station a drumhead of goldbeaters’ skin, about 2¾ inches in diameter,
was placed in front of an electro-magnet. To the middle of the
drumhead, on the side towards the magnet, was glued a circular piece
of clockspring. A similar electro-magnet, drumhead, etc., were placed
at the other station. When notes were sung or words spoken before one
drumhead, the vibrations of the goldbeaters’ skin carried the small
piece of clockspring vibratingly towards and from the electro-magnet,
without producing actual contact. Now, the current which was passing
along the coil round the electro-magnet changed in strength with each
change of position of this small piece of metal. The more rapid the
vibrations, and the greater their amplitude, the more rapid and the
more intense were the changes in the power of the electric current.
Thus, the electro-magnet at the other station underwent changes of
power which were synchronous with, and proportionate to, those changes
of power in the current which were produced by the changes of position
of the vibrating piece of clockspring. Accordingly, the piece of
clockspring at the receiving station, and with it the drumhead there,
was caused by the electro-magnet to vibrate with the same rapidity and
energy as the piece at the transmitting station. Therefore, as the
drumhead at one station varied its vibrations in response to the sounds
uttered in its neighbourhood, so the drumhead at the other station,
varying its vibrations, emitted similar sounds. Later, the receiving
drumhead was made unlike the transmitting one. Instead of a membrane
carrying a small piece of metal, a thin and very flexible disc of
sheet-iron, held in position by a screw, was used. This disc, set in
vibration by the varying action of an electro-magnet, as in the older
arrangement, uttered articulate sounds corresponding to those which,
setting in motion the membrane at the transmitting station, caused the
changes in the power of the electric current and in the action of the
electro-magnet.
At the meeting of the British Association in 1876 Sir W. Thomson gave
the following account of the performance of this instrument at the
Philadelphia Exhibition:—“In the Canadian department” (for Professor
Bell was not at the time an American citizen) “I heard ‘To be or not
to be—there’s the rub,’ through the electric wire; but, scorning
monosyllables, the electric articulation rose to higher flights, and
gave me passages taken at random from the New York newspapers:—‘S. S.
Cox has arrived’ (I failed to make out the ‘S. S. Cox’), ‘the City
of New York,’ ‘Senator Morton,’ ‘the Senate has resolved to print a
thousand extra copies,’ ‘the Americans in London have resolved to
celebrate the coming Fourth of July.’ All this my own ears heard
spoken to me with unmistakable distinctness by the thin circular disc
armature of just such another little electro-magnet as this which I
hold in my hand. The words were shouted with a clear and loud voice
by my colleague judge, Professor Watson, at the far end of the line,
holding his mouth close to a stretched membrane, carrying a piece of
soft iron, which was thus made to perform in the neighbourhood of an
electro-magnet, in circuit with the line, motions proportional to the
sonorific motions of the air. This, the greatest by far of all the
marvels of the electric telegraph, is due to a young countryman of
our own, Mr. Graham Bell, of Edinburgh, and Montreal, and Boston, now
about to become a naturalized citizen of the United States. Who can
but admire the hardihood of invention which devised such very slight
means to realize the mathematical conception that, if electricity is
to convey all the delicacies of quality which distinguish articulate
speech, the strength of its current must vary continuously, and as
nearly as may be in simple proportion to the velocity of a particle of
air engaged in constituting the sound?”
Since these words were spoken by one of the highest authorities in
matters telegraphic, Professor Bell has introduced some important
modifications in his apparatus. He now employs, not an electro-magnet,
but a permanent magnet. That is to say, instead of using at each
station such a bar of soft iron as is shown in Fig. 6, which becomes
a magnet while the electric current is passing through the coil
surrounding it, he uses at each station a bar of iron permanently
magnetized (or preferably a powerful magnet made of several horse-shoe
bars—that is, a compound magnet), surrounded similarly by coils of
wire. No battery is needed. Instead of a current through the coils
magnetizing the iron, the iron already magnetized causes a current
to traverse the coils whenever it acts, or rather whenever its action
changes. If an armature were placed across its ends or poles, at
the moment when it drew that armature to the poles by virtue of its
magnetic power, a current would traverse the coils; but afterwards,
so long as the armature remained there, there would be no current. If
an armature placed near the poles were shifted rapidly in front of
the poles, currents would traverse the coils, or be induced, their
intensity depending on the strength of the magnet, the length of the
coil, and the rapidity and range of the motions. In front of the
poles of the magnet is a diaphragm of very flexible iron (or else
some other flexible material bearing a small piece of iron on the
surface nearest the poles). A mouthpiece to converge the sound upon
this diaphragm substantially completes the apparatus at each station.
Professor Bell thus describes the operation of the instrument:—“The
motion of steel or iron in front of the poles of a magnet creates a
current of electricity in coils surrounding the poles of the magnet,
and the duration of this current of electricity coincides with the
duration of the motion of the steel or iron moved or vibrated in the
proximity of the magnet. When the human voice causes the diaphragm to
vibrate, electrical undulations are induced in the coils around the
magnets precisely similar to the undulations of the air produced by the
voice. The coils are connected with the line wire, and the undulations
induced in them travel through the wire, and, passing through the coils
of another instrument of similar construction at the other end of the
line, are again resolved into air undulations by the diaphragm of this
(other) instrument.”
So perfectly are the sound undulations repeated—though the instrument
has not yet assumed its final form—that not only has the lightest
whisper uttered at one end of a line of 140 miles been distinctly heard
at the other, but the speaker can be distinguished by his voice when he
is known to the listener. So far as can be seen, there is every room
to believe that before long Professor Bell’s grand invention will be
perfected to such a degree that words uttered on the American side of
the Atlantic will be heard distinctly after traversing 2000 miles under
the Atlantic, at the European end of the submarine cable—so that Sir
W. Thomson at Valentia could tell by the voice whether Graham Bell,
or Cyrus Field, or his late colleague Professor Watson, were speaking
to him from Newfoundland. Yet a single wave of those which toss in
millions on the Atlantic, rolling in on the Irish strand, would utterly
drown the voices thus made audible after passing beneath two thousand
miles of ocean.
Here surely is the greatest of telegraphic achievements. Of all the
marvels of telegraphy—and they are many—none are equal to, none
seem even comparable with, this one. Strange truly is the history of
the progress of research which has culminated in this noble triumph,
wonderful the thought that from the study of the convulsive twitchings
of a dead frog by Galvani, and of the quivering of delicately poised
magnetic needles by Ampère, should gradually have arisen through
successive developments a system of communication so perfect and so
wonderful as telegraphy has already become, and promising yet greater
marvels in the future.
The last paragraph had barely been written when news arrived of another
form of telephone, surpassing Gray’s and La Cour’s in some respects as
a conveyor of musical tones, but as yet unable to speak like Bell’s. It
is the invention of Mr. Edison, an American electrician. He calls it
the motograph. He discovered about six years ago the curious property
on which the construction of the instrument depends. If a piece of
paper moistened with certain chemical solutions is laid upon a metallic
plate connected with the positive pole of a galvanic battery, and a
platinum wire connected with the negative pole is dragged over the
moistened paper, the wire slides over the paper like smooth iron over
ice—the usual friction disappearing so long as the current is passing
from the wire to the plate through the paper. At the receiving station
of Mr. Edison’s motograph there is a resonating box, from one face
of which extends a spring bearing a platinum point, which is pressed
by the spring upon a tape of chemically prepared paper. This tape is
steadily unwound, drawing by its friction the platinum point, and
with it the face of the resonator, outwards. This slight strain on
the face of the resonator continues so long as no current passes from
the platinum point to the metallic drum over which the moistened tape
is rolling. But so soon as a current passes, the friction immediately
ceases, and the face of the resonator resumes its normal position. If
then at the transmitting station there is a membrane or a very fine
diaphragm (as in Reuss’s or Bell’s arrangement) which is set vibrating
by a note of any given tone, the current, as in those arrangements, is
transmitted and stopped at intervals corresponding to the tone, and the
face of the resonating box is freed and pulled at the same intervals.
Hence, it speaks the corresponding tone. The instrument appears to have
the advantage over Gray’s in range. In telegraphic communication Gray’s
telephone is limited to about one octave. Edison’s extends from the
deepest bass notes to the highest notes of the human voice, which, when
magnets are employed, are almost inaudible. But Edison’s motograph has
yet to learn to speak.
Other telegraphic marvels might well find a place here. I might speak
of the wonders of submarine telegraphy, and of the marvellous delicacy
of the arrangements by which messages by the Atlantic Cable are read,
and not only read, but made to record themselves. I might dwell, again,
on the ingenious printing telegraph of Mr. Hughes, which sets up its
own types, inks them, and prints them, or on the still more elaborate
plan of the Chevalier Bonelli “for converting the telegraph stations
into so many type-setting workshops.” But space would altogether
fail me to deal properly with these and kindred marvels. There is,
however, one application of telegraphy, especially interesting to the
astronomer, about which I must say a few words: I mean, the employment
of electricity as a regulator of time. Here again it is the principle
of the system, rather than details of construction, which I propose to
describe. Suppose we have a clock not only of excellent construction,
but under astronomical surveillance, so that when it is a second or so
in error it is set right again by the stars. Let the pendulum of this
clock beat seconds; and at each beat let a galvanic current be made and
broken. This may be done in many ways—thus the pendulum may at each
swing tilt up a very light metallic hammer, which forms part of the
circuit when down; or the end of the pendulum may be covered with some
non-conducting substance which comes at each swing between two metallic
springs in very light contact, separating them and so breaking circuit;
or in many other ways the circuit may be broken. When the circuit
is made, let the current travel along a wire which passes through a
number of stations near or remote, traversing at each the coils of a
temporary magnet. Then, at each swing of the pendulum of the regulating
clock, each magnet is magnetized and demagnetized. Thus each, once in
a second, draws to itself, and then releases its armature, which is
thereupon pulled back by a spring. Let the armature, when drawn to the
magnet, move a lever by which one tooth of a wheel is carried forward.
Then the wheel is turned at the rate of one tooth per second. This
wheel communicates motion to others in the usual way. In fact, we have
at each station a clock driven, _not_ by a weight or spring and with a
pendulum which allows one tooth of an escapement wheel to pass at each
swing, but by the distant regulating clock which turns a driving wheel
at the rate of one tooth per second, that is, one tooth for each swing
of the regulating clock’s pendulum. Each clock, then, keeps perfect
time with the regulating clock. In astronomy, where it is often of the
utmost importance to secure perfect synchronism of observation, or the
power of noting the exact difference of time between observations made
at distant stations, not only can the same clock thus keep time for
two observers hundreds of miles apart, but each observer can record by
the same arrangement the moment of the occurrence of some phenomenon.
For if a tape be unwound automatically, as in the Morse instrument,
it is easy so to arrange matters that every second’s beat of the
pendulum records itself by a dot or short line on the tape, and that
the observer can with a touch make (or break) contact at the instant of
observation, and so a mark be made properly placed between two seconds’
marks—thus giving the precise time when the observation was made. Such
applications, however, though exceedingly interesting to astronomers,
are not among those in which the general public take chief interest.
There was one occasion, however, when astronomical time-relations were
connected in the most interesting manner with one of the greatest of
all the marvels of telegraphy: I mean, when the _Great Eastern_ in
mid-ocean was supplied regularly with Greenwich time, and this so
perfectly (and therefore with such perfect indication of her place in
the Atlantic), that when it was calculated from the time-signals that
the buoy left in open ocean to mark the place of the cut cable had been
reached, and the captain was coming on deck with several officers to
look for it, the buoy announced its presence by thumping the side of
the great ship.
_THE PHONOGRAPH, OR VOICE-RECORDER._
In the preceding essay I have described the wonderful instrument called
the telephone, which has recently become as widely known in this
country as in America, the country of its first development. I propose
now briefly to describe another instrument—the phonograph—which,
though not a telegraphic instrument, is related in some degree to the
telephone. In passing, I may remark that some, who as telegraphic
specialists might be expected to know better, have described the
phonograph as a telegraphic invention. A writer in the _Telegraphic
Journal_, for instance, who had mistaken for mine a paper on the
phonograph in one of our daily newspapers, denounced me (as the
supposed author of that paper) for speaking of the possibility of
crystallizing sound by means of this instrument; and then went on to
speak of the mistake I (that is, said author) had made in leaving my
own proper subject of study to speak of telegraphic instruments and
to expatiate on the powers of electricity. In reality the phonograph
has no relation to telegraphy whatever, and its powers do not in the
slightest degree depend on electricity. If the case had been otherwise,
it may be questioned whether the student of astronomy, or of any other
department of science, should be considered incompetent of necessity
to describe a telegraphic instrument, or to discuss the principles
of telegraphic or electrical science. What should unquestionably be
left to the specialist, is the description of the practical effect of
details of instrumental construction, and the like—for only he who is
in the habit of using special instruments or classes of instrument can
be expected to be competent adequately to discuss such matters.
Although, however, the phonograph is not an instrument depending, like
the telephone, on the action of electricity (in some form or other),
yet it is related closely enough to the telephone to make the mistake
of the _Telegraphic_ journalist a natural one. At least, the mistake
would be natural enough for any one but a telegraphic specialist; the
more so that Mr. Edison is a telegraphist, and that he has effected
several important and interesting inventions in telegraphic and
electrical science. For instance, in the previous article, pp. 270,
271, I had occasion to describe at some length the principles of his
“Motograph.” I spoke of it as “another form of telephone, surpassing
Gray’s and La Cour’s in some respects as a conveyer of musical
tones, but as yet unable to speak like Bell’s ... in telegraphic
communication.” I proceeded: “Gray’s telephone is limited to about one
octave. Edison’s extends from the deepest bass notes to the highest
notes of the human voice, which, when magnets are employed, are almost
inaudible; but it has yet to learn to speak.”
The phonograph is an instrument which _has_ learned to speak, though
it does not speak at a distance like the telephone or the motograph.
Yet there seems no special reason why it should not combine both
qualities—the power of repeating messages at considerable intervals of
time after they were originally spoken, and the power of transmitting
them to great distances.
I have said that the phonograph is an instrument closely related to the
telephone. If we consider this feature of the instrument attentively,
we shall be led to the clearer recognition of the acoustical principles
on which its properties depend, and also of the nature of some of the
interesting acoustical problems on which light seems likely to be
thrown by means of experiments with this instrument.
In the telephone a stretched membrane, or a diaphragm of very flexible
iron, vibrates when words are uttered in its neighbourhood. When
a stretched membrane is used, with a small piece of iron at the
centre, this small piece of iron, as swayed by the vibrations of the
membrane, causes electrical undulations to be induced in the coils
round the poles of a magnet placed in front of the membrane. These
undulations travel along the wire and pass through the coils of another
instrument of similar construction at the other end of the wire, where,
accordingly, a stretched membrane vibrates precisely as the first had
done. The vibrations of this membrane excite atmospheric vibrations
identical in character with those which fell upon the first membrane
when the words were uttered in its neighbourhood; and therefore the
same words appear to be uttered in the neighbourhood of the second
membrane, however far it may be from the transmitting membrane, so only
that the electrical undulations are effectually transmitted from the
sending to the receiving instrument.
I have here described what happened in the case of that earlier form
of the telephone in which a stretched membrane of some such substance
as goldbeater’s skin was employed, at the centre of which only was
placed a small piece of iron. For in its bearing on the subject of
the phonograph, this particular form of telephonic diaphragm is
more suggestive than the later form in which very flexible iron was
employed. We see that the vibrations of a small piece of iron at the
centre of a membrane are competent to reproduce all the peculiarities
of the atmospheric waves which fall upon the membrane when words are
uttered in its neighbourhood. This must be regarded, I conceive, as a
remarkable acoustical discovery. Most students of acoustics would have
surmised that to reproduce the motions merely of the central parts
of a stretched diaphragm would be altogether insufficient for the
reproduction of the complicated series of sound-waves corresponding to
the utterance of words. I apprehend that if the problem had originally
been suggested simply as an acoustical one, the idea entertained
would have been this—that though the motions of a diaphragm receiving
vocal sound-waves _might_ be generated artificially in such sort as to
produce the same vocal sounds, yet this could only be done by first
determining what particular points of the diaphragm were centres of
motion, so to speak, and then adopting some mechanical arrangements for
giving to small portions of the membrane at these points the necessary
oscillating motions. It would not, I think, have been supposed that
motions communicated to the centre of the diaphragm would suffice to
make the whole diaphragm vibrate properly in all its different parts.
Let us briefly consider what was before known about the vibrations
of plates, discs, and diaphragms, when particular tones were sounded
in their neighbourhood; and also what was known respecting the
requirements for vocal sounds and speech as distinguished from simple
tones. I need hardly say that I propose only to consider these points
in a general, not in a special, manner.
We must first carefully draw a distinction between the vibrations of a
plate or disc which is itself the source of sound, and those vibrations
which are excited in a plate or disc by sound-waves otherwise
originated. If a disc or plate of given size be set in vibration by a
blow or other impulse it will give forth a special sound, according to
the place where it is struck, or it will give forth combinations of
the several tones which it is capable of emitting. On the other hand,
experiment shows that a diaphragm like that used in the telephone—not
only the electric telephone, but such common telephones as have been
sold of late in large quantities in toy shops, etc.—will respond
to any sounds which are properly directed towards it, not merely
reproducing sounds of different tones, but all the peculiarities which
characterize vocal sounds. In the former case, the size of a disc and
the conditions under which it is struck determine the nature of its
vibrations, and the air responds to the vibrations thus excited; in the
latter, the air is set moving in vibrations of a special kind by the
sounds or words uttered, and the disc or diaphragm responds to these
vibrations. Nevertheless, though it is important that this distinction
be recognized, we can still learn, from the behaviour of discs and
plates set in vibration by a blow or other impulse, the laws according
to which the actual motions of the various parts of a vibrating disc
or plate take place. We owe to Chladni the invention of a method for
rendering visible the nature of such motions.
Certain electrical experiments of Lichtenberg suggested to Chladni
the idea of scattering fine sand over the plate or disc whose motions
he wished to examine. If a horizontal plate covered with fine sand is
set in vibration, those parts which move upwards and downwards scatter
the sand from their neighbourhood, while on those points which undergo
no change of position the sand will remain. Such points are called
_nodes_; and rows of such points are called _nodal lines_, which may be
either straight or curved, according to circumstances.