can be useful to the telegraphic service itself by affording one of
the simplest means of obtaining a number of simultaneous transmissions
through the same wire, and even of being combined in duplex with
the Morse telegraphs. Its applications in the microphonic form are
incalculable, and the proverb which declares that ‘walls have ears’
may in this way be literally true. It is alarming to think of the
consequences of such an indiscreet organ. Diplomatists must certainly
redouble their reserve, and tender confidences will no longer be made
with the same frankness. On this point we cannot think that much will
be gained, but on the other hand the physician will probably soon make
use of this invention to ascertain more readily the processes going on
within the human body.
APPLICATION OF THE TELEPHONE TO SIMULTANEOUS TELEGRAPHIC TRANSMISSIONS.
The simultaneous transmission of several messages through the same wire
is one of the most curious and important applications of the telephone
to telegraphic instruments which can be made, and we have seen that it
was this application which led Messrs. Gray and Bell to the invention
of speaking telephones. The admiration which these instruments have
excited has thrown the original idea into the background, although it
has perhaps a more practical importance. We will now consider these
systems.
An articulating telephone is not necessary in order to obtain
simultaneous transmission: the musical telephones devised by MM.
Petrina, Gray, Froment, &c., are quite sufficient, and a brief
explanation of their principle will make this intelligible.
Suppose that there are seven electro-magnetic vibrators at the two
corresponding stations, which are tuned with the same tuning-fork
on the different notes of the scale, and suppose that a key-board,
resembling the Morse telegraph key, is arranged so that, by lowering
the keys, electric reaction takes place on each vibrator: it is easy
to see that these vibrators may be made to react in the same way on
the corresponding vibrators of the opposite station; but they must be
tuned on the same note, and the sounds emitted will continue while the
keys are lowered. By keeping them down for a shorter or longer time,
the long or short sounds which constitute the elements of telegraphic
language in the Morse system may be obtained, and consequently an
audible transmission becomes possible. Let us now suppose that a
telegraphist accustomed to this mode of transmission is placed before
each of the vibrators, and that they transmit different messages at the
same moment in this way: the telegraphic wire will be instantaneously
traversed by seven currents, broken and massed upon each other, and
they might be expected to produce a medley of confused sounds on the
vibrators at the receiving station; but since they each harmonise with
the corresponding vibrator, they have no sensible influence except on
those for which they are intended. The dominant sound may be made still
more distinct by applying a Helmholtz resonator to each vibrator,[18]
that is, an acoustic instrument which will only vibrate under the
influence of the note to which it is tuned. In this way it is possible
to select the transmitted sounds, and only to allow each _employé_ to
hear that which is intended for him. Consequently, however confused
the sounds may be on the receiving vibrators, the person to whom _do_
is assigned will only receive _do_ sounds, the person to whom _sol_ is
assigned will only receive _sol_ sounds, so that correspondence may be
carried on as well as if they had each a special wire.
In the mode we have described, this telegraphic system only admits
of audible transmissions, and consequently cannot register messages.
To supply this defect, it has been suggested to make the receiving
vibrators react on registers, so arranging the latter that their
electric organ may present such magnetic inertia, that, when it is
influenced by the vibrations of sound, its effect may be maintained
throughout the time of vibration. Experiments show that a Morse
receiver, worked by the current of a local battery, will be enough for
this purpose; so that if the musical vibrator is made to react as a
relay, that is, on a contact in connection with the local battery and
the receiver, the dots and dashes may be obtained on it which are the
constituent elements of the Morse code.
On these principles, and considering that the musical spaces
separating the different notes of the scale are such as may be easily
distinguished by the resonator, seven simultaneous transmissions may be
obtained on the same wire; but experience shows that it is necessary to
be content with a much smaller number. Yet this number may easily be
doubled by applying the mode of transmission in an opposite direction
to the system.
Mr. Bell states that the idea of applying the telephone to multiple
electric transmissions occurred simultaneously to M. Paul Lacour of
Copenhagen, to Mr. Elisha Gray of Chicago, to Mr. Varley of London,
and to Mr. Edison of New York; but there is some confusion here, for
we have already seen, from reference to the patents, that Mr. Varley’s
system dates from 1870, that of M. Paul Lacour from September 1874,
that of Mr. Elisha Gray from February 1875, and those of Messrs.
Bell and Edison were still later. Yet it appears from Mr. Gray’s
specification that he was the first to conceive and execute instruments
of the kind. In fact, in a specification drawn up on August 6, 1874,
he distinctly put forward the system we have described, and which is
the basis of those of which we have still to speak. This specification
was only an addition to two others made out in April and June 1874.
Mr. Varley’s system has only an indirect relation to the one we have
described. It appears from what Mr. Bell said on the subject in a paper
addressed to the Society of Telegraphic Engineers in London, that he
himself only attaches a secondary interest to this invention.
He said that he had been struck with the idea that the greater or
less duration of a musical sound might represent the dot and dash of
the telegraphic alphabet, and it occurred to him that simultaneous
telegraphic transmissions, of which the number should only be
limited by the delicacy of the sense of hearing, might be obtained
by suitable combinations of long and short sounds, and that these
should be effected by a keyboard of tuning-forks applied to one end
of a telegraphic line, and so arranged as to react electrically on
electro-magnetic instruments striking on the strings of a piano. For
this purpose it would be necessary to assign an employé to each of
the keys for the service of transmission, and to arrange that his
correspondent should only distinguish his peculiar note among all
those transmitted. It was this idea, Mr. Bell adds, which led to his
researches in telephony.
For several years he sought for the best mode of reproducing musical
sounds at a distance by means of vibrating rheotomes: the best results
were given by a steel plate vibrating between two contacts, of which
the vibrations were electrically produced and maintained by an
electro-magnet and a local battery. In consequence of its vibration,
the two contacts were touched alternately, and the two circuits
were alternately broken; the local circuit which kept the plate in
vibration, and the other which was connected with the line, and reacted
on the distant receiver, so as to effect simultaneous vibrations in
it. A Morse key was placed in the latter circuit near the sending
instrument, and when it was lowered, vibrations were sent through the
line; when it was raised, these vibrations ceased, and it is easy to
see that, by lowering the key for a longer or shorter time, the short
and long sounds necessary for the different combinations of telegraphic
language could be obtained. Moreover, if the vibrating plate of the
receiving instrument were so regulated as to vibrate in unison with the
sending instrument in correspondence, it would vibrate better with this
sender than with another whose plate was not so adjusted.
It is evident that different sounds might be simultaneously transmitted
with several plates by this arrangement of contact breaker, and that
at the receiving station the sounds might be distinguished by each
employé, since the one which corresponds to the fundamental note of
each vibrating plate is reproduced by that plate. Consequently, the
sounds produced by the vibrating plate of _do_, for example, will only
be audible at the receiving station on the plate tuned to _do_, and the
same will be the case with the other plates; so that the sounds will
reach their destination, if not without confusion, yet with sufficient
clearness to be distinguished by the employés.
Mr. Bell sums up the defects still existing in his system as
follows:--1st. The receiver of the messages must have a good musical
ear, in order to distinguish the value of sounds. 2nd. Since the
signals can only take place when the transmitted currents are in
the same direction, two wires must be employed in order to exchange
messages on each side.
He surmounted the first difficulty by providing the receiver with an
instrument which he called the vibrating contact breaker, and which
registered automatically the sounds produced. This contact breaker was
placed in the circuit of a local battery, which could work a Morse
instrument under certain conditions. When the sounds emitted by the
instrument did not correspond with those for which it had been tuned,
the contact breaker had no effect on the telegraphic instrument: it
only acted when the sounds were those which were to be interpreted, and
its action necessarily corresponded to the length of the sounds.
Mr. Bell adds that he applied the system to electro-chemical
telegraphs; but we need not dwell on this part of the invention, since,
as we have said, it is no longer his special study.
_System of M. Lacour of Copenhagen._--M. Lacour’s system was patented
on the 2nd September, 1874, but his experiments were commenced on the
5th June of the same year. Since M. Lacour believed that the vibrations
would be imperceptible on long lines, his first attempts were made on
a somewhat short line; but in November 1874 fresh experiments were made
between Fredericia and Copenhagen on a line 225 miles in length, and
it was ascertained that vibratory effects could be easily transmitted,
even under the influence of a rather weak battery.
[Illustration: FIG. 59.]
In M. Lacour’s system, the sending instrument is a simple tuning-fork,
placed in a horizontal position, and one of its arms reacts on a
contact breaker, which can produce precisely the same number of
discharges of currents as there are vibrations of the tuning-fork. If
a Morse manipulator is inserted in the circuit, it is evident that
if it is worked so as to produce the dots and dashes of the Morse
alphabet, the same signals will be reproduced at the opposite station,
and the signals will be manifested by long and short sounds, if an
electro-magnetic receiver is connected with the circuit. This sender is
shown fig. 59.
Fig. 60 represents M. Lacour’s receiver. It consists of a tuning-fork F
made of soft iron, not of steel like the sending tuning-fork, and each
of its branches is inserted in the bobbin of an electro-magnetic coil
C C; two distinct electro-magnets M M react close to the extremities
of the fork, in such a way that the polarities developed on the two
branches of the fork under the influence of the coils C C should be of
contrary signs to those of the electro-magnets M M.
[Illustration: FIG. 60.]
If this double electro-magnetic system is inserted in a line circuit,
it follows that, for each discharge of the transmitted current, a
corresponding attraction of the branches of the tuning-fork will take
place, and consequently there will be a vibration, producing a sound,
if the discharges are numerous. This sound will naturally be short or
long in proportion to the duration of the sender’s action, and it
will be the same as that of the tuning-fork in that instrument. Again,
if one branch of the tuning-fork reacts on a contact P inserted in
the circuit of the local battery communicating with a Morse receiver,
traces will be produced on this receiver of length varying with the
duration of the sounds, for the Morse electro-magnet will be so quickly
affected by the successive breaks in the current that its armature
will remain stationary throughout each vibration. ‘I have not yet been
able,’ said M. Lacour in an address delivered before the Danish Academy
of Science in 1875, ‘to calculate the time necessary for the production
of definite vibrations in the tuning-fork. Different factors have to be
considered, but experiment has shown that the time which elapses before
the local circuit is broken is such a small fraction of a second as to
be almost inappreciable, even when the current is very weak.
‘Since intermittent currents only affect a tuning-fork on condition
that it vibrates in unison with the one which produces them, it follows
that if a series of sending tuning-forks, tuned to the different notes
of the scale, is placed at one end of a circuit, and if a similar
series of electro-magnetic tuning-forks, in exact accordance with the
first, is placed at the other end of the circuit, the intermittent
currents transmitted by the sending tuning-forks will be added to each
other without becoming confused, and each of the receiving tuning-forks
will only be affected by the currents emitted by the tuning-fork in
unison with it. In this way the combinations of elementary signals
representing a word may be telegraphed simultaneously.’
M. Lacour enumerates the ways in which this system may be applied as
follows: ‘If the keys in connection with the sending tuning-forks
are placed side by side, and are lowered in succession, or two or
three together, it will be enough to play on the keys as on a musical
instrument, in order that the air may be heard at the receiving
station, or the signals transmitted simultaneously may each belong to
a different message. This system will therefore allow the furthest
station on a line to communicate with one or several intermediate
stations, and _vice versa_, without disturbing the communication
at other stations. In this way two stations can exchange signals,
unperceived by the rest. The power of sending many signals at once
affords a good means of improving the autographic telegraph. In the
instruments now in use, such as those of Caselli and D’Arlincourt,
there is only one tracing stylus, and this stylus must pass over the
whole surface of the telegram in order to obtain a copy of it, but with
the telephone a certain number of styli may be placed side by side
in the form of a comb, and this comb need only be drawn in a certain
direction to pass over the surface of the telegram. In this way a more
faithful copy will be obtained in a shorter time.’
M. Lacour also observes that his system possesses a merit already
pointed out by Mr. Varley, namely, that the instruments permit the
passage of ordinary currents without revealing their presence, whence
it follows that the accidental currents which often disturb telegraphic
transmissions will have no effect on these systems.
M. Lacour began without applying an electro-magnetic system to his
instrument in order to maintain the movement of the tuning-fork,
but he soon saw that this accessory was indispensable, and he made
the tuning-forks themselves electro-magnetic. It also occurred to
him to convert the transmitted currents into pulsatory currents by
inserting an induction coil in the circuit, which was also done by
Mr. Elisha Gray. Finally, in order to obtain the immediate action
of the tuning-forks and the immediate cessation of their action, he
constructed them so as to reduce their inertia as much as possible.
This was effected by inserting the two branches of the tuning-fork in
the same coil and by lengthening its handle, and turning it back so
that it might pass through a second coil, dividing into two branches
and embracing the two vibrating branches, but without touching them.
When a current traverses both coils, it produces, in the kind of
horseshoe magnet formed by the two systems, opposite polarities which
provoke a double reaction in the vibrating branches--a reaction by
repulsion exerted by the two branches in virtue of the same polarity,
and a reaction by attraction by the other two branches in virtue of
their opposite polarities; and this double action is repeated by the
movements of a contact breaker applied to one of the vibrating branches
of the tuning-fork.
[Illustration: FIG. 61.]
_Mr. Elisha Gray’s System._--According to the system originally
patented, each sender, represented fig. 61, consists of an
electro-magnet M M resting below a small copper tablet B S, in such a
way that its poles pass through this tablet and are on a level with
its upper surface. A steel plate A S is fixed above these poles; its
tension can be regulated by means of a screw S; and another screw _c_
is placed on the plate, and is in electric communication with a local
battery R′ by means of a Morse key. Below the plate A S there is a
contact _d_ connected with the line wire L; this contact is met by
the plate at the moment of its attraction by the electro-magnet, and
breaks the current of a line battery P, which acts on the receiver of
the opposite station. Finally, the electric communication established
between the local battery R′ and the electro-magnet, as may be seen
in the figure, produces vibrations in the steel plate A S at each
lowering of the key, as in the case of ordinary vibrations--vibrations
which, with a suitable tension of the plate and a given intensity of
the battery R′, can produce a definite musical note. Moreover, since
at each vibration the plate A S meets the contact, discharges of the
line current take place through the line L, and react on the receiving
instrument, causing it to reproduce exactly the same vibrations as
those of the sending instrument.
[Illustration: FIG. 62.]
The receiving instrument represented fig. 62 exactly resembles the one
we have just described, except that there is no contact _d_ below the
vibrating plate A S, and the contact _c_, instead of communicating
with the line wire, is in electric connection with a register E and
a local battery P. It follows from this arrangement that when the
plate A S vibrates under the influence of the broken currents passing
through the electro-magnet M M, similar vibrations are sent through
the register; but if the electro-magnetic organ of this register is
properly regulated, these vibrations can only produce the effect of
a continuous current, and hence the length of the traces left on the
instrument will vary with the duration of the sounds produced. In this
way the registration of the dashes and dots which constitute the signs
of the Morse vocabulary will be effected.
If it is remembered that the plate A S vibrates under the influence
of electro-magnetic attractions more readily in proportion to their
approximation in number to the vibrations corresponding to the
fundamental sound it can emit, it becomes clear that if this plate is
tuned to the same note as that of the corresponding instrument, it
will be rendered peculiarly sensitive to the vibrations transmitted
by the sender, and the other vibrations which may affect it will only
act faintly. Moreover, a resonator placed above the plate will greatly
increase this predisposition; so that if several systems of this kind,
tuned to different notes, produce simultaneous transmissions, the
sounds corresponding to the different vibrations will be in a certain
sense selected and distributed, in spite of their combination, into the
receivers for which they are specially adapted, and each of them may
retain the traces of the sounds emitted by adding the register, which
may be so arranged as to act as an ordinary Morse receiver. Mr. Gray
states that the number of sending instruments and independent local
circuits may be equal to that of the tones and semitones of two or more
octaves, provided that each vibrating plate be tuned to a different
note of the scale. The instruments may be placed side by side, and
their respective local keys, arranged like the keys of a piano, will
make it easy to play an air combining notes and chords; there may also
be an interval between the instruments, which may be sufficiently far
from each other to allow the employés to work without being distracted
by sounds not intended for them.
In a new arrangement, exhibited at the Paris Exhibition, 1878, Mr. Gray
considerably modified the way of working the various electro-magnetic
organs which we have just described. In this case, the plates consist
of tuning-forks with one branch kept in continual vibration at both
stations, and the signals only become perceptible by intensifying the
sounds produced. This arrangement follows from the necessity of keeping
the line circuit always closed for multiple transmissions of this
nature, so as to react with pulsatory currents, which are alone able,
as we have already seen, to retain the individual character of several
sounds simultaneously transmitted.
[Illustration: FIG. 63.]
Under these conditions, the sender consists, as we see (fig. 63), of
a bar tuning-fork, _a_, which is grooved for the passage of a runner,
heavy enough to tune the fork to the desired note, and it oscillates
between two electro-magnets _e_ and _f_ and two contacts I and G. The
difference of resistance in the electro-magnets is very great: in the
one _f_ the resistance is equal to 2¾ miles of telegraphic wire, in
the other it does not exceed 440 yards. When electric communication is
established as we see in the figure, the following effect takes place.
Since the current of the local battery through the two electro-magnets
is broken by the rest-contact of the Morse key H, the plate _a_ is
subject to two contrary actions; but since the electro-magnet _f_ has
more turns than the electro-magnet _e_, its action is preponderant,
and the plate is attracted towards _f_, and produces a contact with
the spring G, which opens a way of less resistance for the current.
Since the current then passes almost wholly through G, _b_, 1, 2, B,
the electro-magnet is now able to act; the plate _a_ is then attracted
towards _e_, and, by producing a contact on the spring I, it sends
the current of the line B P through the telegraphic line, if the key
H is at the same time lowered on the sending contact: if not, there
will be no effect in this direction, but since the plate _a_ has left
the spring G, the first effect of attraction by the electro-magnet
_f_ will be repeated, and this tends to draw the plate again towards
_f_. This state of things is repeated indefinitely so as to maintain
the vibration of the plate, and to send out signals corresponding with
these vibrations whenever the key H is lowered. The elastic nature of
the plate makes these vibrations more easy, and it ought also to be put
in mechanical vibration at the outset.
The receiver, represented fig. 64, consists of an electro-magnet
M, mounted on a sounding-box C, and having an armature formed by a
tuning-fork L L firmly buttressed on the box by a cross bar T. There
is a runner P on the armature, sliding in a groove, which makes it
possible to tune the vibrations of the tuning-fork to the fundamental
note of the sounding-box C, which is so arranged as to vibrate
in unison with it. Under these conditions, the box as well as the
tuning-fork will act as an analyser of the vibrations transmitted by
the currents, and may set the register at work by itself reacting on
a breaker of the local current. To obtain this result, a membrane of
gold-beater’s skin or parchment must be stretched before the opening
of the box, and a platinum contact must be applied to it, so arranged
as to meet a metallic spring connected with any kind of register or a
Morse instrument, when the membrane vibrates. As, however, in America
the messages are generally received by sound, this addition to the
system is not in use.
[Illustration: FIG. 64.]
The instrument is not only regulated by the runner P, but also by a
regulating screw V which allows the electro-magnet M to be properly
adjusted. The regulating system is made more exact by the small screw
V, and the instrument is connected with the line by the binding screw
B. Of course this double arrangement is necessary for each of the
sending systems.