earlier invention of Mr. Graham Bell, a native of Scotland, who had
settled in the United States. Mr. Bell’s inventions, it may be
mentioned, were by no means the results of fortunate accidents or of
unsought and spontaneous flashes of conception, but rather the outcome
of long, patient and systematic studies. His father, Mr. Alexander
Melville Bell, of Edinburgh, had assiduously cultivated acoustic
science, and had in conjunction with his son, undertaken special
researches into the mechanism of the organs of speech, the elements of
articulate speech in different languages, and the musical components of
vocal sounds. When Graham afterwards pursued these studies in the light
of the fuller investigation carried out by Helmholtz, he was naturally
led to the application of electricity to acoustic transmission. After
some experiments in the production of vowel sounds by combinations of
electric tuning forks, he invented a telephone for reproducing musical
sounds at a distance, which was a great improvement on that of Reiss,
and involved another principle, which indeed is the same as that
utilized in his more mature invention of the speaking telephone. As a
like explanation of the action would apply in both cases, the reader
will find his advantage in following the observations we have to make on
the earlier instrument. This consisted of what was virtually two sets of
electric tuning forks, each set being acted upon by one electro-magnet.
Fig. 302_b_ will suffice to show the general form of the arrangement. A
plate of steel is bent twice at right angles longitudinally, and is
magnetized so that any transverse slice of it would constitute an
ordinary horse shoe magnet. This is seen endways in Fig. 302_b_ at M,
and N. and S. will indicate the north and south poles respectively. To
each limb of this broad magnet is attached a plate of steel, T, cut into
teeth, just in the same way as the steel plate in a common musical box
or mechanical piano, except that the teeth are not pointed. These are
tuned to give severally in pairs the notes of the musical scale when
thrown into vibration. Between the prongs of the series of tuning forks
thus formed is an electro-magnet, L, made of a bar of soft iron, I,
wound longitudinally by a coil, one end of which makes an earth
connection at E and the other is connected by the wire, W W´, to
complete the circuit through the coil of the distant apparatus. It will
be observed that the receiving and transmitting instruments are exactly
alike. Now, suppose one of these teeth is struck or otherwise thrown
into vibration, the result will be, since the free ends of the teeth are
magnetic poles, that alternating electric currents will be generated in
the coil of the electro-magnet (see page 509), and these will flow
through the entire circuit, including the coil of the distant
instrument, where the magnetism generated will alternately attract and
repel the polar extremities of the teeth in the steel plate. It will be
understood, of course, that the fellow prong of the fork will vibrate
also, and will simultaneously approach to and recede from the soft iron
core, so that being of opposite polarity, the effect on the
electro-magnet will be doubled. The action on the distant electro-magnet
will be a rapid series of reversals of the polarities of the core, and
hundreds of times in every second the ends of the steel teeth will be
alternately attracted and repelled. But not all of these will thereby be
thrown into vibration—only the one pair which were tuned into unison
with the former can and will respond to that particular series of
impulses, and the consequence will be that the same note will be emitted
by the receiving instrument. If two or more notes of the transmitter be
simultaneously thrown into vibration, the same notes will be heard from
the receiver, for each series of currents will flow along the wire
independently, just as if the other did not exist, and each will produce
its particular effect on the transmitter. In this way an air played on
the one instrument is heard also from the other, with all its accents
and combinations. But more than this, if a tune be played on a musical
instrument near the sender, or if a song be sung, the air will be
reproduced by the distant receiver. The reason of this is that the steel
tongues take up, or are thrown into movement by, the vibrations that
have the same periodicity. The manner in which a vibratory body responds
to impulses of its own periodicity may be easily shown by exposing the
wires of a piano and raising the dampers, when, if a note be sung near
the instrument, it will be found that a number of the wires respond,
namely, those that are capable of vibrating synchronously with the
constituent vibrations of the voice, for neither a voice nor a sounding
wire gives forth one simple system of vibrations, the audible effect
being due to the superposition or composition of several diverse
elementary systems. With the same arrangement another experiment may be
made, as an illustration of a matter important for our subject. Let the
different vowels be sung to the piano-wire on the same note or pitch,
and in the responses to each a difference of the quality of the sound
will be noticed, although the piano will not distinctly give back the
vowel itself. It would, however, do so if a number of its wires were
strung with certain definite relations in pitch to that of the
fundamental note and in unison with the voice components of the vowel
sound.
[Illustration:
FIG. 302_c_.—_Superposition of Currents._
]
It has been said above that two systems of electrical currents of
different periodicity would flow along one wire independently of each
other, but it should be explained that this takes place by a composition
of the currents, for it is evident that at any given instant the wire
can only be in one of three conditions, viz.: (1) with no current
flowing; (2) with a current in the positive direction; (3) with a
current in the negative direction. Such must always be the case, and,
therefore, it should be clearly understood how this is consistent with
the superposition of currents of different periodicities, a matter which
the diagram, Fig. 302_c_, is intended to illustrate. Suppose the _flow
of time_ to be represented by the dotted lines from _a_ to _b_, the
whole length of which we may call 1/100th of a second, and that the
current passing through the wire is represented in intensity and
direction by the plain lines; the intensity by distance above or below
the dotted line; the direction being positive where the plain line is
above, and negative when it is below the dotted straight line, and of
course no current at all occurs at the instant when the change of
direction takes place. The line A will thus represent alternating
currents, rising and sinking in intensity, and changing from one
direction to the other, going through 600 regularly recurring phases in
one second of time. Similarly, B may represent another series of
currents, having here a periodicity of 500 in one second of time. These
are here supposed to have greater intensity than the former. If the two
currents are sent through one wire their effects are superposed, so that
the actual electrical state of the wire would be represented by the
curve C, which is compounded from the two others, and where it will be
observed the rise and fall of the current, its maxima and minima, no
longer recur at regular intervals within the space of the 1/100th of the
second, the whole of that period being taken up by a less regular series
of changes, the cycle being repeated only 100 times in the second. The
same diagram might serve to illustrate the motions of, say, a particle
of air or the drum of the ear in acoustic vibration, the distances above
and below the straight line being taken to represent the displacements
from the position of rest on one side and the other. If the sounds of an
organ or piano consisted of only these primary vibrations, B would
roughly[8] represent the movements of the wires, the air and the drum of
the ear, when the note _si_{3}_ was sounded alone; A when the note
_re_{4}_ was more faintly sounded alone, and then C, if these notes were
sounded together, would correspond with the movements of the drum of the
ear. The movements it actually makes when we hear speech, or even a
single musical note, are, however, a thousand-fold more complex, for no
musical instrument gives out a note with a single set of vibrations, the
fundamental one being always accompanied by other sets diversely related
to it, according to the class of instrument. In some cases, fifteen or
sixteen sets of vibrations have been distinguished along with the
fundamental note, without exhausting the possible number. Of a like
order of complexity will be the currents which the wire of a speaking
telephone must convey, and the difference between the undulatory nature
of the currents in Bell’s musical telephone and any produced by mere
make and break contacts, as in Reiss’ arrangement, will be obvious, and
recognized as an important step towards the solution of the problem of
transmitting speech. When Mr. Bell invented his instrument, he was
seeking for a method of simultaneously transmitting by one wire several
messages by audible _signs_ merely; and by the method used in his
musical telephone this is practicable, for all that would be required
would be pairs of transmitters and receivers, each adjusted to one
single particular note. Another point that should be noted is that in
the Bell musical telephone no battery is used, for the currents are
those generated by magneto-electric induction, and the circuit through
the wires and coils are completed by earth connections.
Footnote 8:
The lines A and B in the diagram have not harmonic ordinates.
[Music]
[Illustration:
FIG. 302_d_.—_Bell’s Speaking Telephone._
]
In passing from the invention of the musical to that of the speaking
telephone, Mr. Bell passed from the more complex to the more simple
instrument, for of all apparatus by which communication can be carried
on at a distance, the Bell speaking telephone is one of the simplest. He
had only to make its vibrating disc of Scott’s phono-autograph into a
magnetized body, capable of producing currents in an electro-magnet coil
in the same way as did the vibrating plates in his musical telephone.
The Bell speaking telephone was publicly exhibited for the first time at
Philadelphia, in 1876, and was shown the same year to the British
Association by Sir William Thomson, who pronounced it the wonder of
wonders. For the first time in England, the instrument in a still
simpler form was exhibited by Mr. Preece, at the Plymouth meeting of the
British Association in 1877, and of nearly the same construction as is
still often used, although, as we shall presently see, for battery
telephones the transmitting apparatus is now made of larger dimensions,
of a different shape and on a different principle. We shall describe the
simple form in which transmitter and receiver are identical, each
consisting externally of a small cylindrical wooden or ebonite box, and
with a handle three or four inches in length of the same material. Fig.
302_d_ is a section of the instrument where N S is a cylindrical steel
magnet, on one end of which is wound the small coil B, made of fine silk
covered copper wire, the extremities of which pass through the handle M
at _f f_, and are connected by the binding screws _I I´_ with the line
wire C C´. Close to the coil covered end of the magnet is a very thin
diaphragm of iron, L L´, and when this is thrown into vibration by the
voice speaking into the trumpet-mouth opening, R R´, its movements
produce currents in the coil according to the principles that have
already been explained, for it will be observed that the iron disc is
magnetized by the inductive action of the permanent magnet N S. These
currents passing through the coil of the receiving instrument raise or
lower the intensity of the magnetic force in it, so that the distant
disc reproduces the vibrations of the transmitter. Such is at least an
obvious explanation of the action of this very simple arrangement; but
from a number of experiments and observations that have been made with
modifications of the instruments, it would appear that other and much
more complex phenomena concur in producing the effects. It has indeed
been suggested—and the idea is supported by numerous experiments—that,
in these telephonic transmissions of speech, vibrations are concerned
which are not at all of the mechanical kind we have been dealing with in
these explanations, but are _molecular_.
The Bell telephone is used by speaking distinctly before the mouth-piece
of the transmitter, while the listener at the other end of the line
applies the mouth-piece of his instrument to his ear, and one wire is
sufficient with good earth connections, although sometimes a second wire
is employed to complete the circuit. It is also found advantageous to
have two instruments in the circuit at each end, so that one may be held
to the ear while the operator is speaking through the other. In this
way, a rapid conversation can be carried on with the greatest ease, or
again, an instrument may be held at each ear, by which arrangement the
words are more distinctly heard. It is not necessary to shout, as this
has no effect, but to speak with a clear intonation, and some voices are
found to suit better than others. The vowel sounds are best transmitted,
except that of the English _e_, which, with the letters _g_, _j_, _k_,
and _q_, are always somewhat imperfectly transmitted. A song is very
distinctly heard, both in the words and the air, and the voice of the
person singing is readily recognized. Several instruments may be
included in one circuit at different stations, so that half a dozen
persons may take part in a conversation, and questions and answers may
be understood even when crossing each other. If two distinct telephone
circuits have their wires laid for a certain distance (two miles) near
each other, say a foot or more apart, and without any connection
whatever, listeners at the end of the one line will hear the
conversation exchanged through the other line. Other forms of the
instruments have been arranged, by which a large audience may hear
sounds produced at a distance, as, for instance, when a cornet-à-piston
was played in London, it was heard by thousands of people assembled in
the Corn Exchange at Basingstoke.
It would be impossible within our limits to even briefly describe the
great number of improvements and modifications of Bell’s system that
were devised by various persons soon after the invention was brought
out, and many additional complications were introduced into some of the
arrangements. Advantage was also taken to a greater or less extent of
another principle affecting the strength of electric currents, to which
we have now to call the reader’s attention, and to exemplify by one of
the simplest instruments, leaving detailed accounts of the various forms
in which it has been applied to be found in special treatises. The
reader should first turn back to page 400, where he will see an
expression of the strength of a battery current. It will be observed
that the current may be increased or diminished by diminishing or
increasing R, the external resistance, without changing the other terms.
Now M. Du Moncel discovered, as far back as 1856, that an increase of
pressure between two conductors in contact, and conveying a current,
caused a diminution of the electrical resistance, and this discovery was
utilized for telephonic purposes by Mr. Edison in his invention of the
carbon transmitter (1876). In this there is no magnet, and a stretched
membrane may take the place of the metallic plate, although a circle of
photographers’ ferro-type plate gives better results. A pad of
india-rubber, cork, or other material is fixed on the plate, and rests
upon a carbon disc, which again is in contact with a metallic conductor.
Between the latter and the carbon the current from a constant battery
passes. When the plate is thrown into vibration by speaking into the
mouth-piece, the variations of pressure conveyed to the carbon cause
variations in the resistance of its electrical contact, and thus a
series of undulations are produced in the current, and these affect the
electro-magnet of a Bell receiving instrument in the circuit as before,
so that the sounds are reproduced. It is now time to say a word about
the share in the invention of the speaking telephone which has been
claimed by Mr. Elisha Gray, also of the United States, who, at the time
Mr. Bell applied for the patent for his instruments, produced drawings
and descriptions of a plan he had devised for transmitting speech by
undulating electrical currents, and it has been admitted that the plan
he had conceived was perfect in principle. He proposed to use a battery
current, and his receiving instrument was nearly the same as Bell’s. The
undulations of the current were also determined, as in Edison’s
telephone, by changes in the external resistance, but this was effected
in a different, though equally simple manner. To a membrane stretched
across the lower end of a short wide tube that formed the mouth-piece of
the transmitter, and was placed vertically, was attached a piece of
platinum wire, conveying the current and dipping into a liquid of
moderate conductivity, but not quite touching another platinum electrode
fixed at the bottom of the vessel containing the liquid. The space of
liquid traversed by the current being thus varied by the oscillations of
the membrane, the resulting variations of the resistance produced the
requisite undulations in the intensity of the current. Both Mr. Bell and
Mr. Gray applied for patents on the 14th February, 1876, but the
American Patent Office recognized the claim of the former as prior.
[Illustration:
FIG. 302_e_.—_Mr. Hughes’ Microphone._
(B _and_ R _are merely diagrammatic_.)
]
Du Moncel’s observation was applied by Mr. Hughes in the construction of
an instrument, which he named the _microphone_. This was in the same
year that Edison had brought out his carbon telephone, and a certain
similarity, resulting from the identity of the principle employed, led
to an acrimonious controversy on what were supposed to be rival claims.
But the microphone differs so much in arrangement and performance from
the other instrument as to constitute a distinct invention. The
instrument, if it may be so called, is simplicity itself, in the form
represented in Fig. 302_e_, which is one of the most sensitive. There, C
and C´ are two small blocks of carbon, fixed on a small upright piece of
wood. Two cup shaped cavities are hollowed out in the carbon blocks, and
these serve to hold loosely, in a nearly vertical position, a small rod
of gas retort carbon pointed at the ends. This rod is only about one
inch in length, and the lower end merely rests on the bottom of the cup
in C´, while the other is capable of moving about in the upper cavity,
the vertical position being nearly maintained in a state of unstable
equilibrium. The carbons are in the circuit of a voltaic cell or small
battery, B, in the line through a Bell receiving instrument, which may
be at a distance. When the microphone is to be used, it is placed on a
table with a cushion or several folds of wadding beneath its base. If
the receiver be applied to the ear of a listener, he will distinctly
hear every word pronounced by one speaking near the microphone, even in
a low tone; but a loud voice may be heard when the speaker is 20 or 30
feet from the instrument. The minutest vibrations conveyed to the stand
are perceived at the receiver as loud noises. The tread of a fly walking
over the board, S, is heard like the tramp of a horse, and the ticks of
a watch are audible in the receiver when the ear is several inches away
from it. The slight touch of a feather on the stand is distinctly
audible, and a current of air impinging upon it is reproduced as the
noise of a stream of water. The microphone is, in fact, the most
sensitive detector of vibrations that is known, and its employment as a
transmitter has brought the telephone to its present perfection. It has
been constructed in an endless variety of forms, according to the
purposes for which it is intended, and its simplicity is as wonderful as
its extreme sensitiveness. We will further illustrate these qualities by
an experiment of Mr. Willoughby Smith’s on the same principle. Instead
of the two carbon blocks, he laid on the table, in parallel positions,
two small rat tail files, and completed the circuit by a third file,
laid across the others at right angles. This arrangement constituted so
sensitive a transmitter that the listener at the distant Bell receiver
could hear even the faint sound of the speaker’s breathing. Even three
common pins, similarly crossed, make an effective transmitter. The
feebleness of the variations in the current requisite to make the Bell
receiver produce sounds is extraordinary, and a very weak battery
current is sufficient, even under the circumstances of ordinary
practical use. Still more remarkable is the fact that in favourable
conditions the microphone is capable of transmitting sound without any
battery at all, but merely with connections to earth, when the ticking
of a watch placed upon the stand has been distinctly heard at the
distance of nearly one-third of a mile, and speech, also, has been
transmitted with unusual distinctness with the battery left out and
merely a few drops of water placed at the carbon contacts; indeed, it is
said that, even without the water, the voice may be heard. This effect
has been attributed to the carbons and water forming a battery
themselves, and in the latter to the moisture of the speaker’s breath
supplying the fluid element. But, again, the microphone will not only
transmit speech, but, under certain arrangements, it will reproduce it
(when one of the carbon electrodes is attached to a membrane), although
the result is less distinct than with the Bell receiver. It is, however,
not so easy to explain how mere variations of current intensity can
produce the effect where there can be no magnetic attractions and
repulsions. We must, no doubt, look for the cause in some other property
of electric currents. The transmitters used in various lines of
telephonic communication, erected by the Post Office or by companies in
Great Britain, are generally applications of the principle of the
microphone, and not of that of either Mr. Bell’s or Mr. Edison’s
original instrument. But more recently, Mr. Edison has most ingeniously
adapted variations of sliding friction, as modified by the action of the
undulatory current on a liquid electrolyte between the sliding surfaces
to the production of a loud speaking telephonic receiver—that is, one by
which the sounds are made audible to a large assembly. From this
instrument, the notes of a cornet-à-piston, played in Brighton, have
been distinctly heard throughout a large hall in London.
Another curious transmitter is formed of a fine jet of water traversed
by an electric current. Acoustic vibrations are easily set up in the
jet, and these modify its conductivity so as to produce corresponding
undulations of current intensity.
It would take long to point out the many scientific applications of so
sensitive an instrument as the microphone with its Bell receiver. As a
medium for conveying speech to a distance, whether for purposes of peace
or war, its use is sufficiently obvious. Some curiosities of musical
transmission have been noticed, and such experiments are repeated from
time to time with increasing success. It has been applied to many
purposes in surgery and medicine. In many cases of deafness it has made
conversation easy. Even the passage of the molecules of gases, when
diffusing through porous partitions, Mr. Chandler Roberts has by its
means made audible. The distances to which speech can now be transmitted
are considerable, as conversations have been carried on by persons
nearly 300 miles apart.
[Illustration]
LIGHTHOUSES.
Who does not regard with interest the lighthouses which at night throw
their friendly beams across the sea, to guide the mariner in his course,
and warn him of perils from sunken rock or treacherous shoal? The modern
lighthouse, with its beautiful appliances, is entirely the result of the
applied science of our age; and it affords a fine example of the manner
in which experiments, carried on to determine natural laws apparently of
an abstract character and without any obvious direct utility, give rise
to inventions of the highest importance and most extended usefulness.
The lofty structures which were erected near certain ancient harbours,
and of which the Pharos of Alexandria is the most memorable example,
burned on their summits open fires of wood; and whatever beacons existed
from that time down to the end of last century were merely blazing fires
of wood or coal. The lighthouses of the South Foreland, which were
established in 1634, displayed coal fires until 1790, and the
lighthouses in the Isle of Man were first illuminated with oil only in
1816. Down to the beginning of the present century, therefore, the
modern lighthouses showed no improvement on the ancient plan. Even the
Tour de Cordouan, at the mouth of the Garonne river, which was completed
in 1610, and is one of the most famous of modern lighthouses, from its
great height (200 ft.), and the care which has always been given to
render it efficient, showed down to 1780 merely a fire of billets of
wood, the upward loss of the light being diminished by a rude reflector
in the form of an inverted cone. In the improved means of obtaining
artificial light, and in the admirable optical apparatus by which that
light is utilized, we find the vast superiority of modern lighthouses.
But these are sometimes erected on isolated, and almost submerged,
rocks, exposed to the fury of the waves. The difficulties which have to
be overcome in their construction cause some lighthouse towers to rank
among the best specimens of engineering skill. We may, therefore,
consider under the present head—the towers; the sources of light; the
optical apparatus and its accessories.
[Illustration:
FIG. 303.—_The Eddystone Lighthouse._
]