there are two models of it. In the larger one, which makes it possible
to hear as distinctly as if the person with whom conversation is held
were speaking in a loud voice in the same room, the two telephones are
placed parallel to each other, and so as to present their diaphragms
vertically; the space between these two diaphragms is occupied by
a vertical tube, terminating at its lower end in a horizontal tube
corresponding to the centres of the two diaphragms, and on this tube
the mouthpiece is fitted, which projects outside the box in which
the instrument is enclosed. The induction coils, and the magnetic
cores which traverse them, follow the axis of the system, and seem
to constitute the axis of a wheel which is polarised by the poles
of a horseshoe magnet, of which the position with reference to the
surface of the diaphragms can be regulated by moveable screws. The
appearance of the instrument somewhat resembles a gyroscope, resting by
a horizontal axis on two shafts which issue from a flattened horseshoe
magnet.
Above this system there is the electro-magnetic apparatus of the
call-bell, in which there is nothing peculiar, and which is like the
German alarums of which we shall speak at the end of this account.
This instrument is remarkable for strength and clearness of sound, and
especially for its freedom from the Punch and Judy voice so displeasing
in other telephones.
Mr. Phelps’s small model is in the form of an oblong or elliptical
snuff-box, of which the two centres are occupied by two telephonic
systems, influenced by the same magnet. This magnet is placed in a
horizontal position below the snuff-box, and its poles correspond to
the magnetic cores of the coils. These cores are made of iron tubes,
split longitudinally in order to destroy irregular induction reactions,
and the iron diaphragms rest on five spiral springs, which raise them
above the magnetic system. On their other surface the diaphragms are
provided with rings of some semi-elastic substance, which prevent the
central vibrations of the disks from becoming complicated by those of
their edges. The lid, hollowed out in very shallow cavities, is next
placed upon the disks, and there are channels of communication in it
to serve as a sounding-box. The mouthpiece corresponds to one of these
cavities, and the other is closed by a small metallic stopper, which
can be withdrawn to regulate the instrument when necessary. Since the
vibrations of air are transmitted by the channels to both cavities, the
two telephones act together, although at first sight only one of them
seems to be required to produce the effect.
Mr. Phelps praises the simultaneous effects produced on the two
instruments, which he ascribes, first, to the semi-elastic ring
surrounding the rim of each disk, and acting as the hammer of the ear,
that is, as a damper; then, to the longitudinal splits of the magnetic
core, and lastly to the small size of the cavities left above the
vibrating disks. The instrument is made of ebonite, grooved on the
surface in order to give a better grasp to the hand.
Mr. Phelps has a new model, called _the crown telephone_, which is now
in use in America, together with Mr. Edison’s carbon sender. In it
each of the two systems of the large model we have described is worked
by six horseshoe magnets radiating round the magnetic core, and so
arranged that the north poles correspond to this core, and the other
poles to the circular rim of the diaphragm. In this way the magnetic
field is considerably enlarged, and the sound much intensified.
In experiments recently made at Dr. Wells’s church, New York, an
assembly of three hundred people were able to hear speech and vocal or
instrumental music distinctly in different parts of the hall.
_Mr. Cox Walker’s System._--This system, on which we have already said
a few words, has exactly the arrangement of that by Mr. Elisha Gray.
The magnets which act upon the diaphragms are horseshoe, and separate
pipes, issuing from a common mouthpiece, direct the vibrations of
air on the diaphragms. These, indeed, are only defined parts of one
diaphragm, bounded in a circle by mouthpieces corresponding to the
air-pipes, and sufficiently restricted on their edges to limit the
field of vibration.
_M. Trouvé’s System._--M. Trouvé has simplified the arrangement of
telephones with a double diaphragm, by designing the instrument so
as to make Bell’s bar magnet react by both poles at once on several
disks. For this purpose, he employs a tubular magnet, and winds a
helix throughout its whole length, as we see in fig. 36. This magnet
is maintained in a fixed position in the centre of a small cylindrical
box, of which the base is slightly funnel-shaped, thus acting as a
mouthpiece and acoustic tube. It is consequently pierced in the centre
with a hole larger at _a_, the station for speaking, than on the
opposite side _b_. Between the base and the poles of the magnet there
are two vibrating iron plates, M, M′, one of which, M, is pierced with
a hole _a_ of the same diameter as the hollow part of the magnet, and
consequently smaller than that of the mouthpiece. Finally, several
other plates _n_, _n_, _n_, are ranged in parallel lines between these
two plates, so that the magnet and its helix may pass through them.
[Illustration: FIG. 36.]
When anything is said before the mouthpiece _a_, the waves of sound
encountering the edges of the plate M place it in vibration, and,
continuing their passage inside the tubular magnet, they cause the
plate M′ to vibrate at the same time as M. A double inducing action
therefore takes place on the tubular magnet, and this is translated by
the induced currents developed in the helix, which have greater energy
since each of the plates intensifies the magnetic effects produced at
the pole opposite to the one they influence, which is always the case
with bar magnets when the inactive pole is provided with an armature.
This advantage may even be obtained in the case of ordinary telephones,
if the screw which holds the magnet is placed in contact with a mass of
soft iron.
In M. Trouvé’s arrangement, the induced currents therefore possess
greater energy; but he adds that the sounds reproduced will also be
strengthened by the multiplicity of vibratory effects, and by the
enlargement of the magnetic effects, which results from a better
arrangement of the magnets.
‘When the ear is placed at _a_,’ M. Trouvé writes, ‘it perceives
immediately the sounds produced by the first plate M, and those of
the second plate reach the ear through the interior of the magnet.
This new arrangement is well adapted for an experimental comparison
of the results produced by a telephone with a single membrane (a Bell
telephone), and those produced by a telephone with several membranes.
It is in fact enough to listen at the two faces of the telephone
alternately, in order to perceive at once the difference of intensity
in the sounds produced. Those collected at _a_, on the side of the
pierced iron plate, appear manifestly doubled in intensity compared
with those collected at _b_ on the side of the simple membrane which
forms the ordinary telephone.
‘The difference is still more striking if, in transmitting or receiving
a sound of invariable intensity through a multiple telephone, the
unbroken membrane M′ is repeatedly prevented from vibrating.’
Before making this arrangement M. Trouvé had planned another, which he
presented to the Académie des Sciences, November 26, 1877, and which we
have glanced at in the beginning of this chapter. He describes it in
these terms:--
‘In order to increase the intensity of the effects produced in the Bell
telephone, I have substituted for the single membrane a cubic chamber,
of which each face is, with one exception, formed of a vibrating
membrane. Each of these membranes, put in vibration by the same sound,
influences a fixed magnet, which is also provided with an electric
circuit. In this way, by connecting all the currents generated by the
magnets, a single intensity is obtained, which increases in proportion
to the number of magnets influenced. The cube might be replaced by a
polyhedron, of which the faces might be formed of an indefinite number
of vibrating membranes, so as to obtain the desired intensity.’
_M. Demoget’s System._--Several other systems of telephones with
multiple membranes have been proposed. One of them, planned by M.
Demoget, consists in placing before the vibrating disk of the ordinary
Bell telephone, separated by the space of a millimètre, one or two
similar vibrating disks, taking care to pierce in the centre of the
first a circular hole of the same diameter as that of the bar magnet,
and to pierce a larger hole in the second membrane. The inventor
asserts that the distinctness as well as the intensity of sounds is
increased in this way.
‘By this arrangement,’ says M. Demoget, ‘since the vibrating magnetic
mass is larger in proportion to the magnet, the electro-motive force of
the currents generated is increased, and consequently the vibrations of
the disks of the second telephone are more perceptible.’
_Mr. McTighe’s Telephone._--In this telephone, which has several
diaphragms, there is a horseshoe magnet, and instead of placing the
coils upon the poles, there is a single coil fastened to an iron core,
which is inserted between wide polar appendices fitted to the two poles
of the magnet. These appendices consist of thin plates, which act as
vibrating plates.
_Modifications in the arrangement of Telephonic Organs._--We see that
the forms given to the Bell telephone are very varied, and this is
still more the case with its constituent organs, without, however,
producing any remarkable improvements. Mr. Preece observes that little
has been gained by varying the size and strength of the magnets, and
the best effects have been obtained by using the horseshoe magnets
directed by Mr. Bell himself. The telephone was certainly introduced
into Europe with the arrangement which is theoretically the best,
although Mr. Bell is still occupied in improving it. This is also the
opinion of M. Hellesen, who, like Mr. Preece, has made many experiments
on this point; but this has not deterred several people from declaring
that they have discovered the way of making a telephone speak so as to
be audible to an assembly of people.
Of the different instruments made with this object, that of M. Righi
seems to be the most important. It was lately tried with success at the
Académie des Sciences, the Conservatoire des Arts et Métiers, and the
Press pavilion of the Exhibition.
[Illustration: FIG. 37.]
The receiver is only a Bell telephone of large size, with a diaphragm
of parchment L L (fig. 37), in the centre of which there is a
sheet-iron disk F. This membrane is stretched on a large funnel E,
which is fixed on a box C C, containing the electro-magnetic coil B:
and the magnet N S, much larger than in the ordinary instruments,
issues from the box, and serves as its support.
[Illustration: FIG. 38.]
The sender resembles the one represented in fig. 19, except that,
instead of liquid, M. Righi employs plumbago mixed with powdered
silver, and the platinum needle is replaced by a metallic disk D (fig.
38). The receiver I, which contains the powder, is supported on a
spring R, which can be pushed up and down by a regulating screw V,
and the whole is fitted into a box C C, and supported on a foot P.
The speaker places himself above the mouthpiece E, and the vibrations
transmitted to the membrane L L cause the variations of resistance in
I which are necessary for the transmission of speech, as in the Edison
system. Two Bunsen cells are enough to set the instrument at work, and
it will make the sound of a trumpet or flute audible throughout a room.
Vocal music, which is less intense, is necessarily transmitted to a
rather less distance, and words spoken in the natural voice are heard
by those standing about two yards and a half from the instrument.
The maximum distance at which the instrument has been worked with the
battery only is twenty-eight miles, the distance between Bologna and
Ferrara, and for greater distances it is necessary to have recourse to
induction coils.
In this case, an induction coil is introduced into the circuit at each
station, and its primary wire is traversed by a current from the local
battery, and so also is the sender, which is elsewhere connected with
the receiver by a commutator. The secondary circuit of these coils
is completed through the earth and line wire. From this arrangement
it follows that the induced current which influences the receiver in
correspondence, only produces its effect after a second induction,
produced on the primary wire of the local coil, and it appears that
this is a sufficient effect; but the advantage of this arrangement is,
that it is possible to receive and transmit sounds without the aid of
anything but the commutator.
Among other arrangements which have been suggested, we may mention one
in which, instead of the bar magnet, a horseshoe magnet is used, with
a vibrating plate placed between its poles. For this purpose the poles
are tipped with iron, and one of them is pierced with a hole which
corresponds to the mouthpiece of the instrument. The two branches of
the magnet are also furnished with magnetising helices. When anything
is spoken before the hole, the vibrating plate causes induced currents
in the two helices: these currents would be of opposite direction if
the poles were of like nature, but, since the magnetic poles are of
contrary nature, they are in the same direction. The vibrating plate
then acts like the two plates of M. Trouvé’s instrument, which we have
described above.
In another arrangement, lately made by Ader, the receiver is only an
ordinary two-branched magnet, of which the armature is supported, at
about two millimètres from its poles, by a glass plate to which it is
glued, and the plate itself is fastened to two rigid supports. In order
to hear it is only necessary to apply the ear to the plate. The sender
is a moveable rod of iron or carbon, which rests on a fixed piece
of carbon, with no pressure except its own weight, and it supports a
concave disk, to which the speaker applies his mouth. These two parts
are so arranged as to move horizontally, so that, when the instrument
is suspended, the circuit is forcibly disconnected by the fact of its
position, and is therefore closed until anyone takes it up to speak.
Speech is well reproduced by this system, and may be transmitted to
some distance if it is made on a larger scale.
Again, an anonymous inventor, in a little note inserted in ‘Les
Mondes,’ February 7, 1878, writes as follows: ‘Since the intensity of
the currents produced in the telephone is in proportion to the mass of
soft iron which vibrates before the pole of the magnet, and since, on
the other hand, the plate is sensitive in proportion to its tenuity, I
employ, instead of the ordinary plate, one reduced by nitric acid to
the least possible thickness, and I fix it to a circle of soft iron,
which keeps it stretched and forms part of the same substance. This
circle is placed in a circular opening made inside the compartment. The
intensity of a telephone is much increased when such a system replaces
the ordinary plate, even at one end of the line.’
In order to obtain vibrating plates of extreme tenuity, M. E. Duchemin
thought of employing very thin plates of mica, sprinkled with
pulverised iron fixed to the plate by a layer of silicate of potash.
The inventor asserts that it is possible to correspond in a low voice
with the aid of this system; but it has this inconvenience, that the
plate will be broken by speaking too loud.
Professor Jorgenson, of Copenhagen, has also made a Bell telephone
which produces very intense sounds, and which has permitted him to
observe some curious effects. In this instrument, the magnet is made
in a mode analogous to Nicklès’ tubular magnets. There is first a
cylindrical magnet with a core of soft iron at its upper end, to which
the coil is fitted; next, a magnetised tube, formed of a steel ring,
which encloses the first magnetic system, and is connected with it by
an iron tube. Finally, above the polar extremities of this system,
there is the vibrating disk, with the same arrangement as that of
ordinary telephones, and of which the superficies is large. If this
plate is only a millimètre in thickness, the words spoken can be heard
throughout a room; but the sounds lose their clearness when the ear
is approached to the vibrating plate, the words are confused, and
there is the reverberation which is observed on speaking in a place
apt to produce echoes: the listener is, in fact, stunned by the sounds
produced. On using a thicker plate--one, for example, of three or four
millimètres--the telephone only produces the effect of the ordinary
instruments, and it is necessary to apply the ear to it.
M. Marin Maillet, of Lyons, has suggested that the sounds reproduced by
the telephone might be increased by reflecting them through a certain
number of reflectors, which, by concentrating them in a focus on a
resonator, would considerably enlarge them. Since this idea was not
accompanied by experiments, it can hardly be regarded as serious.
TELEPHONIC EXPERIMENTS.
Since Mr. Bell’s experiments of which an account has been given in the
early part of this work, much study has been given by men of science
and inventors to the effects produced in this curious instrument, so as
to ascertain its theory and deduce improvements in its construction. We
will take a glance at these researches in succession.
_Experiments on the Effects produced by Voltaic and Induced
Currents._--The comparative study of the effects produced in the
telephone by voltaic and induced currents was one of the first and
most important. In 1873, as we have seen, Mr. Elisha Gray converted
the voltaic currents, which he employed to cause the vibrations of his
transmitting plate, into induced currents by means of an induction
coil, such as Ruhmkorff’s. The voltaic currents then traversed the
primary helix of the coil, and the induced currents reacted on the
receiving instrument, producing on its electro-magnetic system the
vibrations excited at the sending station. When Mr. Edison designed
his battery telephone, he had recourse to the same means to work his
receiving telephone, since he had ascertained that induced currents
were superior to voltaic currents. But this peculiarity of Mr. Edison’s
arrangement was not clearly understood from the descriptions which
reached Europe, so that several persons believed that they had invented
this arrangement--among others, Colonel Navez and MM. Pollard and
Garnier.
Colonel Navez, in an interesting paper on the new telephonic system,
presented to the Belgian Royal Academy, February 2, 1878, only suggests
this arrangement as a mode of reproducing speech at a great distance;
but he quotes no experiment which distinctly shows the advantages
of this combination. Twenty days later, MM. Pollard and Garnier,
unacquainted with Colonel Navez’s researches, sent to me the results
they had obtained by similar means, and these results appeared to me
so interesting that I communicated them to the Académie des Sciences,
February 25, 1878. In order that the importance of these results may
be clearly understood, I will repeat the text of M. Pollard’s letter,
addressed to me on February 20, 1878:
‘With the object of increasing the variations of electric intensity
in the Edison system, we induce a current in the circuit of a small
Ruhmkorff coil, and we fix the receiving telephone to the extremities
of the induced wire. The current received has the same intensity as
that of the inducing current, and consequently the variations produced
in the current which works the telephone have a much wider range. The
intensity of the transmitted sounds is strongly increased, and the
value of this increase depends upon the relative number of spirals in
the inducing and induced circuits. Our attempts to determine the best
proportions have been laborious, since it is necessary to make a coil
for each experiment; we have hitherto obtained excellent results with
a small Ruhmkorff coil reduced to its simplest form, that is, without
condenser or contact-breaker. The inducing wire is No. 16, and is wound
in five layers; the induced wire is No. 32, and in twenty layers. The
length of the coil is seven centimètres.
‘The following is the most remarkable and instructive experiment: When
setting the sender to work with a single Daniell cell, there is no
appreciable effect at the receiving station, at least in the telephone
which I have made, when it is in immediate connection with the circuit;
after inserting the small induction coil, sounds become distinctly
audible, and their intensity equals that of good ordinary telephones.
Since the battery current is only moderately intense, the points of
plumbago are not worn down, and the regulating apparatus lasts for a
long while. When a stronger battery is used, consisting of six cells
of bichromate of potash (in tension) or twelve Leclanché cells,
sufficient intensity is obtained by the direct action to make sounds
nearly as audible as in ordinary telephones; but when the induction
coil is inserted, the sounds become much more intense, and may be heard
at a distance of from fifty to sixty centimètres from the mouthpiece.
Songs may, under such circumstances, be heard at a distance of several
yards; but the relative increase does not appear to be so great as in
the case of the single Daniell cell.’
On the other hand, ‘Les Mondes,’ March 7, 1878, contains an account of
a series of experiments made by Signor Luvini, Professor of Physics
at the Military Academy of Turin, which proved that the introduction
of electro-magnets into the circuit which connects the two telephones
sensibly increases the intensity of sound. The maximum effect is
produced by placing one close to the transmitting, and the other close
to the receiving telephone, and the introduction of other magnets is
of no use. The inducing wire of a Ruhmkorff coil, when introduced into
such a circuit, excited no sensible effects of induction in the induced
circuit, and consequently could not set the telephone in connection
with this circuit at work. But the current of a Clarke machine produces
sounds resembling the beats of a drum, which are deafening when the ear
is applied to the instrument: they become very faint, however, at the
distance of a mètre. The currents of a Ruhmkorff machine are still
more energetic, and the sound fills a whole room. By modifying the
position of the lever of the coil, the sound passes through different
tones, which are always in unison with the breaks of the current, at
least up to a certain pitch.
This property of currents induced by the Ruhmkorff coil has enabled
M. Gaiffe to obtain by their means a very simple mode of regulating
telephones, so as to produce in them the maximum amount of sensibility.
For this purpose he places the telephone he proposes to regulate in the
circuit of an induction instrument with moveable helices and graduated
intensities. The sounds which result from the vibrator are then
reverberated from the telephone, and are audible at a distance from the
instrument; by using a screw-driver, it is possible to adjust the screw
to which the free end of the bar magnet of the instrument is fixed. It
can be tightened or loosened, so as to advance or withdraw the other
end of the magnet from the vibrating plate of the telephone, and the
process is repeated until the maximum intensity of sound is obtained.
On the other hand, as the sounds given out by the two telephones in
correspondence are intense in proportion to the degree of unison in
the vibrations produced by them, it is necessary to select those which
emit the same sounds for the same given note; and the mode we have just
described may be employed with advantage, since it will be enough to
observe what instruments give the same note in the condition of maximum
sensibility, when regulated in the same way by the induction machine.
It is very important that the telephones in correspondence should
be well matched, not only to ensure clear transmissions, but also
with reference to the tone of voice of those who are to use it. The
sound becomes more audible when the tone of voice corresponds to the
telephonic tone; and for this reason some telephones repeat the voices
of women and children better than those of men, and with others the
reverse takes place.
The telephonic vibrations vary in different instruments, and these
variations may be noted in the way we have indicated.