NEWTON, FARADAY AND BELL AT WORK
Newton, the supreme generalizer . . . Faraday, the master of
experiment . . . Bell, the inventor of the telephone, transmits
speech by a beam of light.
Having now taken a rapid general view of observation and experiment, of
the faculty of sound theorizing, let us enter the presence of two great
masters of research and invention, beginning with a man who united the
loftiest powers as a mathematician, a physicist, and a generalizer.
How Newton Discovered the Law of Gravitation.
How Sir Isaac Newton discovered the law of gravitation is thus told in
his Life by Sir David Brewster:--“It was either in 1665 or 1666 that
Newton’s mind was first directed to the subject of gravity. He appears
to have left Cambridge some time before August 8, 1665, when the college
was dismissed on account of the plague, and it was, therefore, in the
autumn of that year, and not in that of 1666, that the apple is said to
have fallen from the tree at Woolsthorpe, and suggested to Newton the
idea of gravity. When sitting alone in the garden, and speculating on
the power of gravity, it occurred to him that, as the same power by
which the apple fell to the ground was not sensibly diminished at the
greatest distance from the centre of the earth to which we can reach,
neither at the summits of the loftiest spires, nor on the tops of the
highest mountains, it might extend to the moon and retain her in her
orbit, in the same manner as it bends into a curve the path of a stone
or a cannon ball, when projected in a straight line from the surface of
the earth. If the moon was thus kept in her orbit by gravitation, or, in
other words, its attraction, it was equally probable, he thought, that
the planets were kept in their orbits by gravitating towards the sun.
Kepler had discovered the great law of the planetary motions, that the
squares of their periodic times were as the cubes of their distances
from the sun, and hence Newton drew the important conclusion that the
force of gravity, or attraction, by which the planets were retained in
their orbits, varied as the square of their distances from the sun.
Knowing the force of gravity at the earth’s surface, he was, therefore,
led to compare it with the force exhibited in the actual motion of the
moon, in a circular orbit; but having assumed that the distance of the
moon from the earth was equal to sixty of the earth’s semi-diameters, he
found that the force by which the moon was drawn from its rectilinear
path in a second of time was only 13.9 feet, whereas at the surface of
the earth it was 16.1 in a second. This great discrepancy between his
theory and what he then considered to be the fact, induced him to
abandon the subject, and pursue other studies with which he had been
occupied.
“It does not distinctly appear at what time Newton became acquainted
with the more accurate measurement of the earth, executed by Picard in
1670, and was thus led to resume his investigations. Picard’s method of
measuring his degree, and the precise result which he obtained, were
communicated to the Royal Society, January 11, 1672, and the results of
his observations and calculations were published in the Philosophical
Transactions for 1675. But whatever was the time when Newton became
acquainted with Picard’s measurement, it seems to be quite certain that
he did not resume his former thoughts concerning the moon until 1684.
Pemberton tells us, that ‘some years after he laid aside’ his former
thoughts, a letter from Dr. Hooke put him on inquiring what was the real
figure in which a body, let fall from any high place, descends, taking
the motion of the earth round its axis into consideration, and that this
gave occasion to his resuming his former thoughts concerning the moon,
and determining, from Picard’s recent measures, that ‘the moon appeared
to be kept in her orbit purely by the power of gravity.’ But though
Hooke’s letter of 1679 was the occasion of Newton’s resuming his
inquiries, it does not fix the time when he employed the measures of
Picard. In a letter from Newton to Hailey, in 1686, he tells him that
Hooke’s letters in 1679 were the cause of his ‘finding the method of
determining the figures, which, when I had tried in the ellipsis, I
threw the calculations by, being upon other studies; and so it rested
for about five years, till, upon your request, I sought for the papers.’
Hence Mr. Rigaud considers it clear, that the figures here alluded to
were the paths of bodies acted upon by a central force, and that the
same occasion induced him to resume his former thoughts concerning the
moon, and to avail himself of Picard’s measures to correct his
calculations. It was, therefore, in 1684, that Newton discovered that
the moon’s deflection in a minute was sixteen feet, the same as that of
bodies at the earth’s surface. As his calculations drew to a close, he
is said to have been so agitated that he was obliged to desire a friend
to finish them.”
Michael Faraday’s Method of Working.
With no mathematics beyond simple arithmetic, Michael Faraday displayed
powers of experiment and generalization so extraordinary that in these
respects he stands at the same height as Newton himself. In the life of
Michael Faraday, by Dr. J. H. Gladstone, we are given his account of the
great physicist’s method of working:--
“The habit of Faraday was to think out carefully beforehand the subject
on which he was working, and to plan his mode of attack. Then, if he saw
that some new piece of apparatus was needed, he would describe it fully
to the instrument maker with a drawing, and it rarely happened that
there was any need of alteration in executing the order. If, however,
the means of experiment existed already, he would give Anderson, his
assistant, a written list of the things he would require, at least a day
before--for Anderson was not to be hurried. When all was ready, he would
descend into the laboratory, give a quick glance round to see that all
was right, take an apron from the drawer, and rub his hands together as
he looked at the preparations made for his work. There must be no tool
on the table but such as he required. As he began his face would be
exceedingly grave, and during the progress of an experiment all must be
exceedingly quiet; but if it was proceeding according to his wish, he
would commence to hum a tune, and sometimes to rock himself sideways,
balancing alternately on either foot. Then, too, he would often talk to
his assistant about the result he was expecting. He would put away each
tool in its own place as soon as done with, or at any rate as soon as
the day’s work was over, and he would not unnecessarily take a thing
away from its place. No bottle was allowed to remain without its proper
stopper; no open glass might stand for a night without a paper cover; no
rubbish was to be left on the floor; bad smells were to be avoided if
possible; and machinery in motion was not to be permitted to grate. In
working, also, he was very careful not to employ more force than was
wanted to produce the effect. When his experiments were finished and put
away, he would leave the laboratory, and think further about them
upstairs.
“It was through this lifelong series of experiments that Faraday won his
knowledge and mastered the forces of nature. The rare ingenuity of his
mind was ably seconded by his manipulative skill, while the quickness of
his perceptions was equalled by the calm rapidity of his movements. He
had indeed a passion for experimenting. This peeps out in the preface to
the second edition of his ‘Chemical Manipulation,’ where he writes,
‘Being intended especially as a book of instruction, no attempts were
made to render it pleasing, otherwise than by rendering it effectual;
for I concluded that, if the work taught clearly what it was intended to
inculcate, the high interest always belonging to a well-made or
successful experiment would be sufficient to give it all the requisite
charms, and more than enough to make it valuable in the eyes of those
for whom it was designed.’
“He could scarcely pass a gold leaf electrometer without causing the
leaves to diverge by a sudden flick from his silk handkerchief. I
recollect, too, his meeting me at the entrance to the lecture theatre at
Jermyn Street, when Lyon Playfair was giving the first, or one of the
first lectures ever delivered in the building. ‘Let us go up here,’ said
he, leading me far away from the central table. I asked him why he chose
such an out-of-the-way place. ‘Oh,’ he replied, ‘we shall be able here
to find out what are the acoustic qualities of the room.’
“The simplicity of the means with which he made his experiments was
often astonishing, and was indeed one of the manifestations of his
genius. A good instance is thus narrated by Sir Frederick Arrow:--‘When
the electric light was first permanently exhibited at Dungeness, on 6th
June, 1862, a committee of the Elder Brethren, of which I was one,
accompanied Faraday to observe it. Before we left Dover, Faraday showed
me a little common paper box and said, “I must take care of this; it’s
my special photometer,”--and then, opening it, produced a lady’s
ordinary black shawl pin (jet, or imitation, perhaps)--and then holding
it a little way off the candle, showed me the image very distinct; and
then, putting it a little further off, placed another candle near it,
and the relative distance was shown by the size of the image.’
“In lecturing to the young he delighted to show how easily apparatus
might be extemporized. Thus, in order to construct an electrical
machine, he once inverted a four-legged stool to serve for the stand,
and took a white glass bottle for the cylinder. A cork was fastened into
the mouth of this bottle, and a bung was fastened with sealing wax to
the other end: into the cork was inserted a handle for rotating the
bottle, and in the centre of the bung was a wooden pivot on which it
turned: while with some stout wire he made crutches on two of the legs
of the stool for the axles of this glass cylinder to work upon. The silk
rubber he held in his hand. A japanned tea cannister resting on a glass
tumbler formed the conductor, and the collector was the head of a
toasting fork. With this apparently rough apparatus he exhibited all the
rudimentary experiments in electricity to a large audience.”
Faraday’s Orderliness and Imagination.
Faraday, in addition to the rarest ability in experiment, had an
orderliness of mind which gave the utmost effectiveness to his work in
every department. His successor, Professor John Tyndall, says:--
“Faraday’s sense of order ran like a luminous beam through all the
transactions of his life. The most entangled and complicated matters
fell into harmony in his hands. His mode of keeping accounts excited the
admiration of the managing board of the Royal Institution. And his
science was similarly ordered. In his Experimental Researches he
numbered every paragraph, and welded their various parts together by
incessant reference. His private notes of the Experimental Researches
which are happily preserved, are similarly numbered; their last
paragraph bears the number 16,041. His working qualities, moreover,
showed the tenacity of the Teuton. His nature was impulsive, but there
was a force behind the impulse which did not permit it to retreat. If in
his warm moments he formed a resolution, in his cool ones he made that
resolution good. Thus his fire was that of a solid combustible, not that
of a gas, which blazes suddenly, and dies as suddenly away.”
Faraday had exalted powers of imagination and as he gazed at the curves
in which iron-filings disposed themselves when tapped on a card held
above a magnet, he saw similar “lines of force” surrounding every
attracting mass of whatever kind. Other observers had confined their
attention to what takes place, or is supposed to take place, in a
conductor; he closely scanned what took place around a conductor. He was
thus addressed in a letter from that remarkable physicist, Professor
James Clerk Maxwell of Cambridge:--
“As far as I know you are the first person in whom the idea of bodies
acting at a distance by throwing the surrounding medium into a state of
constraint has arisen, as a principle to be actually believed in. We
have had streams of hooks and eyes flying around magnets, and even
pictures of them so beset; but nothing is clearer than your description
of all sources of force keeping up a state of energy in all that
surrounds them, which state by its increase or diminution measures the
work done by any change in the system. You seem to see the lines of
force curving round obstacles and driving plump at conductors, and
swerving toward certain directions in crystals, and carrying with them
everywhere the same amount of attractive power, spread wider or denser
as the lines widen or contract. You have seen that the great mystery is,
not how like bodies repel and unlike attract, but how like bodies
attract by gravitation. But if you can get over that difficulty either
by making gravity the residual of the two electricities or by simply
admitting it, then your lines of force can ‘weave a web across the sky’
and lead the stars in their courses without any necessarily immediate
connection with the objects of their attraction. . . .”
How Light Becomes a Bearer of Speech.
Michael Faraday, as we have seen, by researches of consummate ability
laid the foundation of modern electrical science and art. In that field
there is to-day no inventor more illustrious than Professor Alexander
Graham Bell, the creator of the telephone, that simplest and most
important of electrical devices.[34] Not content with obliging a wire to
carry speech in electric waves, Professor Bell has impressed beams of
light into the same service. The successive steps by which he arrived at
the photophone are of extraordinary interest. His story as given in the
proceedings of the American Association for the Advancement of Science,
1880, is here somewhat condensed:--
[34] Professor Bell’s narrative of how he invented the telephone is
given in “Invention and Discovery,” one of the six volumes of
“Little Masterpieces of Science,” Doubleday, Page & Co., New York.
In “Flame, Electricity and the Camera” by the present writer,
published by the same firm, is a chapter describing the telephone in
its later developments. This chapter was revised by the late
Professor Alexander Melville Bell, father of the inventor.
“In bringing before you some discoveries by Mr. Sumner Tainter and
myself, which have resulted in the production and reproduction of sound
by means of light, let me sketch the state of knowledge which formed the
starting point of our experiments. I shall first describe selenium, and
the uses of it devised by previous experimenters; our researches have so
widened the class of substances sensitive, like selenium, to
light-vibrations that this sensitiveness seems to be a property of all
matter. We have found this property in gold, silver, platinum, iron,
steel, brass, copper, zinc, lead, antimony, german-silver, ivory,
celluloid, gutta percha, hard and soft rubber, paper, parchment, wood,
mica, and silvered glass. At first carbon and microscope glass seemed
insensitive; later experiments proved them to be no exceptions to the
rule.
“We find that when a vibratory beam of light falls upon these substances
they emit sounds, the pitch of which depends upon the frequency of the
vibratory change in the light. We also find that when we control the
form or character of the light-vibrations, we control the quality of the
sound, and obtain all varieties of articulate speech. We can thus speak
from station to station wherever we can project a beam of light.
Selenium, indispensable in the apparatus, was discovered by Berzelius in
1817. It is a metalloid resembling tellurium; they differ, however, in
electrical properties; tellurium is a good conductor, selenium in its
usual forms is a non-conductor. Knox, in 1837, discovered that selenium
is a conductor when fused; in 1851, Hittorf showed that it conducts when
in one of its allotropic forms. When selenium is rapidly cooled from a
fused condition it is a non-conductor. In this vitreous form it is dark
brown, almost black by reflected light, having an exceedingly brilliant
surface; in thin films it is transparent, and appears of a beautiful
ruby red by transmitted light. When selenium is cooled from fusion with
extreme slowness, it presents an entirely different appearance, being of
a dull lead color, and having throughout a granular or crystalline
structure and looking like a metal. It is now opaque even in very thin
films. It was this kind of selenium that Hittorf found to be a conductor
of electricity at ordinary temperatures. He also noticed that its
resistance to the passage of electricity diminished continuously by
heating up to the point of fusion; and that the resistance suddenly
increased as the solid passed to liquidity. It was early discovered that
exposure to sunlight hastens the change of selenium from one allotropic
form to another; an observation of significance in the light of recent
discoveries.
The Cardinal Discovery.
“Mr. Willoughby Smith, an engineer engaged in the laying of submarine
cables, had devised a system of testing and signalling during their
submersion. For this system, in 1872, it occurred to him that he might
employ crystalline selenium, on account of its high resistance, at the
shore end of a cable. On experiment the selenium was found to have all
the resistance required; some of the bars displayed a resistance of 1400
megohms, as much as would be offered by a telegraph wire long enough to
reach from the earth to the sun. But this resistance was found to be
extremely variable; the reason was disclosed when Mr. May, an assistant,
observed that the resistance of selenium is less in light than in
darkness. This discovery created widespread interest throughout the
world. Among the investigators who at once turned their attention to
the subject was Professor W. G. Adams of King’s College, London, who
proved that the action on selenium is chiefly due to the luminous rays
of the spectrum, the ultra-red and ultra-violet rays having little or no
effect. Dr. Werner Siemens, the eminent German physicist, produced a
variety of selenium fifteen times more conductive in sunlight than in
darkness. This extraordinary sensitiveness was brought about by heating
for some hours at a temperature of 210° C., followed by extremely slow
cooling.
[Illustration: Telephones receiving sounds through a beam of light.]
The Telephone Brought in.
“Observations concerning the effect of light upon the conductivity of
selenium had employed the galvanometer solely; it occurred to me that
the telephone, from its extreme sensitiveness, might be substituted with
advantage. On consideration I saw that the experiments could not be
conducted in the ordinary way with continuous light, for a good reason:
the law of audibility of the telephone is precisely analogous to the law
of electrical induction. No effect is produced during the passage of a
continuous and steady current. It is only at the moment of change from a
stronger to a weaker state, or, vice versa, that any audible effect is
produced; this effect is exactly proportional to the amount of variation
in the current. It was, therefore, evident that the telephone could only
respond to the effect produced in selenium at the moment of change from
light towards darkness, or _vice versa_, and that it would be advisable
to intermit the light with great rapidity so as to produce a succession
of changes in the conductivity of the selenium corresponding in
frequency to musical vibrations within the limits of the sense of
hearing. For I had often noticed that currents of electricity, so feeble
as hardly to produce any audible effects from a telephone when the
circuit was simply opened and closed, caused very perceptible musical
sounds when the circuit was rapidly interrupted; and that the higher the
pitch of the sound the more audible was its effect. I was much struck by
the idea of producing sound in this way by the action of light.
Accordingly I proposed to pass a bright light through one of the
orifices in a perforated screen consisting of a circular disk with holes
near its circumference. Upon rapidly rotating the disk an intermittent
beam of light would fall on the selenium, and from a connected telephone
a musical tone would be produced, its pitch depending upon the rapidity
with which the disk spun round.
Variations of Light Necessary.
“Upon further consideration I saw that the effect could not only be
produced at the extreme distance at which selenium would normally
respond to the action of a luminous body, but that this distance could
be indefinitely increased by using a parallel beam of light, so that we
might telephone from one place to another with no conducting wire
between the transmitter and the receiver. To reduce this idea to
practice it was necessary to devise an apparatus to be operated by the
voice of a speaker, by which variations could be produced in a parallel
beam of light, corresponding to variations in the air produced by the
voice. I proposed, therefore, to pass light through two plates
perforated by many small orifices. One of these plates was to be fixed,
the other was to be attached to the centre of a diaphragm actuated by
the voice. In its vibrations the diaphragm would cause the movable plate
to slide to and fro over the surface of the fixed plate, by turns
enlarging and contracting the free orifices for the passage of light.
The parallel beam emerging from this apparatus could be received at some
distant place on a lens focussing it upon a sensitive piece of selenium
placed in a local circuit, with a telephone and a galvanic battery. The
variations in the light produced by a speaker’s voice should cause
corresponding variations in the electrical resistance of the selenium at
the distant place, and the telephone in circuit with the selenium should
reproduce audibly the tones and articulations of the speaker’s voice. It
is greatly due to the genius and perseverance of my friend, Mr. Sumner
Tainter, that the problem thus entered upon has been successfully
solved.
Special Treatment of the Selenium.
“The first point to which we devoted our attention was reducing the
resistance of crystalline selenium within manageable limits. The
resistance of selenium cells, employed by former experimenters, was
counted in millions of ohms; there is no record of a cell measuring less
than 250,000 ohms in the dark. We have succeeded in producing cells
measuring only 300 ohms in the dark and 150 in the light. Our
predecessors all seemed to have used platinum for the conducting part of
their cells, excepting Werner Siemens, who found that iron and copper
would do. We have discovered that brass, although chemically acted upon
by selenium, forms an excellent material; indeed, we are inclined to
believe that the chemical action between brass and selenium has
contributed to the lowness in resistance of our cells, an intimate union
taking place between the two substances. In brass we have constructed
many cells of diverse forms. One of them (two are described by Professor
Bell), is cylindrical so that it may be used with a concave reflector
instead of with a lens. It is composed of many metallic disks separated
by mica disks slightly smaller in diameter. The spaces between the brass
disks over the mica are filled with selenium, and the alternate brass
disks are metallically connected. The selenium is applied to the cell
duly heated: next comes annealing. To effect this an oven is inserted in
a pot of linseed oil standing upon glass supports in another similar pot
of linseed oil. The whole is then heated to about 214° C., and kept
there for twenty-four hours, then allowed to cool down during forty to
sixty hours until the temperature of ordinary air is reached.
A Perfected Transmitter.
“We have devised more than fifty forms of photophonic transmitters. In
one of them (several others are described by Professor Bell), a beam of
light passes through a lens of variable focus formed of two sheets of
thin glass or mica containing between them a transparent liquid or gas.
When vocal vibrations are communicated to this gas or liquid, they cause
a vibratory change in the convexity of the glass surfaces with a
corresponding change in the intensity of the light as it falls upon the
selenium. We have found the simplest apparatus to consist in a plane
mirror of flexible material, such as silvered mica or microscope glass,
against the back of which the speaker’s voice is directed.
[Illustration: Selenium cylinder with reflector.]
“A large number of trials of this apparatus have been made with the
transmitting and receiving instruments so far apart that sounds could
not be heard directly through the air. In a recent experiment Mr.
Tainter operated the transmitting instrument, placed on the top of the
Franklin School House in Washington, D. C.; the receiver being arranged
in a window of my laboratory, at a distance of 213 metres. Upon placing
the telephone to my ear, I heard distinctly from the illuminated
receiver: ‘Mr. Bell, if you hear what I say, come to the window and wave
your hat.’
“We have found that articulate speech can be reproduced by the
oxyhydrogen light, and even by a beam from a kerosene lamp. The loudest
effects follow upon interrupting the light by means of a perforated disk
swiftly rotated. Because this apparatus is noiseless it allows a close
approach of the receiver while not interfering with its message.
“We have endeavored to ascertain the nature of the rays which affect
selenium, placing in the path of an intermittent beam various absorbing
substances. In these experiments Professor Cross has rendered us aid.
When a solution of alum, or bisulphide of carbon, is employed, there is
but slight reduction in loudness, but a solution of iodine in bisulphide
of carbon cuts off most of the audible effect. Even an opaque sheet of
hard rubber is less obstructive.
[Illustration: A perforated disc rotated yields a succession of sounds
from light.]
Experiments Without a Telephone.
“It is a well known fact that the molecular disturbance produced in a
mass of iron by the magnetizing influence of an intermittent electrical
current can be observed as sound by placing the ear in close contact
with the iron. It occurred to us that the molecular disturbance produced
in crystalline selenium by the action of an intermittent beam of light
should be audible in a similar manner with no telephone or battery. Many
experiments were made to verify this theory; at first without definite
results. The behavior of the hard rubber just mentioned suggested
listening to it also. This was tried with an extraordinary result. I
held the sheet in close contact with my ear while a beam of intermittent
light was focussed upon it through a lens. A distinct musical note was
immediately heard. Other substances, as enumerated at the outset of my
address, were now successively tried in the form of thin disks, in every
case with success. On the whole, we feel warranted in announcing as our
conclusion that sounds can be produced by the action of a variable light
from substances of all kinds in the form of thin diaphragms. The reason
why thin diaphragms are more effective than masses appears to be that
the molecular disturbance produced by light is chiefly a surface action,
and that the vibration has to be transmitted through the mass of the
substance in order to affect the ear. We have led air, directly in
contact with an illuminated surface, to the ear by throwing the luminous
beam upon the interior of a tube. We have thus heard from interrupted
sunlight very perceptible musical tones through tubes of ordinary
vulcanized rubber, of brass, and of wood. These were all the materials
at hand in tubular form, and we have had no opportunity since of
extending the observations to other substances. A musical tone can be
heard by throwing the intermittent beam of light into the ear itself.
This experiment was at first unsuccessful on account of the position in
which the ear was held.”