranged as to leave a certain space between the
disk and the edges of the hole V, which is open in its centre. The
size of the box should be so calculated as to permit of its acting as
a sounding-box, without however provoking echoes and a confusion of
sounds.
[Illustration: FIG. 22.]
When the instrument is properly made, it will produce very marked
effects; and M. Pollard, one of the first Frenchmen to take up the
study of telephones, has written as follows on the subject:
‘The instrument which I have prepared gives results which are truly
astonishing. In the first place, when considering the resistance, the
introduction into the circuit of five or six persons does not sensibly
diminish the intensity of sounds. On putting an instrument to each
ear, the sensation is precisely the same as if the correspondent were
speaking some yards behind. The intensity, the clearness, the purity of
tone are irreproachable.
‘I can speak to my colleague in quite an undertone, scarcely breathing
as I may say, and persons placed within two yards of me will be unable
to catch a single word of our conversation.
‘On the part of the receiver, if anyone raises his voice to call me, I
hear the call in all parts of my office, at least when silence prevails
there; at any rate, when I am seated at my table with the instrument
some yards off, I can always hear the call. In order to increase the
intensity of sound, I fitted the mouthpiece with a copper horn of
conical shape, and under these conditions words spoken in my bureau two
or three yards from the mouthpiece can be heard at the other end of the
line; from my station, a little more than a yard from the tube, I can
hear and speak to my colleague without effort.’
In using the ordinary Bell telephone, it is necessary to speak
distinctly before the mouthpiece of the telephone which is handled,
while the listener placed at the corresponding station keeps the
mouthpiece of the receiver to his ear. These two instruments form
a closed circuit with the two wires which connect them, but one is
enough to make the transmission perfect, if care is taken to place
both instruments in connection with the earth, which thus takes the
place of the second wire. M. Bourbouze asserts that the intensity of
sound in the telephone is much increased by employing this expedient,
but we believe that this increase depends upon the conditions of the
circuit, although he asserts that the fact can be proved in a circuit
not exceeding eighty yards.
For practical purposes it is necessary to have two telephones at each
station, so as to hold one to the ear while speaking through the other,
as in fig. 23. It is also much more easy to hear with a telephone
applied to each ear, in which case they are held as in fig. 24. In
order not to fatigue the arms, an arrangement has been made by which
they are held before the ears by a strap and spring which goes round
the head.
[Illustration: FIG. 23.]
The sending power of the telephone varies with different voices.
Mr. Preece asserts that shouting has no effect, and that, in order
to obtain a favourable result, the intonation must be clear, the
articulation distinct, and the sounds emitted must resemble musical
sounds as much as possible.
Mr. Wilmot, one of the electricians employed by the Post Office,
says that he has been able to make himself heard on circuits through
which no other voices were audible. The vowel sounds are most readily
transmitted, and among other letters _e_, _g_, _j_, _k_, and _q_ are
always repeated more imperfectly. The ear requires practice, and
the faculty of hearing varies in a surprising degree in different
people. Singing is very distinctly heard, as well as wind instruments,
especially the cornet-à-piston, which, when played in London, was heard
by thousands of people in the Corn Exchange at Basingstoke.
[Illustration: FIG. 24.]
According to Mr. Rollo Russell, it is not necessary to isolate the
circuit of a telephone when the distance is relatively slight; thus,
with a circuit of about 430 yards, it is possible to use a simple
copper wire, laid on the grass, without destroying the telephonic
transmission from a small musical box, as long as the two wires do not
touch each other. Transmission took place, even when the circuit was
buried in moist earth for a length of thirty-five yards, or immersed in
a well for a length of forty-eight yards. The words transmitted under
such conditions did not differ from those transmitted by an isolated
circuit.