vibrations produced by the voice on the carbon of a telephonic sender
would be magnified if the moveable part of the rheotome were subjected
to an electro-magnetic action resulting from the vibrations themselves,
and he has contrived a sender, which is based on the principle shown
in fig. 33, and which has the merit of forming in itself the inducing
apparatus intended to transform the voltaic currents employed. This
instrument is composed of a vertical iron tube, supported on a
magnetic bar N S, and surrounded by a magnetising coil B B, above
which is fixed an inducing helix of fine wire I I, communicating with
the circuit. Within the tube there is a lead pencil C, held by a
porte-crayon which can be raised or lowered by means of a screw V fixed
below the magnetic bar. Finally, above this pencil, there is an iron
vibrating plate L L, with a platinum point in communication with the
battery in its centre; the local circuit communicates with the pencil
by means of the magnetising helix B, and for this purpose one end is
soldered to the iron tube.
From this arrangement it follows that the vibrations of the plate L
L, at the moment when it comes nearest to the pencil, tend to become
greater in consequence of the attractive force exerted on the plate,
and as the pressure of the lead pencil is increased, it increases the
differences of resistance which result from it, and consequently causes
greater variations in the intensity of the transmitted currents.
_Reaction Telephone of Messrs. Thomson and Houston._--The telephonic
arrangement we have described has lately been adopted by Mr. Elihu
Thomson and Mr. Edwin J. Houston, who, on June 21, 1878, two months
after M. Hellesen explained his system to me,[7] published an article
in ‘The English Mechanic and World of Science’ about an instrument
very similar to that of M. Hellesen.
In their instrument, the current, which passes through a body of
moderately conducting capacity, acts on an electro-magnet provided with
an induction coil, and this electro-magnet reacts on the diaphragm,
in order to increase the range of its vibrations, and to create at
the same moment two electric actions in the same direction: the only
difference lies in the arrangement of the contact of this indifferent
conductor with the vibrating plate. Instead of a simple contact
effected by pressure between this plate and a carbon pencil, a fragment
of the same substance with a sharpened point is fixed on the vibrating
plate, and it dips into a drop of mercury which has been poured into
the receptacle made for it at the upper end of the electro-magnet.
In other respects, the arrangement of the instrument is that of an
ordinary telephone, and the iron rod of the electro-magnet represents
the magnetised bar of the Bell telephone. The inventors assert that
this instrument can be used both as a sender and receiver, and it is in
the following manner that it is worked in each case.
When the instrument is transmitting, the morsel of carbon dips more or
less into the mercury, and consequently differences are produced in the
surfaces of contact, according to the range of vibrations made by the
plate; the current varies in intensity in proportion to this range, and
induced currents in the induction coil result from these variations;
the induced currents react on the receiving telephone, as in Bell’s
instrument, and are further strengthened by those which are produced
electrically by the movement of the diaphragm before the induction
coil, and the iron of the electro-magnet.
When the instrument is used as a receiver, the usual effects are
displayed, for since the iron of the electro-magnet is magnetised by
the current, its conditions are precisely those of the ordinary Bell
telephone, and the induced currents reach it in the same manner, only
with greater intensity. Messrs. Thomson and Houston assert that their
system has produced excellent results, and that by it the sound of the
voice is much less altered than in other telephones.
_Telephones with batteries and liquid senders._--We have seen that in
1867 Mr. Gray conceived the idea of a telephonic system based on the
differences of resistance effected in a circuit completed by a liquid,
when the layer of liquid interposed between the electrodes varies in
thickness under the influence of the vibrations of the telephonic plate
which is in communication with one of these electrodes. This system
has since been the subject of study by several inventors, among others
by MM. Richemond and Salet; and I give some of the accounts which have
been published respecting their researches.
Another telephone for the reproduction of articulate sounds, which
M. Richemond terms the _electro-hydro telephone_, has been recently
patented in the United States. It resembles that of Mr. Edison in
some respects, but instead of making use of carbon disks to modify
the resistance of the circuit, water is employed, and this water is
placed in communication with the circuit and battery by means of two
platinum points, one of which is fixed on the metallic diaphragm which
vibrates under the influence of the voice. As the vibrations of the
diaphragm transport the point which is attached to it to different
parts of the interpolar layer of liquid, they diminish or increase the
electric resistance of this layer, and cause corresponding variations
in the intensity of the current traversing the circuit. The receiving
telephone is of the usual kind. (See ‘Telegraphic Journal,’ September
15, 1877.)
M. Salet writes: ‘I thought it would be interesting to construct a
telephone in which there should be absolute solidarity in the movements
of the two membranes, and for this purpose I have availed myself of the
great resistance of liquids. Mr. Bell had already obtained some results
by attaching to the vibrating membrane a platinum wire communicating
with a battery, and dipping more or less into a metallic vessel, itself
connected by the line with the receiving telephone and containing some
acidulated water. I have substituted for the platinum wire a small
aluminium lever supporting a disk of platinum, and at a very slight
distance from it there is a second disk in connection with the line.
The vibrations of the membrane, tripled or quadrupled in their range,
are not altered in form, thanks to the small size and light weight of
the lever: they cause variations in the thickness of the liquid layer
traversed by the current, and consequently in its intensity, and these
variations cause corresponding differences in the attractive force
of the receiving electro-magnet. Under its influence the receiving
membrane executes movements which are identical with those of the
sending membrane. The sound transmitted is very distinct, and its
_timbre_ is perfectly maintained, a result which might have been
anticipated. The consonants, however, are not so clearly pronounced
as those transmitted by Mr. Bell’s instrument. This inconvenience is
most apparent when the lever is heavy, and might easily be obviated.
The electrolysis also produces a continual murmur, but this does not
interfere with the distinctness of the sound.
‘Since on this system the voice is not required to _produce_, but only
to _direct_ the electric current generated by a battery, the intensity
of the sound received might in theory be increased at pleasure. I have
in fact been able to make the receiver emit very powerful sounds, and
I think that this advantage greatly counterbalances the necessity of
employing a battery, and a somewhat delicate sending instrument.
Unfortunately it can only be used for moderate distances. Assuming that
any displacement of the transmitting membrane increases the resistance
to a degree equivalent to five or six hundred yards of wire: if the
line is five hundred yards long, the intensity of the current will be
reduced by one half, and the receiving membrane will take up a fresh
position, considerably differing from the first; but if the line is
three hundred miles in length, the intensity of the current will only
be modified by a thousandth part. An immense battery must therefore be
employed in order that this variation may be translated by a sensible
change in the position of the receiving membrane.’ (See ‘Comptes Rendus
de l’Académie des Sciences,’ February 18, 1878.)
M. J. Luvini, in an article inserted in ‘Les Mondes,’ March 7,
1878, has suggested a system of rheotome by means of a current, for
battery telephones, which, although complicated, possibly offers some
advantages, since it produces currents alternately reversed. In this
system, the vibrating disk of the sender, which should be in a vertical
position, reacts on a moveable horizontal wire, turned back at a right
angle, and supporting on each of its branches two platinum points
which dip into two bulbs, filled with a liquid of moderate conducting
capacity. The two branches of this wire, insulated from each other, are
placed in communication with the two poles of the battery, and the
four cups into which the platinum wire dips communicate inversely with
the line and the earth by means of platinum wires immoveably fixed in
the cups. It follows from this arrangement, that when the distances
are duly regulated between the fixed and moveable wires, two equal
currents will be opposed to each other across the line circuit when the
diaphragm is motionless; but as soon as it vibrates, the respective
distances of the wires will vary, and it follows from this that there
will be a differential current, of which the intensity will correspond
with the extent of the displacement of the system, or with the range
of vibrations, and the direction will vary with the movements above or
below the line of the nodes of vibration. In this way the advantage of
the induced currents is obtained.
_Telephones with a battery and voltaic arcs._--In order to obtain
variations of resistance of still greater sensitiveness than is the
case with liquids or pulverised substances, the idea has been suggested
of employing conductors of heated gas, and several arrangements of
battery telephones have been made in which the circuit was completed
by a stratum of air, separating the vibrating disk from a platinum
point, which serves to excite an electric discharge of high tension.
Under these conditions, the stratum of air becomes the conductor, and
the intensity of the current which traverses it corresponds to its
thickness. This problem has been solved, either by means of voltaic
currents of high tension, or by a Ruhmkorff coil.
The former system was arranged by M. Trouvé, and he writes as follows
on the subject in the journal ‘La Nature’ of April 6, 1878: ‘A metallic
vibrating membrane forms one of the poles of a high tension battery;
the other pole is fastened before the disk by a micrometer screw which
can be adjusted so as to vary the distance from the disk according to
the tension of the battery, but without ever coming in contact with it.
The distance must not in any case exceed that to which the discharge
of the battery can extend. Under these conditions, the membrane which
vibrates under the influence of the waves of sound has the effect of
constantly modifying the distance between the two poles, and thus
of continually varying the intensity of the current: consequently
the receiving instrument (a Bell telephone, or telephone with an
electro-magnet) is subjected to magnetic variations, corresponding to
the variations of the current which affect it, and this has the effect
of making the receiving instrument vibrate at the same moment. This
kind of telephonic instrument relies, therefore, on the possibility of
varying within wide limits the resistance of the outer circuit of a
high-tension battery, in which the poles are not in contact. In order
to vary the conditions of this resistance, it is also possible to
interpose some vapour or other medium, such as air, or gas of greater
or less rarity.’
M. Trouvé thinks that he was successful with his battery of small
disks, moistened with sulphate of copper and sulphate of zinc,
arranging these elements, to the number of five or six hundred, in
glass tubes of small diameter. It is well known that it is unnecessary
for the elements to be of large size in order to obtain tension
currents.
M. de Lalagade has suggested an analogous mode by employing for the
formation of the arc a current of which the tension is increased by
inserting a strong electro-magnet into the circuit. This electro-magnet
acts on a Hughes magnet in order to produce induction currents capable
of making the receiving instrument act. M. de Lalagade says that a
Bunsen battery, or one of six cells with bichromate of potash, will
be enough to produce a continuous voltaic arc between the vibrating
plate of a telephone and a platinum point which is sufficiently remote
to avoid contact. It is necessary, however, to begin with a contact,
in order to produce the formation of this arc. In M. de Lalagade’s
system, the vibrating plate should have in its centre a small platinum
plate, in order to obviate the oxidising effects of the spark. The
inventor asserts that sounds transmitted in this way, and reproduced
in a telephone of which the electro-magnetic system is set upon a
sounding-box, will have greater intensity than the sounds transmitted
by an ordinary telephone, and the speaker will appear to be close to
the ear.
_Mercury Telephones._--These systems are based on the physical
principle discovered by M. Lippmann, that if a layer of acidulated
water is placed above mercury, and connected with it by an electrode
and wire, every mechanical action which exerts pressure on the surface
of the mercury, and alters the form of its meniscus, will cause an
electric reaction, capable of producing a current with a force which
corresponds to the mechanical action exerted. Conversely, every
electric action produced on the circuit of such a system will occasion
a displacement of the meniscus, and consequently its movement, which
will be more marked in proportion to the smallness of the tube in which
the mercury is placed, and to the greatness of the electric action.
This electric action may result from a difference of potential in
the electric condition of the two extremities of the circuit, which
communicate with the electric source employed, or with some electric
generator.[8]
[Illustration: FIG. 34.]
In accordance with these effects, it is intelligible that if two tubes
T T, pointed at the end, and containing mercury, are plunged into
two vessels V V (fig. 34) containing acidulated water and mercury,
and metallic wires, P P, Q Q, are used, first to connect the columns
of mercury in the tubes, and secondly the layers of mercury at the
bottom of the two vessels, the tubes being a little removed from
the surface of the mercury in the vessels, we shall then have a
metallic circuit, completed by two electrolytes, one of which will
be subjected to the mechanical or electrical effects produced in the
other. If two vibratory plates B B are placed above the tubes, and
one of these is caused to vibrate, the other will reproduce these
vibrations, influenced by the vibratory movements communicated by the
corresponding column of mercury. The vibrations themselves will be in
connection with the electrical discharges resulting from the movements
of the column of mercury in the first tube, which are mechanically
produced. If an electric generator is introduced into the circuit, the
effect which we have just analysed will be caused by modifications in
the potential of this generator, in consequence of electro-capillary
effects. But if no generator is employed, the action will result from
electric currents determined by the electro-capillary attraction
itself. In the latter case, however, the instrument must be more
delicately made, in order to obtain more sensitive electric reaction,
and M. A. Bréguet describes his instrument as follows.
‘The instrument consists of a tube of thin glass, a few centimètres in
length, containing alternate drops of mercury and acidulated water,
so as to constitute so many electro-capillary elements, connected in
tension. The two ends of the tube are fused together, yet so as to
allow a platinum wire to touch the nearest drop of mercury on each
side. A small circle of thin deal is fixed at right angles to the tube
by its centre, thus providing a surface of some extent, which can be
applied to the ear when the instrument is a receiver, and to make the
tube more mobile under the influence of the voice when the instrument
is a sender. The following are the advantages offered by instruments of
this construction:--
‘1. They do not involve the use of a battery.
‘2. The disturbing influence of the resistance of a long line is almost
destroyed in these instruments, although it is still appreciable in the
Bell telephone.
‘3. Two mercury telephones, coupled together as we described above, are
absolutely correlative, in this sense, that even different positions
in the equilibrium of the mercury in one of them produce different
positions of equilibrium in the opposite instrument. It is therefore
possible to reproduce at a distance, without a battery, not merely
faithful indications of oscillatory movements, which is done by the
Bell telephone, but also the exact image of the most general movements.’
_Friction Telephones._--Mr. E. Gray has quite recently applied the
principle of producing sounds by the friction of animal tissues to the
construction of a speaking telephone which may be heard through a whole
room, like the singing condenser. He obtains this result by means of
clockwork, which causes the rotation of the metallic disk of which we
have spoken (p. 23), and on which a piece of skin is so arranged as to
produce friction. A carbon or liquid telephone is placed at the sending
station, in such a way as to react on an induction coil, as in the
systems of Edison, Navez, or Pollard, and speech is reproduced on the
rotating disk, and is audible, as we have said, without the necessity
of approaching the ear to the instrument.
The best arrangement of the metallic disk on which the animal tissue
rubs is that of a cylindrical box, of which the outer lid is made of a
thin sheet of zinc with a highly polished, slightly oxidised surface;
for the agent of friction, glove-leather slightly moistened with
acidulated water may be used, or a sinew of an ox, or skin taken from
the ear or tail of a pig.
MODIFICATIONS INTRODUCED IN THE CONSTRUCTION OF THE BELL TELEPHONES.
The modifications which we have been considering relate to the
principle of the instrument; those which we have now to consider are
only modifications in the form and arrangement of the different organs
which form the Bell telephone itself, and which have been designed with
the object of increasing the intensity and distinctness of the sounds
produced.
_Telephones with several diaphragms._--When we remember that the
induced currents caused in a magnet result from the vibratory movements
of the diaphragm, and that these are produced by the vibrations of
the stratum of air interposed between this diaphragm and the vocal
organ, it necessarily follows that if these vibrations of the air react
on several diaphragms, each attached to its electro-magnetic organ,
several induced currents might be caused simultaneously, and if these
were properly connected, their effects on the receiver would be so
much the more intense, since the sounds produced would result from the
combination of several sources of sound. Several inventors, starting
from this argument, have planned instruments of varying ingenuity,
which we will now describe, but without being able to declare who
was the first to realise this idea. It is in fact so simple, that it
probably suggested itself to the minds of several inventors at the
same time, and we see that while M. Trouvé proposed this improvement
in France in November 1877, it was tried in America and discussed in
England, where indeed it was not expected to produce very favourable
results. Mr. Preece wrote on the subject in a paper entitled ‘On some
Physical Points connected with the Telephone,’ which was published
in April 1878. He observes that all the attempts to improve the
telephone have ended in disappointment and failure. One of the first
attempts of the kind was made by Mr. Wilmot, who expected to obtain
favourable results by augmenting the number of diaphragms, helices,
and magnets, connecting the helices in a series, and causing them
to act simultaneously, so as to increase the energy of the currents
developed by the influence of the voice; but experience showed that
when the instrument acted directly, the vibratory effect of each of the
diaphragms decreased in proportion to their number, and the general
effect remained the same as with a single diaphragm. Mr. Wilmot’s
instrument was made in the beginning of October 1877, and that of M.
Trouvé was only an imitation of it.
On the other hand, we see that if the telephones with several membranes
were not successful in England, this was not the case in America, for
the telephones which experience has shown to give the best results in
that country are those of Mr. Elisha Gray and Mr. Phelps, and these
have several diaphragms. It is evident that there are details of
construction in these instruments which may appear insignificant in
theory, and which are notwithstanding very important from a practical
point of view, and we believe that it is to this circumstance that
instruments of this kind owe their success or failure. Thus, for
example, it seems that the vibrations of air caused in the mouthpiece
ought to be immediately directed on the surface of the diaphragms by
means of distinct channels; it is necessary that the empty space round
each diaphragm should be sufficiently limited to prevent echoes and
interruptions, unless the case is so large that there is no danger of
such effects. Above all, it is necessary that the organs should be
fixed in some material unsusceptible of reverberation, and for this
reason a preference is given to iron or ebonite. It is certain that
when the instrument is properly made, its effects are superior to
those of the Bell telephones, and it is asserted in the ‘Telegraphic
Journal’ that experiments were made with one of these instruments
before the Royal Society, in London, May 1, 1878, and that the
intensity of sound was in proportion to the number of diaphragms. This
instrument was designed by Mr. Cox Walker, of York, and possessed eight
diaphragms. He considers that this is the arrangement which gives the
best results.
[Illustration: FIG. 35.]
_Mr. Elisha Gray’s System._--Mr. Elisha Gray’s last system, which
we represent in fig. 35, is one of those which have given the best
effects. It is made, as we see, of two telephones, side by side, to
which correspond two tubes, issuing from a common mouthpiece E. One
of these telephones is seen in section in the plate, the other in
elevation, and they correspond to the two branches of a nickel-plated
horseshoe magnet N U S, which may serve as a suspension ring. In that
part of the plate which represents the section, the induction coil is
shown in B, and the magnetic core, of soft iron, in A, which is screwed
to the polar end of the magnet S; the vibrating plate is in L L, and,
as we see, the tube of the mouthpiece terminates on its surface.
In another model there are four telephones side by side, instead of
two, and the effects produced are still more marked.