light as it enters the atmosphere of the earth, of the further
impediment which the rays encounter if they pass from the air into the
sea. In the main the causes which hinder a pulse committed to a cable
are two: induction, and the electrostatic capacity of the wire, that is,
the capacity of the wire to take up a charge of its own, just as if it
were the metal of a Leyden jar.
Let us first consider induction. As a current takes its way through the
copper core it induces in its surroundings a second and opposing
current. For this the remedy is one too costly to be applied. Were a
cable manufactured in a double line, as in the best telephonic circuits,
induction, with its retarding and quenching effects, would be
neutralized. Here the steel wire armour which encircles the cable plays
an unwelcome part. Induction is always proportioned to the conductivity
of the mass in which it appears; as steel is an excellent conductor, the
armour of an ocean cable, close as it is to the copper core, has induced
in it a current much stronger, and therefore more retarding, than if the
steel wire were absent.
A word now as to the second difficulty in working beneath the sea--that
due to the absorbing power of the line itself. An Atlantic cable, like
any other extended conductor, is virtually a long, cylindrical Leyden
jar, the copper wire forming the inner coat, and its surroundings the
outer coat. Before a signal can be received at the distant terminus the
wire must first be charged. The effect is somewhat like transmitting a
signal through water which fills a rubber tube; first of all the tube
is distended, and its compression, or secondary effect, really transmits
the impulse. A remedy for this is a condenser formed of alternate sheets
of tin-foil and mica, _C_, connected with the battery, _B_, so as to
balance the electric charge of the cable wire (Fig. 60). In the first
Atlantic line an impulse demanded one-seventh of a second for its
journey. This was reduced when Mr. Whitehouse made the capital discovery
that the speed of a signal is increased threefold when the wire is
alternately connected with the zinc and copper poles of the battery. Sir
William Thomson ascertained that these successive pulses are most
effective when of proportioned lengths. He accordingly devised an
automatic transmitter which draws a duly perforated slip of paper under
a metallic spring connected with the cable. To-day 250 to 300 letters
are sent per minute instead of fifteen, as at first.
[Illustration: Fig. 60.--Condenser]
In many ways a deep-sea cable exaggerates in an instructive manner the
phenomena of telegraphy over long aerial lines. The two ends of a cable
may be in regions of widely diverse electrical potential, or pressure,
just as the readings of the barometer at these two places may differ
much. If a copper wire were allowed to offer itself as a gateless
conductor it would equalize these variations of potential with serious
injury to itself. Accordingly the rule is adopted of working the cable
not directly, as if it were a land line, but indirectly through
condensers. As the throb sent through such apparatus is but momentary,
the cable is in no risk from the strong currents which would course
through it if it were permitted to be an open channel.
[Illustration: Fig. 61.--Reflecting galvanometer L, lamp; N, moving spot
of light reflected from mirror]
A serious error in working the first cables was in supposing that they
required strong currents as in land lines of considerable length. The
very reverse is the fact. Mr. Charles Bright, in _Submarine Telegraphs_,
says:
"Mr. Latimer Clark had the conductor of the 1865 and 1866 lines joined
together at the Newfoundland end, thus forming an unbroken length of
3,700 miles in circuit. He then placed some sulphuric acid in a very
small silver thimble, with a fragment of zinc weighing a grain or two.
By this primitive agency he succeeded in conveying signals through twice
the breadth of the Atlantic Ocean in little more than a second of time
after making contact. The deflections were not of a dubious character,
but full and strong, from which it was manifest than an even smaller
battery would suffice to produce somewhat similar effects."
[Illustration: Fig. 62.--Siphon recorder]
At first in operating the Atlantic cable a mirror galvanometer was
employed as a receiver. The principle of this receiver has often been
illustrated by a mischievous boy as, with a slight and almost
imperceptible motion of his hand, he has used a bit of looking-glass to
dart a ray of reflected sunlight across a wide street or a large room.
On the same plan, the extremely minute motion of a galvanometer, as it
receives the successive pulsations of a message, is magnified by a
weightless lever of light so that the words are easily read by an
operator (Fig. 61). This beautiful invention comes from the hands of Sir
William Thomson [now Lord Kelvin], who, more than any other electrician,
has made ocean telegraphy an established success.
[Illustration: Fig. 63.--Siphon record. "Arrived yesterday"]
In another receiver, also of his design, the siphon recorder, he began
by taking advantage of the fact, observed long before by Bose, that a
charge of electricity stimulates the flow of a liquid. In its original
form the ink-well into which the siphon dipped was insulated and charged
to a high voltage by an influence-machine; the ink, powerfully repelled,
was spurted from the siphon point to a moving strip of paper beneath
(Fig. 62). It was afterward found better to use a delicate mechanical
shaker which throws out the ink in minute drops as the cable current
gently sways the siphon back and forth (Fig. 63).
Minute as the current is which suffices for cable telegraphy, it is
essential that the metallic circuit be not only unbroken, but unimpaired
throughout. No part of his duty has more severely taxed the resources of
the electrician than to discover the breaks and leaks in his ocean
cables. One of his methods is to pour electricity as it were, into a
broken wire, much as if it were a narrow tube, and estimate the length
of the wire (and consequently the distance from shore to the defect or
break) by the quantity of current required to fill it.
FOOTNOTES:
[1] Henry M. Field, "History of the Atlantic Telegraph." New York:
Scribner, 1866.
[2] "A Century of Electricity." Boston, Houghton, Mifflin & Co., 1887.
BELL'S TELEPHONIC RESEARCHES
[From "Bell's Electric Speaking Telephones," by George B. Prescott,
copyright by D Appleton & Co., New York, 1884]
In a lecture delivered before the Society of Telegraph Engineers, in
London, October 31, 1877, Prof. A. G. Bell gave a history of his
researches in telephony, together with the experiments that he was led
to undertake in his endeavours to produce a practical system of multiple
telegraphy, and to realize also the transmission of articulate speech.
After the usual introduction, Professor Bell said in part:
It is to-night my pleasure, as well as duty, to give you some account of
the telephonic researches in which I have been so long engaged. Many
years ago my attention was directed to the mechanism of speech by my
father, Alexander Melville Bell, of Edinburgh, who has made a life-long
study of the subject. Many of those present may recollect the invention
by my father of a means of representing, in a wonderfully accurate
manner, the positions of the vocal organs in forming sounds. Together we
carried on quite a number of experiments, seeking to discover the
correct mechanism of English and foreign elements of speech, and I
remember especially an investigation in which we were engaged
concerning the musical relations of vowel sounds. When vocal sounds are
whispered, each vowel seems to possess a particular pitch of its own,
and by whispering certain vowels in succession a musical scale can be
distinctly perceived. Our aim was to determine the natural pitch of each
vowel; but unexpected difficulties made their appearance, for many of
the vowels seemed to possess a double pitch--one due, probably, to the
resonance of the air in the mouth, and the other to the resonance of the
air contained in the cavity behind the tongue, comprehending the pharynx
and larynx.
I hit upon an expedient for determining the pitch, which, at that time,
I thought to be original with myself. It consisted in vibrating a tuning
fork in front of the mouth while the positions of the vocal organs for
the various vowels were silently taken. It was found that each vowel
position caused the reinforcement of some particular fork or forks.
I wrote an account of these researches to Mr. Alex. J. Ellis, of London.
In reply, he informed me that the experiments related had already been
performed by Helmholtz, and in a much more perfect manner than I had
done. Indeed, he said that Helmholtz had not only analyzed the vowel
sounds into their constituent musical elements, but had actually
performed the synthesis of them.
He had succeeded in producing, artificially, certain of the vowel sounds
by causing tuning forks of different pitch to vibrate simultaneously by
means of an electric current. Mr. Ellis was kind enough to grant me an
interview for the purpose of explaining the apparatus employed by
Helmholtz in producing these extraordinary effects, and I spent the
greater part of a delightful day with him in investigating the subject.
At that time, however, I was too slightly acquainted with the laws of
electricity fully to understand the explanations given; but the
interview had the effect of arousing my interest in the subjects of
sound and electricity, and I did not rest until I had obtained
possession of a copy of Helmholtz's great work "The Theory of Tone," and
had attempted, in a crude and imperfect manner, it is true, to reproduce
his results. While reflecting upon the possibilities of the production
of sound by electrical means, it struck me that the principle of
vibrating a tuning fork by the intermittent attraction of an
electro-magnet might be applied to the electrical production of music.
I imagined to myself a series of tuning forks of different pitches,
arranged to vibrate automatically in the manner shown by Helmholtz--each
fork interrupting, at every vibration, a voltaic current--and the
thought occurred, Why should not the depression of a key like that of a
piano direct the interrupted current from any one of these forks,
through a telegraph wire, to a series of electro-magnets operating the
strings of a piano or other musical instrument, in which case a person
might play the tuning fork piano in one place and the music be audible
from the electro-magnetic piano in a distant city.
The more I reflected upon this arrangement the more feasible did it seem
to me; indeed, I saw no reason why the depression of a number of keys at
the tuning fork end of the circuit should not be followed by the audible
production of a full chord from the piano in the distant city, each
tuning fork affecting at the receiving end that string of the piano with
which it was in unison. At this time the interest which I felt in
electricity led me to study the various systems of telegraphy in use in
this country and in America. I was much struck with the simplicity of
the Morse alphabet, and with the fact that it could be read by sound.
Instead of having the dots and dashes recorded on paper, the operators
were in the habit of observing the duration of the click of the
instruments, and in this way were enabled to distinguish by ear the
various signals.
It struck me that in a similar manner the duration of a musical note
might be made to represent the dot or dash of the telegraph code, so
that a person might operate one of the keys of the tuning fork piano
referred to above, and the duration of the sound proceeding from the
corresponding string of the distant piano be observed by an operator
stationed there. It seemed to me that in this way a number of distinct
telegraph messages might be sent simultaneously from the tuning fork
piano to the other end of the circuit by operators, each manipulating a
different key of the instrument. These messages would be read by
operators stationed at the distant piano, each receiving operator
listening for signals for a certain definite pitch, and ignoring all
others. In this way could be accomplished the simultaneous transmission
of a number of telegraphic messages along a single wire, the number
being limited only by the delicacy of the listener's ear. The idea of
increasing the carrying power of a telegraph wire in this way took
complete possession of my mind, and it was this practical end that I had
in view when I commenced my researches in electric telephony.
[Illustration: Fig. 1]
In the progress of science it is universally found that complexity leads
to simplicity, and in narrating the history of scientific research it is
often advisable to begin at the end.
In glancing back over my own researches, I find it necessary to
designate, by distinct names, a variety of electrical currents by means
of which sounds can be produced, and I shall direct your attention to
several distinct species of what may be termed telephonic currents of
electricity. In order that the peculiarities of these currents may be
clearly understood, I shall project upon the screen a graphical
illustration of the different varieties.
The graphical method of representing electrical currents shown in Fig. 1
is the best means I have been able to devise of studying, in an accurate
manner, the effects produced by various forms of telephonic apparatus,
and it has led me to the conception of that peculiar species of
telephonic current, here designated as _undulatory_, which has rendered
feasible the artificial production of articulate speech by electrical
means.
A horizontal line (_g g'_) is taken as the zero of current, and impulses
of positive electricity are represented above the zero line, and
negative impulses below it, or _vice versa_.
The vertical thickness of any electrical impulse (_b_ or _d_), measured
from the zero line, indicates the intensity of the electrical current at
the point observed; and the horizontal extension of the electric line
(_b_ or _d_) indicates the duration of the impulse.
Nine varieties of telephonic currents may be distinguished, but it will
only be necessary to show you six of these. The three primary varieties
designated as intermittent, pulsatory and undulatory, are represented in
lines 1, 2 and 3.
Sub-varieties of these can be distinguished as direct or reversed
currents, according as the electrical impulses are all of one kind or
are alternately positive and negative. Direct currents may still
further be distinguished as positive or negative, according as the
impulses are of one kind or of the other.
An intermittent current is characterized by the alternate presence and
absence of electricity upon the circuit.
A pulsatory current results from sudden or instantaneous changes in the
intensity of a continuous current; and
An undulatory current is a current of electricity, the intensity of
which varies in a manner proportional to the velocity of the motion of a
particle of air during the production of a sound: thus the curve
representing graphically the undulatory current for a simple musical
note is the curve expressive of a simple pendulous vibration--that is, a
sinusoidal curve.
And here I may remark, that, although the conception of the undulatory
current of electricity is entirely original with myself, methods of
producing sound by means of intermittent and pulsatory currents have
long been known. For instance, it was long since discovered that an
electro-magnet gives forth a decided sound when it is suddenly
magnetized or demagnetized. When the circuit upon which it is placed is
rapidly made and broken, a succession of explosive noises proceeds from
the magnet. These sounds produce upon the ear the effect of a musical
note when the current is interrupted a sufficient number of times per
second....
[Illustration: Fig. 2]
For several years my attention was almost exclusively directed to the
production of an instrument for making and breaking a voltaic circuit
with extreme rapidity, to take the place of the transmitting tuning fork
used in Helmholtz's researches. Without going into details, I shall
merely say that the great defects of this plan of multiple telegraphy
were found to consist, first, in the fact that the receiving operators
were required to possess a good musical ear in order to discriminate the
signals; and secondly, that the signals could only pass in one direction
along the line (so that two wires would be necessary in order to
complete communication in both directions). The first objection was got
over by employing the device which I term a "vibratory circuit breaker,"
whereby musical signals can be automatically recorded....
I have formerly stated that Helmholtz was enabled to produce vowel
sounds artificially by combining musical tones of different pitches and
intensities. His apparatus is shown in Fig. 2. Tuning forks of different
pitch are placed between the poles of electro-magnets (_a1_, _a2_, &c.),
and are kept in continuous vibration by the action of an intermittent
current from the fork _b_. Resonators, 1, 2, 3, etc., are arranged so as
to reinforce the sounds in a greater or less degree, according as the
exterior orifices are enlarged or contracted.
[Illustration: Fig. 3]
Thus it will be seen that upon Helmholtz's plan the tuning forks
themselves produce tones of uniform intensity, the loudness being varied
by an external reinforcement; but it struck me that the same results
would be obtained, and in a much more perfect manner, by causing the
tuning forks themselves to vibrate with different degrees of amplitude.
I therefore devised the apparatus shown in Fig. 3, which was my first
form of articulating telephone. In this figure a harp of steel rods is
employed, attached to the poles of a permanent magnet, N. S. When any
one of the rods is thrown into vibration an undulatory current is
produced in the coils of the electro-magnet E, and the electro-magnet E'
attracts the rods of the harp H' with a varying force, throwing into
vibration that rod which is in unison with that vibrating at the other
end of the circuit. Not only so, but the amplitude of vibration in the
one will determine the amplitude of vibration in the other, for the
intensity of the induced current is determined by the amplitude of the
inducing vibration, and the amplitude of the vibration at the receiving
end depends upon the intensity of the attractive impulses. When we sing
into a piano, certain of the strings of the instrument are set in
vibration sympathetically by the action of the voice with different
degrees of amplitude, and a sound, which is an approximation to the
vowel uttered, is produced from the piano. Theory shows that, had the
piano a very much larger number of strings to the octave, the vowel
sounds would be perfectly reproduced. My idea of the action of the
apparatus, shown in Fig. 3, was this: Utter a sound in the neighbourhood
of the harp H, and certain of the rods would be thrown into vibration
with different amplitudes. At the other end of the circuit the
corresponding rods of the harp H would vibrate with their proper
relations of force, and the _timbre_ [characteristic quality] of the
sound would be reproduced. The expense of constructing such an apparatus
as that shown in figure 3 deterred me from making the attempt, and I
sought to simplify the apparatus before venturing to have it made.
[Illustration: Fig. 4]
[Illustration: Fig. 5]
[Illustration: Fig. 6]
I have before alluded to the invention by my father of a system of
physiological symbols for representing the action of the vocal organs,
and I had been invited by the Boston Board of Education to conduct a
series of experiments with the system in the Boston school for the deaf
and dumb. It is well known that deaf mutes are dumb merely because they
are deaf, and that there is no defect in their vocal organs to
incapacitate them from utterance. Hence it was thought that my father's
system of pictorial symbols, popularly known as visible speech, might
prove a means whereby we could teach the deaf and dumb to use their
vocal organs and to speak. The great success of these experiments urged
upon me the advisability of devising method of exhibiting the vibrations
of sound optically, for use in teaching the deaf and dumb. For some time
I carried on experiments with the manometric capsule of Koeenig and with
the phonautograph of Leon Scott. The scientific apparatus in the
Institute of Technology in Boston was freely placed at my disposal for
these experiments, and it happened that at that time a student of the
Institute of Technology, Mr. Maurey, had invented an improvement upon
the phonautograph. He had succeeded in vibrating by the voice a stylus
of wood about a foot in length, which was attached to the membrane of
the phonautograph, and in this way he had been enabled to obtain
enlarged tracings upon a plane surface of smoked glass. With this
apparatus I succeeded in producing very beautiful tracings of the
vibrations of the air for vowel sounds. Some of these tracings are shown
in Fig. 4. I was much struck with this improved form of apparatus, and
it occurred to me that there was a remarkable likeness between the
manner in which this piece of wood was vibrated by the membrane of the
phonautograph and the manner in which the _ossiculo_ [small bones] of
the human ear were moved by the tympanic membrane. I determined
therefore, to construct a phonautograph modelled still more closely
upon the mechanism of the human ear, and for this purpose I sought the
assistance of a distinguished aurist in Boston, Dr. Clarence J. Blake.
He suggested the use of the human ear itself as a phonautograph, instead
of making an artificial imitation of it. The idea was novel and struck
me accordingly, and I requested my friend to prepare a specimen for me,
which he did. The apparatus, as finally constructed, is shown in Fig. 5.
The _stapes_ [inmost of the three auditory ossicles] was removed and a
pointed piece of hay about an inch in length was attached to the end of
the incus [the middle of the three auditory ossicles]. Upon moistening
the membrana tympani [membrane of the ear drum] and the ossiculae with a
mixture of glycerine and water the necessary mobility of the parts was
obtained, and upon singing into the external artificial ear the piece of
hay was thrown into vibration, and tracings were obtained upon a plane
surface of smoked glass passed rapidly underneath. While engaged in
these experiments I was struck with the remarkable disproportion in
weight between the membrane and the bones that were vibrated by it. It
occurred to me that if a membrane as thin as tissue paper could control
the vibration of bones that were, compared to it, of immense size and
weight, why should not a larger and thicker membrane be able to vibrate
a piece of iron in front of an electro-magnet, in which case the
complication of steel rods shown in my first form of telephone, Fig. 3,
could be done away with, and a simple piece of iron attached to a
membrane be placed at either end of the telegraphic circuit.