WAVES AND RIPPLES IN THE ÆTHER.
Having in the last chapter explained the nature and mode of production
of electric oscillations, and shown that when these take place in an
open electric circuit or long straight rod they give rise to certain
actions at a distance, rendered evident by the changes taking place
in the conductivity of metallic powders, we have now to present the
outlines of a proof that these actions are really due to a wave-motion
of some description set up in the æther, which in nature is essentially
the same as that which constitutes the agency we call light.
We shall begin by studying a few of the epoch-making discoveries we
owe to the celebrated Heinrich Hertz, announced in a series of famous
researches with which he surprised and delighted the scientific world
in the years 1887 and 1888. These investigations opened a new and
remarkable field of experimental work.
The precise form of apparatus used by Hertz in these researches
is, however, unsuited for lecture demonstration, and I shall use
on this occasion some arrangements of my own, which are only
convenient modifications of appliances previously employed by other
experimentalists. The devices here shown are, however, very convenient
for public demonstrations.
This apparatus consists of two parts, a part for generating electric
waves, which we shall call the radiator, and a part for detecting them,
which is called the receiver.
The radiator consists of a zinc box, A (see Fig. 73), which is provided
with hollow trunnions, and can be fixed to a suitable stand and turned
in any direction. The box has an open end to it, and in its interior
there are two brass rods about 4 inches long, each terminating in brass
balls, S, 1 inch in diameter. These rods are thrust through corks
fixed in the end of two ebonite tubes, which pass through the hollow
trunnions of the box. The rods have their ends attached to very closely
wound spirals of gutta-percha-covered wire contained in the ebonite
tubes. These spirals are called _choking coils_. When the balls are
arranged in the interior of the box in their proper position, they are
about ¹⁄₁₆ inch apart, and the rods to which they are attached are in
line with each other.
[Illustration: FIG. 73.—Electric wave radiator (A) and receiver (B).]
The outer ends of the choking coils are connected to an induction
coil or electrical machine, say a small Wimshurst machine, suitable
for producing electric sparks about 2 or 3 inches in length. If then
sparks are taken between the balls, we have an arrangement which is,
in fact, a small Hertz oscillator or radiator. It has been fully
explained in the last chapter that the action of the induction coil
or electrical machine is first to create a difference in the electric
condition of the balls, such that one is positively electrified and
the other negatively. The balls and rods and the surrounding air, as
already explained, then form a sort of Leyden jar or condenser, and
in virtue of the electromotive force the air is electrically strained
around the balls. When this strain reaches a particular value, the
air between the balls passes at once into a conductive condition, and
we have a discharge which is oscillatory in nature produced between
the conductors. We may consider that the electrical charges on the two
rods rush backwards and forwards, setting up on the rods an oscillatory
surface electric current, and that this is accompanied by a very rapid
reversal of the strain in the surrounding non-conductor or dielectric.
This state of affairs results in sending out into space an effect
called an electric wave.
[Illustration: FIG. 74.—Electric radiation detector (Miller).]
Turning, then, to the receiver B (Fig. 73), we notice that this
consists of a similarly shaped metal box, having in it a board to which
are fixed two short nickel wires. These are crossed without touching
in the interior of a small ebonite box (see Fig. 74). The wires are
just covered inside the box with a very small quantity of fine nickel
filings. To the end of the zinc receiver-box is fixed a long lead pipe,
in the interior of which are two insulated wires, _c_, _d_.
These wires are joined to the extremities of the nickel wires in the
receiver-box and then, passing through the lead pipe, they enter
another metal box which contains a battery and electric bell. The pinch
of nickel filings in the small ebonite box is not an electric conductor
in its ordinary condition, and hence the electric circuit, including
the battery and bell, is not complete. If, however, an electric
oscillation is set up in the nickel receiver-wires, the mass of metal
particles connecting them at once becomes a conductor, because little
metallic granules stick or cohere together. The battery is thus able
to send an electric current through the circuit, which includes the
coherer, and the electric bell is caused to ring. It may be mentioned
that in the actual apparatus employed the arrangement is not quite
so simple. The coherer would be permanently injured if we were to
attempt to send through it an electric current strong enough to ring an
electric bell. Hence we associate with the coherer a contrivance called
a _relay_. A single voltaic cell, E (a dry cell) (see Fig. 75), is
joined up in series with the coherer C and this relay R. The latter is
a sort of switch or circuit-closer of such kind that when a very feeble
current passes through it it closes a second circuit through which a
much stronger current can pass. The transition of the nickel filings
from a non-conductive to a conductive condition is, therefore, only the
means by which a very small current of electricity is allowed to pass
through the circuit of an electro-magnet which forms the circuit of the
relay. This action causes a piece of iron to be attracted, and this
again in turn closes another circuit, and so enables the current from a
second battery, F, of five or six cells to actuate the electric bell G.
The arrangement of the two batteries, the relay coherer, and bell will
be understood by studying the diagram of connections in Fig. 75.
The really important condition in securing success in the performance
of the experiments made with this apparatus is that the long wires
which connect the receiver-box with the metal box containing the bell,
battery, and relay shall be entirely enclosed in a lead pipe without
joint, which is soldered at one end into the receiver-box and at the
other into the battery-box. Another practical point is that these
wires, where they enter the battery-box, must have included in their
circuit two little coils of insulated wire of a good many turns, which
are called “choking coils.” A third element of success is that the
coherer or sensitive conductor shall be sensitive enough, but not too
sensitive. This condition can only be obtained by a process of trial
and failure. Being provided with these two pieces of apparatus, we can
now proceed to exhibit a series of experiments of great interest.
[Illustration: FIG. 75.]
In the first place, let the radiator-box and receiver-box be placed a
few feet apart with their open mouths facing each other, like two guns
arranged to fire down each other’s throats. Then, if all is in order
when we make an electric spark between the two balls of the radiator,
the electric bell in connection with the receiver will begin to ring,
showing that the coherer in the receiver-box has been affected and
made conductive by the electric wave sent out from the radiator-box.
If a smart rap is then given to the receiver-box the clinging metallic
filings in the ebonite box will be separated again and, the circuit
being interrupted, the bell will stop ringing.
This being done, the radiator-box is then turned a little on one side
by rotating it round its hollow trunnions like a gun until the open
mouths of the two boxes no longer face each other. It will then be
found, on repeating the former experiment, that the bell will not ring
when a spark is made between the balls. A little experimenting will
show that the action which affects the coherer is propagated out from
the radiator-box in straight lines like the light from a lamp, and
that we are here dealing with something which has all the character
of radiation. In the next place, let the receiver- and radiator-boxes
be again arranged with their open mouths facing each other. We make
a spark and again secure the responsive action of the bell. We shall
now proceed to prove that this effect, which is called _electric
radiation_, passes quite freely through certain substances, but is
more or less completely stopped by others. For instance, if we hold a
sheet of iron, tinfoil, or even paper covered with silver leaf between
the open mouths of the radiator and receiver, we find that the bell
of the receiver will not ring even when a rapid series of oscillatory
sparks are made in the radiator. These sheets of metal, thick or
thin, are quite _opaque_ to the electric radiation proceeding from
the spark-balls. On the other hand, we find a sheet of paper or card,
a wooden board, a sheet of glass, a slab of wax or bitumen, sulphur,
marble, or slate, are all quite pervious or transparent, and when held
between the radiator and receiver do not hinder at all perceptibly
the action of the former on the latter. We conclude, therefore, that
some bodies are opaque and some transparent to the electric radiation.
But the classification does not agree with the classification as
regards opacity or transparency for light. Wood, marble, and pitch
are optically opaque, but electrically transparent. The general law,
however, which decides the question of opacity or transparency for
electric radiation, is as follows: _All good electrical conductors are
opaque to electric radiation, and all good insulators or non-conductors
are transparent._
Hence we see at once why metal sheets are opaque, and wood, wax, or
glass transparent, to the electric radiation from the spark-balls.
We may go one step further. If we take some sheets of perforated zinc
or wire gauze, or even a large packet of pins, or paper bag full of
iron filings, we shall find that all these bodies are practically
opaque to the electric rays. Moreover, we can show that not only is the
above law true for solids, but it holds good for liquids as well. I
have provided here a number of flat glass bottles which are filled with
various liquids, salt water, fresh water, solution of soda, paraffin
oil, olive oil, turpentine and methylated spirits.
If we test an empty glass bottle between the radiator and receiver,
we can assure ourselves that the bottle itself is transparent to the
electric radiation.
If, then, we take the bottles containing the various liquids and test
them one by one between the radiator and the receiver, we find that the
bottles containing the paraffin oil, the olive oil, and the turpentine
are transparent to the electric radiation, but that the bottles
containing the salt water, the fresh water, the solution of soda, and
the methylated spirits are all opaque. The oils and liquids similar to
them are all good non-conductors, whereas water and various aqueous
solutions are fairly good conductors of electricity, and hence these
liquids, although they are all about equally transparent to light,
behave very differently to electric radiation. As regards the electric
ray, a bottle full of pure water is as opaque to the electric radiation
we are here using as it would be to light if it were filled with black
ink.
Experiment shows that every object containing water, or which is wet,
is exceedingly opaque to the electric radiation we are employing. Thus,
for instance, if I take a dry duster folded in four, and hold it in the
path of the electric ray, you see that it is quite transparent, and
that the bell attached to the receiver rings as easily as if there were
no duster there at all. If, however, we dip the duster in water, and
then hold it between the radiator and receiver, we find that the wet
duster is perfectly opaque.
The human body consists largely of water which exists in the tissues,
and hence it is not surprising to find that the hand or any part of the
body placed between the radiator and receiver intercepts the electric
ray. You see, if I hold my hand in front of the radiator, that nothing
is able to escape from it, when sparks are made between the balls,
which can affect the receiver. In the same way it can be shown by
experiment that the human head is perfectly opaque—in fact, much more
opaque than an equally thick block of wood; and this opacity to the
electric ray is due in a veritable sense to the water in the brain.
All dry animal tissues, such as leather, bone, gelatine, and flesh, if
dry, are very transparent to electric radiation of the kind we are now
using, but if these objects are made thoroughly wet, then they become
intensely opaque.
[Illustration: FIG. 76.—The reflection of an electric ray.]
We can, then, proceed to show that this electric radiation can be
reflected, just like light or sound, by metal or other conducting
surfaces, and that the law of reflection of the electric ray is the
same as the law of reflection for rays of light or sound. If we place
the radiator A with its mouth upwards, still preserving the receiver
B in a horizontal position, it is possible to adjust the two very
near to one another, but yet so that the radiation from the radiator
does not affect the receiver. If I now hold a metal plate, P, at an
angle of 45° above the mouth of the radiator, you will notice that the
bell at once rings, thus showing that the electric radiation has been
reflected into the receiver-box (see Fig. 76). Also we find that a very
small deviation from the angle of 45° is sufficient to prevent the
effect. Careful experiments in the laboratory show that the electric
ray is reflected according to the optical law, viz. that the angle of
reflection is equal to the angle of incidence. We find that any good
conducting surface will, in this manner, affect the electric radiation.
Thus I can reflect it from a sheet of tinfoil or even from my hand, and
the fact that I can, so to speak, take hold of this electric radiation,
and deflect it in different directions by the palm of my hand, produces
in the mind a very strong conviction that we are dealing with
something of a very real nature in experimenting with this electric
radiation.
It will be in your remembrance that, in the chapter in which we
were dealing with waves in the air, I showed you a very interesting
experiment illustrating the refraction of rays of sound by means of
a carbonic acid prism, and I have now to bring before you an exactly
analogous experiment performed with electric radiation. Here, for
instance, is a prism made of paraffin wax, a substance which you
have already seen is transparent to the electric ray. If we arrange
the radiator- and receiver-boxes at an angle to one another, it is
possible so to adjust them that the electric radiation projected from
the radiator-box A just escapes the receiver-box B, and does not
therefore cause the bell to ring (see Fig. 77). When this adjustment
has been made we introduce the paraffin prism P into the path of the
electric ray, and if the adjustments are properly made, we find that
the electric ray is bent round or refracted, and that it then enters
the receiver-box and causes the bell to ring. This experiment was first
performed by Hertz with a very large pitch prism, but his apparatus was
too cumbersome for lecture purposes, and the smaller and more compact
arrangement you see before you is therefore preferable for present
purposes.
[Illustration: FIG. 77.—The refraction of an electric ray.]
I have it in my power to show you a still more remarkable experiment in
electric refraction. It is found that dry ice is very transparent to
these electric rays, but if the ice is wetted on the surface, then, as
you have already learnt, the film of moisture is opaque. We have had
constructed for the purposes of this lecture a prism of ice by freezing
water in a properly shaped zinc box. This prism is now being arranged
between the radiator and the receiver, and its surfaces must next be
dried carefully with dusters and white blotting paper to remove every
trace of moisture. When this is done we find we can repeat with the ice
prism the same experiment performed just now with the paraffin prism,
and we can refract the electric ray. If you will recall to your memory
the statements which were made in connection with the refraction of
rays of sound and waves of water, you will remember that it was pointed
out that the refraction of a ray of sound and the bending of a train
of water waves was due to the passage of the waves in the air or in
the water from a region where they were moving quickly to a region in
which they were compelled to move more slowly; and it was furthermore
shown that this bending must take place whenever a plain wave of any
kind passes in an oblique direction from one region to another region
where it undergoes an alteration in velocity. In other words, it was
shown that the bending or refraction of the direction of motion of a
wave, whether in air or water, is a proof that there is a difference
in its velocity in the two places bounded by the surface at which the
refraction takes place. If this bending takes place in such fashion
that the ray is bent towards the perpendicular line drawn to the
bounding surface, which is the same thing as saying if the line of
the wave is bent so as to make a less angle with the bounding surface
after it has passed from one region to the other, then it shows that
the wave-motion travels more slowly after it has passed the bounding
surface than before.
If we now return to the consideration of the electric experiment
with the prism of paraffin or ice, we shall find that this, properly
interpreted, gives us a proof that the electric radiation travels
more slowly in paraffin wax or ice than it does in air, and the ratio
between its velocity in air or in empty space and its velocity in any
non-conductor is called the _electric index of refraction_ for that
non-conductor. This index can be determined by making two measurements.
First, that of the refracting angle of the prism; and secondly, that of
the deviation of the ray.[26] I have made these two experiments for the
prisms of paraffin and ice in my laboratory, and I find the electric
refractive index of paraffin to be 1·64, and the electric refractive
index of ice to be 1·83.
In connection with the refraction of rays of sound, it was pointed
out that a curved surface has the power to diverge or converge rays
of sound, and you will remember that we employed a sound-lens for
converging the rays of sound diverging from a whistle, just as an
ordinary burning-glass, or double convex lens, can be employed to
bring the rays of sunlight to a focus. We shall now attempt a similar
experiment with the electric ray. A block of paraffin is fashioned
into the shape of a semi-cylinder, flat on one side and convex on the
other, and this plano-convex paraffin lens has a convex surface having
a radius of 6 inches. If I place the radiator A and receiver B about
4 feet apart, then by making a few little adjustments it is possible
to so arrange matters that the radiation which proceeds from the
radiator is not powerful enough at a distance of 4 feet to sensibly
affect the coherer and make the bell ring (see Fig. 78). If, however,
I adjust the paraffin lens =L= halfway between, I shall converge this
electric radiation to a focus just about the place where the coherer
is situated, and the consequence is that on making sparks between the
balls of the radiator, we find that the bell attached to the receiver
at once rings.
[Illustration: FIG. 78.—Converging a beam of electric radiation.]
We have, therefore, here brought to a focus, by means of a paraffin
lens, the electrical radiation just in the same manner that an ordinary
burning-glass focuses the rays of light and heat of the sun, and
enables us to light with it some paper or a cigar. We have, therefore,
indubitable proof in all these experiments that we have something
proceeding from the radiator which is capable of being reflected or
refracted just like the rays of sound or ripples on the surface of
water; and, moreover, we find that this electric radiation passes
through some substances but not through others. There is, therefore,
a strong presumption that we are here dealing with something which
is similar in nature to light, although it cannot affect the eye. In
order that we may complete the proof we must show that this radiation
is susceptible of interference. This proof may be partly obtained from
the consideration of the following facts connected with the opacity or
transparency of wire grating to the electric radiation:—
I have here a wooden frame across which are strained some wires about
a quarter of an inch apart (see Fig. 79). If we hold this frame or
grid in front of the radiator so that the direction of these wires
is at right angles to the direction of the radiator rods which carry
the balls, we find that the grid is quite transparent to the electric
radiation, but if we turn the grid round so that the wires of the
grid are parallel to the radiator rods, we find at once that the grid
becomes perfectly opaque. The same experiment can be prettily shown by
means of a paper of pins. Here are some large carpet pins arranged in
rows in paper, and if I hold this paper of pins in between the radiator
and receiver with the pins parallel to the radiator, it is perfectly
opaque to the electric ray, but if I turn it so that the pins are at
right angles, it is quite transparent. The same experiment succeeds
with a paper of ordinary pins, but not so well with a paper of midget
pins.
[Illustration: FIG. 79.]
The explanation of this action of a grid is as follows: You have
already seen that an alternating current in one electric circuit
can produce another alternating current in a secondary circuit
placed parallel with the first. It is not difficult to show, either
experimentally or from theory, that when the primary current is an
electrical oscillation—that is, a very rapid alternating current—the
current in the secondary circuit is also an electrical oscillation
of the same frequency or rapidity, but that the currents in the
two circuits, primary and secondary, are always moving in opposite
directions at the same moment. Accordingly, if we hold a grid in
front of the radiator, the wires of the grid have what are called
_induced oscillations_ set up in them, and these induced oscillations
themselves create electric radiation. Accordingly, it is clear that
if a grid of this kind is held near to a radiator with the wires of
the grid parallel to the radiator rods, we have two sets of radiations
produced which, at any point on the side of the grid furthest from the
radiator rods, must neutralize one another, and therefore destroy each
other’s effect. Hence it is possible to cause the electric radiations
proceeding from two electric circuits parallel with each other to
destroy one another at a distant point; and we may, therefore, make use
of the same arguments as in the case of a similar experiment with light
to prove that this electric radiation must be a wave-motion.
It would occupy too much of our time, and it would involve the
discussion of matters which are rather beyond the scope of elementary
lectures, if we were to enter into a complete analysis of all the
arguments proving that this electric radiation, which proceeds from
an electric oscillator, is really a wave-motion. I may, however,
mention one fact, which has been the outcome of an enormous amount of
experimental research, and that is, that the velocity of this electric
radiation through space is identical with that of light. It has already
been mentioned that a ray of light flits through space at the rate of
1,000,000,000 feet, or nearly 186,500 miles a second. By suitable and
very ingenious arrangements, physicists have been able to measure the
velocity of electric radiation, and have found in every case that its
velocity of propagation is precisely the same as a ray of light.
Let us, then, summarize briefly what we have learnt. We find that when
we set up an electrical oscillation in an open circuit consisting of
two metallic rods placed in one straight line, we have proceeding
from this circuit an electrical radiation which is capable of being
propagated through space, which moves in straight lines, can be
reflected and refracted, can exhibit the phenomena of interference,
and moreover which is propagated with exactly the same velocity as
light. Is it possible to resist the conclusion that this effect which
we call electric radiation, and the similarly behaving physical agency
which we call light, must both be affections of the same medium? It
is hardly necessary to occupy time with experiments in showing that a
ray of light can be reflected and refracted by mirrors and prisms, and
converged or diverged by transparent lenses. These are simple optical
facts, and if you are not familiar with them it will be easy for you to
make their acquaintance by studying any simple book upon optics; but
I should like to draw your attention to the fact that, in addition to
rays of light and electric radiation, we are acquainted with another
kind of radiation, which is also susceptible of being refracted, and
that is commonly called _dark heat_.
Supposing that we take an iron ball and make it red hot in a furnace,
then, in a perfectly dark room, we see the ball glowing brilliantly,
and we are conscious by our sensations that it is throwing off heat.
Let us imagine that the ball is allowed to cool down to a temperature
of about 500° C.; it will then just cease to be visible in a perfectly
dark room, but yet if we hold our hand or a thermometer near to it,
we can detect its presence by the dark radiant heat it sends out.
Experiments show that even when the ball is brilliantly incandescent,
nearly 98 or 99 per cent. of all the radiation it sends out is dark
heat, and only 1 or 2 per cent. is radiation which can affect the eye
as light. It is quite easy to show that this dark heat can be reflected
just like light. If I fix this red-hot ball in the focus of a metallic
mirror and lift up ball and mirror nearly to the ceiling and then
place upon the table another convex, polished, metallic mirror, the
top mirror will gather up and project downwards the radiation from the
iron ball and the bottom mirror will converge that to a focus. If then
we fix a red-hot ball in the focus of the upper mirror and allow it
to cool until it is just not visible in the dark, we shall find that
we can still ignite a piece of phosphorus or some other inflammable
substance by holding it in the focus of the bottom mirror, thus showing
that the dark radiation from the iron ball is susceptible of reflection
just as are rays of light or electric rays. In fact, if time permitted,
it would be possible to show a whole series of experiments with dark
radiant heat which would prove that this radiation possesses similar
properties of luminous or electric radiation in its behaviour as
regards reflection, refraction, and interference.
A vast body of proof has been accumulated that all these forms of
radiation are merely varieties of one and the same thing, and that the
only thing in which they really differ from one another is in what is
called their wave-length. At this point I will remind you once more of
that general law which connects together the velocity of propagation of
a wave-motion, the wave-length and the frequency. It is expressed in
the formula: _wave-velocity_ (V) _equals frequency_ (_n_) _multiplied
by wave-length_ (λ), or in symbolical language—
V = _n_λ
Accordingly, if the velocity of propagation can be determined, and if
the frequency or periodicity of the wave-motion is known, then the
wave-length can be found from the above simple rule; or conversely,
if the velocity of propagation and the wave-length are known, the
frequency is determined.
The wave-length of various kinds of monochromatic (one-colour)
light can be easily determined by means of Young’s experiment on
interference. If the distance between the two small holes from which
the two streams of light emerge is measured, and if the distance from
them to the screen and also the distance of the first dark band from
the central line is determined, it is then very easy to calculate the
difference in the distances from the two holes to the dark band. This
difference, however, must, as already explained, be equal to one-half
wave-length of the light employed. Experiments made in various ways
have shown that the wave-length of yellow light is not far from the
fifty thousandth part of an inch.
Hence as the velocity of visible light is 186,500 miles per second,
or 1000 million feet, or 12,000 million inches per second, whilst
the wave-length is something like ¹⁄₅₀₀₀₀ inch, it is clear that the
frequency, or number of light waves which enter the eye per second,
must be reckoned in _millions of millions_. In fact, it ranges from 400
to 700 billions. There is a certain difference of opinion as to what is
meant by a billion. We here use the word to signify a million times a
million, a million being a thousand times a thousand.
The following table shows us the frequency or number of waves per
second, corresponding to light rays producing colour-sensations of
various kinds:—
VIBRATION RATES OF ÆTHER WAVES AFFECTING THE EYE AS LIGHT.
Colour sensation. Vibrations per second.
Deep red 400 billions.
Red-orange 437 ”
Yellow-orange 457 ”
Yellow 509 ”
Green 570 ”
Blue-green 617 ”
Blue-violet 696 ”
Violet 750 ”
Investigation has shown that the quality in a light ray which causes
it to affect our eye with a particular colour-sensation is its
wave-length, whereas the quality which affects our eyes as brightness
or brilliancy is due to the amplitude of the waves. It is somewhat
difficult to realize at first that, outside of ourselves, there is no
such thing as colour. Colour is a sensation produced when æther waves
of a certain wave-length enter the eye and fall on the retina. If the
retina is stimulated 400 billions of times per second, we experience
a sensation of redness, and if it is stimulated 700 billion times
per second, we experience a sensation of blueness; but externally,
there is no such thing as red and blue, there is only a difference in
wave-frequency. It is astonishing when we learn for the first time that
400 millions of millions of times per second something in the back of
our eyes is moved or stimulated whenever we look at a lady’s red dress,
a surgeon’s red lamp, or the red petal of a geranium flower.
You will notice, on referring to the above table of frequencies, that
the range of sensibility of the human eye is very much smaller than
that of the ear. Our eyes are wonderful instruments for detecting
wave-motion in the æther, and our ears are appliances for detecting
wave-motion in the air. The ear, however, is, as explained in a
previous chapter, sensitive to air-vibration forming musical tones
which lie between 30 and 30,000 per second, and these numbers are
in the ratio of 1000 to 1, and cover a range of about ten octaves.
The eye, however, is only sensitive to æther-vibrations which lie in
frequency between 400 and 700 billions per second, and these numbers
are in the ratio of nearly 2 to 1, or comprise only _one_ octave.
The question, of course, immediately arises—What are the properties of
æther waves the frequency of which lies outside the above limits?
Scientific investigation has made us acquainted with a vast range or
gamut of æther-vibrations, and we are able to summarize our present
knowledge as follows:—
The physical effect we call light, and that which we have up to
the present moment merely called electric radiation, are really
identical in nature, and both consist in waves propagated through the
space-filling æther, the only difference between them is in wave-length
and wave-amplitude. In between these two classes of radiation comes a
third, which is called the dark-heat radiation, and beyond the limits
of visible radiation we are acquainted with another group of æther
waves which cannot affect the eye as light, but which from their power
to affect a photographic plate, is called _actinic radiation_. Hence,
briefly speaking, four great groups of æther waves are known to us,
called respectively—
1. Actinic, or photographic rays.
2. Luminous, or light rays.
3. Ultra-red, or dark-heat rays.
4. Electric, or Hertz rays.
Convincing proof has been afforded that these various rays are
essentially the same in nature, and that they consist in periodic
disturbances or waves propagated through the æther in every case with
the velocity of 186,500 miles, or 1000 million feet, or 30,000 million
centimetres per second.
We may, therefore, say that these classes of æther waves differ from
each other only in the same sense in which a bass note in music differs
from a treble one; that is, the difference is a difference in frequency.
Just, therefore, as we have a gamut, or scale of musical tones, or
air-vibrations of increasing frequency, so we may arrange a gamut
or scale of æther waves progressively placed according to their
vibration-rates. Our present knowledge concerning æther waves can best
be exhibited by arranging in a chart a series of numbers showing the
wave-lengths of the waves with which we are so far acquainted. As a
limit of length we shall take the one-thousandth part of a millimetre.
Most persons know that a millimetre is a thousandth part of a metre,
and is a short length nearly equal to one twenty-fifth of an inch. The
thousandth part of a millimetre is called a _micron_, and is denoted
by the symbol 1μ. This last is therefore an exceedingly short length,
nearly equal to one twenty-five thousandth part of an inch.
Following, also, the musical nomenclature, we shall speak of all
those waves included between two wave-lengths, one of which is double
or half the other, as _an octave_. Thus all the various waves whose
wave-lengths lie between 1μ and 2μ in length are said to be an octave
of radiation. As a preliminary to further discussion let us consider,
in the first place, the simple facts about the radiation which affects
our eyes as light.
The light which comes to us from the sun is not a simple thing.
It consists of æther waves of many different wave-lengths mingled
together. Sir Isaac Newton first revealed to us the compound nature of
white light by his celebrated experiment with a glass prism, and his
optical discoveries were the starting-point for our information on this
subject. If a beam of sunlight is allowed to fall on a glass prism,
the rays of light of different wave-lengths which compose it are each
bent or refracted to a different degree. In free space æther waves of
various wave-lengths all travel, as far as we know, at the same rate.
This equality in speed is, however, disturbed the moment the waves
enter a transparent material substance such as glass. The velocity of
propagation is then reduced in all cases, but it is generally more
reduced for the shorter waves than for the longer ones; and as a
consequence the rays of shorter-wave lengths are more bent or refracted
than the rays of longer wave-length. We have, therefore, a _dispersion_
of the component rays, or a sorting out or analysis of the mixture of
rays of various wave-lengths, and if we receive the light on a screen
after passing through the prism we have a band of coloured light
called a _spectrum_, which consists of a series of patches of light
each of a different wave-length. The component rays of the original
beam of light are spread out fan-fashion by the prism. We may note, in
passing, that it is not every transparent body when fashioned into a
prism which thus analyzes the light into a fan-shaped beam with rays of
various wave-lengths arranged in the order of their wave-lengths. The
substances which behave as does glass or water when made into prisms
are said to exhibit _normal dispersive power_. There are, however, some
bodies, such as iodine or an alcoholic solution of fuschine, which
exhibit _anomalous dispersion_ and refract some longer waves of light
more than some shorter ones. The arrangements for forming a normal
spectrum are as follows: We pass a beam of light from the electric lamp
through a lens, and place in front of this lens a metal plate with a
narrow vertical slit-shaped opening in it. At a proper distance in
front of the slit we place another lens, and project upon the screen a
sharp image of the slit in the shape of a bar of white light. Placing
a hollow glass prism filled with bisulphide of carbon in front of
the last lens, we find that the various rays in the white light are
dispersed, and we produce on the screen a band of rainbow-coloured
light, called the spectrum. This spectrum is in reality a series
of differently coloured images of the slit placed side by side. By
making use of the principle of interference as disclosed by Young, it
is possible to make a measurement of the wave-length of the rays of
light which produce the sensation of various colours when they fall
upon the eye. Thus the wave-length of those æther waves which produce
the sensation of deep red is 0·75μ, and that of the waves producing
the sensation of violet when they fall upon the retina of the eye is
0·43μ. The whole of the visible spectrum is therefore included within
a single octave of æther radiation. Within these limits any change in
the wave-lengths makes itself felt in our eyes as a change of colour.
It is commonly said that there are seven colours in the spectrum—red,
orange, yellow, green, blue, indigo, and violet. As a matter of fact,
a highly trained eye can discover about a thousand different tints in
the spectrum of white light. Time will not allow us to enter into any
discussion of what is called colour-vision and the theory of sensations
of colour. The fact I wish to impress upon you here is that, outside of
ourselves, there is no such thing as colour. The rays of light which
produce these sensations of colour when they enter the eye differ
from one another only in wave-length and wave-amplitude. Hence there
is a complete analogy between light of different colours and sounds
of different pitches or tone. Red light differs from blue light only
as a bass note in music differs from a treble note. Hence you must
distinguish very carefully between a ray of light in itself, and the
sensation it produces when it falls upon the retina of the eye. Our
eyes are gifted with a marvellous power of detecting slight differences
between the wave-length and the amplitude of the rays which may
stimulate two adjacent portions of the retina of our eyes.
That range of sensibility is, however, very limited. Supposing we allow
a ray having a wave-length greater than 0·75 or less than 0·43 to enter
the human eye. It produces no sensation of light at all. Accordingly,
if we form a spectrum with sunlight, we find a tolerably sharp limit
to the visible spectrum. Supposing, however, we allow the spectrum to
fall upon a sensitive photographic plate, we find that the plate will
be chemically acted upon far beyond the limits of the visible violet
end of the spectrum. Hence we learn that beyond the violet there is
radiation of a kind which is invisible to the eye, yet can affect a
photographic plate. This is called the _ultra-violet_, or _actinic_
radiation.
Schumann, in 1893, measured waves in actinic radiation of a wave-length
as short as 0·1μ, or one two hundred and fifty thousandth part of an
inch, and hence we may say that we are acquainted with at least two
octaves of invisible ultra-violet or actinic radiation, or æther waves
have wave-lengths lying between the limits 0·1μ and 0·4μ.
In a similar manner very delicate heat-detecting instruments or
thermometers called bolometers, or thermopiles, show us that beyond the
visible-red end of the normal spectrum there is radiation called the
_ultra-red_ radiation, or _dark-heat_, which cannot affect the eye.
The wave-length of dark-heat radiation has been measured up to a limit
of 67μ by Professor Rubens and Professor Nichols in 1897 and 1898.
Accordingly, we can assert that beyond the red end of the spectrum we
are acquainted with six octaves or more of ultra-red radiation, viz.
that lying in wave-length between 0·75μ and 67μ.
We may represent the above facts in another way as follows: In most
pianos the keyboard extends over a range of seven or eight octaves.
Imagine a piano having a keyboard with nine octaves, and that each key
was labelled to correspond with a light wave of a particular length. At
the extreme treble end let the first key be labelled 0·1, and at the
extreme base end let the last key be labelled 51·2. Then the various
octaves will be comprised between the keys marked 0·1, 0·2, 0·4, 0·8,
1·6, 3·2, 6·4, 12·8, 25·6, and 51·2 (see Fig. 77).
Suppose that each key when struck caused some kind of electric radiator
to emit an æther wave whose wave-length reckoned in microns or
thousandths of a millimetre, is indicated by the number on the key. Of
all this great gamut of æther waves only the notes of one octave, viz.
the third from the treble end, the wave-lengths of which lie between
0·4μ and 0·8μ, affect the retina of the human eye as light.
Those waves in the two octaves higher up, that is, of wave-length
less than 0·4μ, are able powerfully to affect a photographic plate,
and so, indeed, do some of the waves in the visible octave. We may,
in fact, say that all the æther waves with which we are acquainted,
the wave-length of which is less than about 0·5μ, are able to make
an impression upon a photographic plate. These rays, whatever their
wave-lengths, are called the actinic rays.
On the other hand, all the æther waves with wave-length greater than
about 0·8μ, and for six octaves further down, can only be recognized
by their ability to heat a delicate thermopile or other heat-measuring
instrument. They cannot affect the eye, and they have little or
no effect in decomposing silver salts and impressing a sensitive
photographic surface.
[Illustration: GAMUT OF ÆTHER WAVES.
Wave-lengths in microns. μ = ¹⁄₁₀₀₀ of a millimetre.
0·1μ } Shortest ultra-violet wave-length measured by
} Schumann, 1893.
0·2 } Two octaves of ultra-violet waves.
0·4 }
.............} 0·43μ = violet light }
V I B G Y O R} one octave of light waves.
.............} 0·75μ = red light }
0·8 }
1·6 }
3·2 }
6·4 }
12·8 } Seven octaves of dark-heat waves.
25·6 }
51·2 }
.............} Longest dark-heat wave-length = 67μ, measured
102·4 } by Nichols in 1898.
204·8 }
409·6 }
819·2 } Five octaves of unknown radiation.
1638·4 }
3276·8 } Shortest Hertz wave made by Lampa = 4 mm.
} (1897).
.............}
6553·6 }
13 mm. }
26 }
52 }
104 }
208 } Hertz radiation, 12 octaves of most frequently
416 } used Hertz waves.
832 }
1664 }
3328 }
6656 }
13312 }
............. Longer waves still up to 1000 feet or more in
wave-length are used in wireless telegraphy.
FIG. 80.]
It should be noted, however, that whilst there are more or less
definite limits to the wave-lengths of the eye-affecting radiation,
and probably also to the actinic, or photographic radiation (radiation
of some wave-lengths being both visible and actinic), rays of every
wave-length are in some degree thermal, or heat-producing. The term
_dark-heat radiation_ is, however, generally restricted to radiation
of that wave-length which is non-visible and non-actinic. This mode
of presenting the facts will call your attention again to the narrow
limits of sensibility of the human eye as compared with those of the
ear.
The above-mentioned range of wave-lengths does not, however, exhaust
our powers of æther-wave production. If we skip over six octaves
lying below the limits of the longest dark-heat wave with which we
are acquainted, we should arrive at a wave whose wave-length is about
4000μ, or 4 millimetres. At this point we encounter the shortest æther
waves which have yet been made by means of electrical oscillations in
the fashion first discovered by Hertz.
It is not possible to define exactly the wave-length limits of
radiation as yet made by means of electrical oscillations. Lampa
has experimented with æther waves made by the Hertz method, the
wave-length of which was not more than 4 millimetres. Professors Lodge,
Rhigi, Bose, Trouton, the author, and many others, have carried out
quasi-optical experiments with electrically made æther waves, the
wave-length of which ranged from a few millimetres to several inches.
Hertz’s own work was chiefly done with æther waves from 1 or 2 feet to
30 or 40 feet in wave-length. More recently, æther waves of 800 to 1000
feet in wave-length have been employed in wireless telegraphy. Perhaps
we shall not be wrong in saying that we are acquainted with sixteen or
seventeen octaves of æther-wave radiation which is made electrically,
and is usually called the Hertz radiation.
Between the radiation of greatest wave-length which proceeds from hot
or incandescent bodies such as the sun, the electric arc, or a hot
ball, and that of the shortest wave-length which has been created
by means of electrical oscillations set up in some form of Hertz
oscillator, there is a range of six octaves of æther waves which, so
far as we know, have not yet been manufactured or detected. Herein lies
an opportunity for much future scientific work. We have to discover
how to create and recognize these interconnecting wave-lengths. From
the fact that all Hertz waves travel with the same speed as light,
and from our ability to imitate, as you have seen, the well-known
optical phenomena with Hertz radiation of short wave-length, the great
induction has been made that all æther waves have the same essential
nature, and that invisible actinic rays, light rays, dark-heat rays,
and Hertz rays are all of them æther waves of various wave-lengths and
amplitudes. Thus we see, as Maxwell long ago predicted, that light
in all probability is an electro-magnetic phenomenon, and therefore
all optical effects must be capable of receiving an electro-magnetic
explanation. The inclusion thus made of the whole science of Optics
within the domain of Electricity and Magnetism is one of the grandest
achievements of Physical Science. It stands second only to Newton’s
great discovery of universal gravitation, which reduced all Physical
Astronomy to pure Dynamics, and showed that the force concerned in the
falling of a stone is identical with that which holds the planets in
their orbits, and controls the motions of galaxies of suns.
At the end of the last chapter it was explained that these Hertz
radiations are created in the æther by the suddenly starting, stopping,
or reversing the motion of crowds of electrons, which are, as it were,
instantly released from a state of pressure or tension, and set moving
inside a straight insulated conductor, which forms an open electric
circuit. The radiations we call light and dark heat are probably,
therefore, started in a similar manner by vibrations of the electrons
which form parts of, or which build up, atoms. There are many physical
phenomena which seem to show that the electrons which we can detach
from atoms in a high vacuum tube are capable of vibrating freely in
definite periods when in connection with their atom. If the atoms are
able to move freely, and if each is practically independent, as is the
case in a gas, and if they are then caused to radiate by any means,
the radiation emitted by the vibration of these electrons consists
of certain definite wave-lengths. Hence, when we form the spectrum
of an incandescent gas, we find it to consist of several detached
bright lines, each corresponding to one particular wave-length, and
we do not obtain a uniformly graduated band of coloured light. If an
atom is struck by colliding with another, and then left to itself, it
appears as if the electrons which compose it and form part of it are
set in vibration, and each executes its oscillation in some definite
period of time. An atom has, therefore, been compared to a “collection
of small tuning-forks,” which, if rudely struck, would result in
the emission of a set of air-wave trains, each one corresponding in
wave-length to one particular tuning-fork which emitted it. Hence, if
we could administer a blow to such a congeries of tuning-forks, and
then analyze the compound sound, we should obtain a sound spectrum
consisting of separated tones—in other words, a bright line spectrum
of the complex sound. Supposing, however, that we have a mass of atoms
much more closely in contact, as in the case of a solid body, the
continual collisions between the atoms and the closer contact between
them cause the vibrations of the electrons to be “forced,” and not
“free.” Hence the electrons are compelled to execute all varieties of
irregular motion, and these predominate over their regular free natural
vibrations. Accordingly, the waves emitted are of a large variety of
wave-length, and when the radiation is analyzed by a prism, we obtain a
continuous spectrum, or band of many-coloured light, as the result of
the separation of the rays of different wave-lengths present in it.
It is this fact which renders our present method of creating artificial
light so excessively uneconomical.
All our practical methods for making light consist in heating a solid
body in one way or another. In the case of the electric light we heat
electrically a carbon rod or filament, or else, as in the Nernst lamp,
a rod composed of magnesia and the rare earths. In the case of the
lime-light we heat a cylinder of lime. In an ordinary gas or candle
flame we heat small particles of carbon, and the same is the case even
in the sun itself.
But this process manufactures not only the single octave of radiation
which can affect our eyes as light, but a dozen other octaves of
radiation to which they are insensible. Hence it follows that of
the whole radiation from a gas flame, only about 3 per cent. is
eye-affecting light, the remainder is dark heat. In the case of an
incandescent electric lamp, this _luminous efficiency_ may amount to
5 per cent., and in the electric arc to 10 or 15 per cent. There is,
however, always a great dilution of the useful light by useless dark
heat.
The proportion of the light or eye-affecting radiation to the dark heat
in the total radiation from any source of light increases with the
temperature, but it is not always merely a question of temperature.
Thus the electric arc is hotter than a candle flame, and the sun is
hotter than the electric arc. Hence, whilst the luminous rays only form
3 parts out of 100, or 3 per cent. of the radiation of a candle, they
constitute 10 to 15 per cent. of those of the electric arc, and more
than 30 per cent. of those of the sun. On the other hand, the glow-worm
and the fire-fly seem to have possession of a knowledge and an art
which is as yet denied to man. It has been shown by Professor Langley
and Mr. Very that nearly the whole of the radiation from the natural
torch of the fire-fly is useful light, and none of it is useless dark
heat. Hence these photogenic (light-producing) insects have the art,
which we have not, of creating _cold_ light, or unadulterated luminous
radiation.
At the present moment in ordinary incandescent or glow-lamp electric
lighting we require to expend an amount of power, called one
horse-power, to produce illumination equal to that of 600 candles.
Supposing, however, that all our power could be utilized in generating
merely the rays useful for vision, or which can impress our eyes, we
might be able to create by the expenditure of one horse-power more than
twenty times as much illumination, that is, a light equal to 12,000
candles.
These figures show us what rewards await the inventor who can discover
a means of generating æther waves having wave-lengths strictly limited
to the range lying between the limits 0·4μ and 0·7μ without, at the
same time, being obliged to create radiation comprising longer waves
which are not useful for the purpose of rendering objects visible to
us. For the purposes of artificial illumination we require only the
æther waves in this one particular octave, and nothing else.
This increase in the efficiency of our sources of artificial
illumination is only likely to be brought about when we abandon the
process of heating a solid substance to make it give out light, and
adopt some other means of setting the electrons in vibration.
It is almost impossible to discuss the subject of æther waves without
some reference to the most modern utilization of them in the so-called
wireless telegraphy. Without entering upon the vexed questions of
priority, or on the historical development of the art, we shall simply
confine our attention here to a consideration of the methods employed
by Mr. Marconi, who has accomplished such wonderful feats in this
department of invention.
We have already seen that when two insulated conductors are placed
with their ends very near together, and are then electrified, one
positively and the other negatively, and then allowed to be suddenly
connected by an electric spark, they constitute an arrangement called
an electrical oscillator. If the conductors take the form of two long
rods placed in one line, and if their contiguous ends are provided
with spark-balls separated by a small gap, we have seen that we have
shown that, under the above-mentioned conditions, electric currents
of very high frequency are set up in these rods. For creating these
oscillations, an instrument called an induction coil or spark-coil is
generally employed. You will understand the arrangements better if a
brief description is given first of the spark-coil itself as used in
wireless telegraphy.
[Illustration: FIG. 81.—A 10-inch induction coil for wireless
telegraphy (Newton).]
The appliance consists of a large bundle of fine iron wires, which
are wound over with a long coil of insulated wire. This forms the
_primary coil_. It is enclosed entirely in a tube of ebonite. One end
of this coil is a _contact-breaker_, which automatically interrupts
an electric current flowing from a battery through the primary coil
(see Fig. 81). A _hand-key_ is also placed in the circuit to stop
and start the primary current as desired. Over the primary coil is
a very long coil of much finer silk-covered copper wire, called the
_secondary coil_. The length of this coil is very considerable, and
may amount to many miles. The secondary coil is divided into sections
all carefully insulated from each other. Another important part is the
_condenser_. This consists of sheets of tinfoil laid between sheets of
waxed paper, alternate tinfoil sheets being connected. The arrangement
forms virtually a Leyden jar, and one set of tinfoils is connected to
one side of the automatic break, and the other to the adjacent side.
When, therefore, the primary circuit is interrupted by the break,
the condenser is at that moment thrown into series with the primary
coil. The rapid interruption of the primary current causes a secondary
current in the fine-wire coil. The automatic contact-breaker makes from
ten to fifty such interruptions per second. At every “break” of the
primary a very high electromotive force is generated in the secondary
circuit, which may amount to many hundreds of thousands of volts. This
very high secondary electromotive force is able to cause an electric
discharge in the form of a spark between brass balls connected to the
secondary circuit terminals. Coils are generally rated by the length
(in inches) of the spark they can produce between brass balls about ¹⁄₂
inch in diameter. The coil most commonly used in wireless telegraphy is
thus technically termed a “10-inch induction coil,” from the length of
the spark this particular type of coil can produce.
If the insulated brass balls, called the spark-balls, connected to the
secondary terminals, are placed an inch or so apart, and the hand-key
in the primary circuit is closed, a battery connected to the primary
circuit will send a rapidly interrupted current through the primary
coil, and a torrent of sparks will pass between the spark-balls. The
primary current of the 10-inch coil is usually a current of 10 ampères,
supplied at a pressure of 10 volts.
If the hand-key is raised or pressed, it is possible to make long or
short torrents of secondary sparks.
Suppose, then, that we connect to the secondary spark-balls two long
insulated rods, and place the spark-balls about ¹⁄₄ inch apart. On
pressing the hand-key, we obtain a peculiarly bright crackling spark
between the balls, which is an oscillatory spark, and at the same
time, as already described, electrical oscillations are set up in
the rods and electric waves given off. We may represent to ourselves
these electrical oscillations in the rods as similar to the mechanical
vibrations which would be set up in a long elastic wooden rod, clamped
at the middle and set in vibration at the ends. Or we may consider them
similar to the fundamental vibrations of an open organ-pipe, the middle
of the pipe corresponding with the middle of the rod. In comparing
the mechanical vibrations of the rod or the acoustic vibration of the
air in the organ-pipe with the electrical oscillations in the long
rods, we must bear in mind that the displacement of the rod or the air
in the organ-pipe at any point corresponds with electrical pressure,
or potential, as it is called, at any point in the long oscillator.
Hence, bearing in mind the remarks in the fourth lecture, it will be
evident to you that just as the length of the air wave emitted by the
open organ-pipe is double the length of the pipe, so the length of the
electric wave thrown off from the pair of long rods is double their
total length.
Instead of using a pair of rods for the electrical oscillator, it was
found by Mr. Marconi to be an improvement to employ only one insulated
rod, held vertically, and to connect it to one spark-ball of the coil,
and to connect the opposite spark-ball to a metal plate buried in the
earth. Then, when the spark-balls are placed a little apart and the
hand-key pressed, we have a torrent of oscillatory sparks between the
“earthed ball” and the insulated rod ball. This sets up in the rod
electrical oscillations, which run up and down the rod. It is easy to
show that there is a strong electric current passing into and out of
the rod by connecting it to the spark-ball by means of a piece of fine
wire. When the sparks are taken, we find this wire will become hot, it
may be red hot, or sometimes it may be melted.
By applying the principles already explained, it is not difficult to
demonstrate that in the case of an oscillator consisting of a single
rod connected to one spark-ball the electric waves thrown off are in
wave-length four times the length of the rod.
The electrical actions taking place, therefore, are as follows: At
each interruption of the primary current of the spark-coil there is an
electromotive force created in the secondary circuit, which gradually
charges up the insulated rod until it attains a state in which it is
said to be at a potential or electrical pressure of several thousand
volts. The spark then happens between the balls, and the rod begins to
discharge.
This process consists, so to speak, in draining the electric charge out
of the rod, and it takes the form of an electric current in the rod,
which has a zero value at the top insulated end, and has its maximum
value at the spark-ball end.
Also, when the oscillations take place, we have variations of electric
pressure, or potential, which are at a maximum at the upper or
insulated end, and have a zero value at the spark-ball end. From the
rod we have a hemispherical electric wave radiated. In the language
of wireless telegraphists, such a simple insulated rod is called an
insulated _aerial_, or an insulated _antenna_.
A simple insulated aerial has, however, a very small electrical
capacity, and it can store up so little electric energy that the whole
of it is radiated in the first oscillation. Hence, strictly speaking,
we have no train of electric waves radiated, but merely a solitary
wave or electric impulse. The effect on the æther thus produced
corresponds to the effect on the air caused by the crack of a whip
or an explosion, and not to a musical note or tone as produced by an
organ-pipe.
We can, however, make an arrangement which is superior in electric
wave-making power, as follows:—
[Illustration: FIG. 82.—Transmitter for wireless telegraphy.]
The vertical rod, or antenna, A, is not insulated, but is connected
by its lower end with one end of a coil of insulated wire, S, wound
on a wooden frame (see Fig. 82). The other end of this last coil is
connected to a metal plate, _e_, buried in the earth. Around the
wooden frame is wound a second insulated wire, P, one end of which is
connected to one spark-ball of the induction coil, and the other end
to the outside of a Leyden jar, L, or collection of jars. This double
coil on a frame is called an oscillation transformer. The inside of
this condenser is connected to the second spark-ball of the induction
coil I. When these spark-balls S are placed a short distance apart,
and the coil set in action, we have a torrent of oscillatory electric
sparks between these balls, and powerful oscillations set up in one
circuit of the oscillation-transformer. These oscillations induce other
oscillations in the second circuit of the oscillation-transformer, viz.
in the one connected to the aerial. The oscillations produced in the
air-wire, or aerial, are therefore induced, or secondary oscillations.
The aerial wire, or antenna, has therefore a much larger store of
electric energy to draw upon, viz. that stored up in the Leyden jars,
than if it was itself directly charged by the coil.
In order, however, to obtain the best results certain adjustments
have to be made. It has already been explained that every open
electrical circuit has a certain natural time-period for the electrical
oscillations which can be set up in it. This is technically called its
_tune_.
If we administer a blow to a suspended pendulum we have seen that, if
left to itself, it vibrates in a definite period of time, called its
natural period. In the same manner, if we have a condenser or Leyden
jar having electrical _capacity_ which is joined in series with a coil
of wire having electrical _inertia_ or _inductance_, and apply to the
circuit so formed a sudden electromotive force or impulse, and then
leave the circuit to itself, the electric charge in it vibrates in a
certain definite period, called its natural electrical periodic time.
The aerial, or antenna, is simply a rod connected to the earth, but
it has a certain inductance, and also a certain electrical capacity,
and hence any metal rod merely stuck at one end in the earth has a
perfectly definite periodic time for the electrical oscillations which
can be produced in it. We may compare the rod in this respect with a
piece of steel spring held at one end in a vice. If we pull the spring
on one side, and let it vibrate, it does so in accordance with its
natural time-period for mechanical vibrations. The sound waves given
out by it have a wave-length equal to four times the length of the
spring. In the same manner the fundamental wave-length of the electric
waves emitted by an “earthed aerial,” or rod stuck in the earth,
when an electric impulse is applied to its lower end, and electrical
oscillations are set up in it, have a wave-length equal to four times
that of the rod. Hence to obtain the best result the circuit, including
the aerial A, must be “tuned” electrically to the circuit including the
Leyden jar L.[27]
A consideration of these arrangements will show you that if the
hand-key in the primary circuit of the induction coil is pressed for a
long or short time, we have long or short torrents of sparks produced
between the secondary balls, and long or short trains of electric waves
emitted from the aerial, or earthed vertical wire.
Whenever we have any two different signals, we can always make an
alphabet with them by suitable combinations of the two. In the
well-known Morse alphabet, with which every telegraphist is as
familiar as we all are with the printed alphabet, the sign for each of
the letters of the alphabet is composed of groups of long and short
symbols, called dots and dashes, as follows: Each letter is made by
selecting some arrangements of _dots_ or _dashes_, these being the
technical names for the two signs. The Morse code, as used all over the
world, is given in the table below—
[Illustration: THE MORSE ALPHABET.
A — ———
B ——— — — —
C ——— — ——— —
D ——— — —
E —
F — — ——— —
G ——— ——— —
H — — — —
I — —
J — ——— ——— ———
K ——— — ———
L — ——— — —
M ——— ———
N ——— —
O ——— ——— ———
P — ——— ——— —
Q ——— ——— — ———
R — ——— —
S — — —
T ———
U — — ———
V — — — ———
W — ——— ———
X ——— — — ———
Y ——— — ——— ———
Z ——— ——— — —
]
[Illustration: THE MORSE NUMERALS.
1 — ——— ——— ——— ———
2 — — ——— ——— ———
3 — — — ——— ———
4 — — — — ———
5 — — — — —
6 ——— — — — —
7 ——— ——— — — —
8 ——— ——— ——— — —
9 ——— ——— ——— ——— —
0 ——— ——— ——— ——— ———
Full Stop — ——— — ——— — ———
Signal for calling up — — — ——— — — — ———
]
The process of sending a wireless message consists in so manipulating
the key in the primary circuit of the induction coils that a rapid
stream of sparks passes between the secondary balls for a shorter
or for a longer time. This gives rise to a corresponding series of
electric waves, radiated from the aerial. The dash is equal in duration
to about three dots, and a space equal to three dots is left between
each letter, and one equal to five dots between each word. Thus, in
Morse alphabet the sentence “How are you?” is written—
[Illustration:
— — — — ——— ——— ——— — ——— ——— — ——— — ——— — —
H O W A R E
——— — ——— ——— ——— ——— ——— — — ———
Y O U
]
We have, in the next place, to explain how the signals sent out are
recorded.
[Illustration: FIG. 83.—Marconi receiving arrangement for wireless
telegraphy.]
At the receiving station is erected a second insulated aerial, antenna,
or long vertical rod, A (see Fig. 83), and the lower end is connected
to the earth through a coil of fine insulated wire, P, which forms one
circuit of an oscillation-transformer. The secondary circuit, S, of
this oscillation-transformer, which is called a _jigger_, is cut in
the middle and has a small condenser, C_{1}, inserted, consisting of
two sheets of tinfoil separated by waxed paper (see Fig. 83), and to
the ends of this circuit is connected the coherer, or metallic filings
tube, T, which acts as a sensitive receiver. The Marconi sensitive
tube (see Fig. 84) is made as follows. A glass tube about ¹⁄₄ inch in
diameter and 2 inches long has two silver plugs put in it, and these
are soldered to two platinum wires which are sealed into the closed
ends of the tube. The ends of the plugs are cut in a slanting fashion
and made very smooth. These ends very nearly touch each other. A very
small quantity of very fine metallic powder consisting of nineteen
parts nickel and one part silver is then placed between the plugs. The
quantity of this powder is scarcely more than could be taken up on the
head of a large pin. The glass tube is then exhausted of its air and
sealed. The tube is attached to a bone rod by means of which it is held
in a clip.
[Illustration: FIG. 84.—Marconi coherer.]
To the two sides of the above-mentioned condenser are connected two
wires which lead to a circuit including a single voltaic cell, V, and
a relay, E. The relay is connected to another circuit which includes a
battery, B, and a piece of apparatus called a _Morse printer_, M, for
marking dots and dashes on a strip of paper.
The working details of the above rather complicated system of apparatus
devised by Mr. Marconi would require for its full elucidation a large
amount of explanation of a technical character. The general reader may,
however, form a sufficiently clear idea of its performance as follows:—
When the electrical waves from the distant transmitting station
reach the aerial at the receiving station, they set up in it
sympathetic electrical oscillations. The most favourable conditions
are when the two aerials at the distant stations are exactly
similar. These electrical oscillations, or rapid electric currents,
set up an electromotive force in the secondary circuit of the
oscillation-transformer, and this acts, as already explained, upon
the metallic filings in the coherer-tube and causes it to become an
electrical conductor. The cell attached to the relay then sends a
current through the conductive circuit so formed and operates the
relay. This last contrivance is merely a very delicate switch or
circuit-closer which is set in action by a small current sent through
one of its circuits, and it then closes a second circuit and so enables
another much larger battery to send a current through the _Morse
printer_. The printer then prints a _dot_ upon a moving strip of paper
and records a signal. One other element in this rather complicated
arrangement remains to be noticed, and that is the _tapper_. Underneath
the coherer-tube is a little hammer worked by an electro-magnet like
an electric bell. This tapper is set vibrating by the same current
which passes through the Morse printer, and hence almost as soon as
the latter has begun to print, the sensitive tube receives a little
tap which causes the metallic filings to become again a non-conductor,
and so arrests the whole of the electric currency. If it were not
for this tapper, the arrival of the electric wave would cause the
printer to begin printing a line which would continue. The _dot_ is,
so to speak, an arrested line. If, however, trains of electric waves
continue to arrive, then dots continue to be printed in close order,
and form a _dash_ on the paper strip. It will thus be seen that the
whole arrangements constitute an exceedingly ingenious device of such
a nature that a single touch on the hand-key at one station causing
a spark or two to take place between the spark-balls makes a _dot_
appear upon a band of paper at the distant station; whilst, if the
hand-key is held down so that a stream of sparks takes place at the
transmitting station, a dash is recorded at the receiving station. The
means by which this distant effect is produced is the train of electric
waves moving over the earth’s surface setting out from one aerial and
arriving at the other.
[Illustration: FIG. 85.]
The reader who has difficulty in following the above explanations may
perhaps gather a sufficiently clear notion of the processes at work by
considering a reduced, or simplified, arrangement. Imagine two long
insulated rods, A, A′ (see Fig. 85), like lightning-conductors set up
at distant places. Suppose each rod cut near the bottom, and let a pair
of spark-balls, S, be inserted in one gap and a coherer or sensitive
tube, C, in the other. At one station let an electrical machine have
its positive and negative terminals connected to the two spark-balls,
and at the other let a battery and electric bell be connected to
the ends of the coherer. Then, as long as the coherer remains in a
non-conductive condition, the electric bell does not ring. If, however,
a spark is made between the balls, in virtue of all that has been
explained, the reader will understand that the coherer-tube becomes at
once conductive by the action of the electric wave sent out from the
transmitter-rod. The battery at the receiver-rod then sends a current
through the coherer, and rings the bell.
All the other complicated details of the receiver are for making the
process of stopping the bell and beginning over again self-acting,
and also for the production of two kinds of signals, a _long_ and
a _short_, by means of which an alphabet is made. In order that we
may have _telegraphy_ in any proper sense of the word, we must be
able to transmit any _intelligence_ at pleasure, and not merely one
single arbitrary signal. This transmission of intelligence involves
the command of an _alphabet_, and that in turn requires the power of
production of two kinds of signals.
It remains to notice a few of the special details which characterize
Mr. Marconi’s system of wireless telegraphy. In establishing wireless
communication between two places, the first thing to be done is to
equip them both with aerials. If one station is on land, it is usual
to erect a strong mast about 150 feet high, and to the top of this is
attached a sprit. From this sprit a stranded copper wire is suspended
by means of an insulator of ebonite, so that the upper end of the wire
is insulated. The lower end of the wire is led into a little hut or
into some room near the foot of the mast in which is the receiving and
transmitting apparatus.
If the apparatus is to be installed on board ship, then a similar
insulated wire is suspended from a yardarm or from a sprit attached to
a mast. Each station is provided with the transmitting apparatus and
the receiving apparatus, and the attendant changes over the aerial
from one connection to the other so as to receive or send at pleasure.
In the case of long-distance wireless telegraphy, the aerial is not a
single wire, but a collection of wires, suspended so as to space them
a little from each other. Thus in the case of the first experiments
made by M. Marconi across the Atlantic, the aerial erected on the coast
of Cornwall consisted of fifty stranded copper wires each 150 feet in
length suspended in a fan-shaped fashion from a long transverse stay
upheld between two masts. The wires were spaced out at the top and
gathered in together at the bottom.
The question which almost immediately occurs to most people to ask is
how far it is possible to prevent the electric waves emanating from
one station affecting all receiving instruments alike within a certain
radius. The answer to this is that considerable progress has been made
in effecting what is called “tuning” the various stations. In speaking
of acoustic resonance it has been pointed out that a train of air waves
can set up vibration in other bodies which have the same natural period
of vibration. Thus, if we open a piano so as to expose the strings,
and if a singer with a strong voice sings a loud true note and then
stops suddenly, it will be found that one particular string of the
piano is vibrating, viz. that which would give out if struck the note
which was sung, but all the rest of the strings are silent. It has been
pointed out that every open electric circuit has a natural electrical
time-period of vibration in which its electric charge oscillates if
it is disturbed by a sudden electromotive force and then left to
itself. If the two aerials at two stations are exactly alike, and if
the various circuits constituting the oscillation-transformers in the
transmitting and receiving appliances are all adjusted to have the same
electrical period, then it is found that the stations so tuned are
sympathetic at distances vastly greater than they would be if not so
tuned. Hence it is possible to arrange wireless telegraph apparatus so
that it is not affected by any electric waves arriving from a distance
which have not a particular time-period.
Mr. Marconi has also proved that it is possible to receive on the same
aerial, at the same time, two different messages on separate receiving
instruments from two distant but properly tuned transmitting stations.
Since the date of these pioneer inventions many different forms of
wave detector have been discovered, and wireless telegraphy has shown
itself to be of the greatest utility in effecting communication between
ship and ship, and ship and shore. Its value in enabling intelligence
to be transmitted from lightships or lighthouses to coast stations
cannot be over estimated. One very remarkable feature of the apparatus
as arranged by Mr. Marconi is the small space it occupies. It is in
this respect most admirably adapted for use on board ship. It only
requires a long, insulated, vertical wire which can easily be suspended
from a mast, and the whole receiving and transmitting apparatus can be
placed on board ship in a small cabin. Employing the sensitive tube and
Marconi receiving arrangements, messages can easily be sent 150 miles
over the sea-surface by means of an aerial 150 feet high and a 10-inch
induction coil.
It is a curious fact that better results are obtained over a
water-surface than over land. Two similar stations with the same
appliances can communicate at two or three times greater distance if
they are separated by sea than if they are on land and have no water
between. This is connected with the fact that electric waves are not
able to pass through sea-water, but can diffuse through dry earth.
The sea-surface acts somewhat like an optical reflector or mirror,
and the electric waves glide along its surface. The rotundity of the
earth within certain limits hardly makes any perceptible effect upon
the ease of communication. The waves sent out by the transmitter of
a long-distance wireless station are from 3000 to 20,000 feet in
length, and there is, therefore, a considerable amount of bending
or diffraction. It is a familiar fact, as already explained, that a
wave-motion, whether on water or in air, spreads round an obstacle to
a certain extent. Thus an interposing rock or wall does not form a
sharply marked _sound-shadow_, but there is some deflection of the air
waves by the edge of the obstacle. The amount of bending which takes
place depends on the length of the wave.
If we take two places on the sea-surface 200 miles apart, the surface
of the sea at the halfway distance is just 1¹⁄₄ miles above the
straight line joining the places. In other words, the rotundity of
the earth interposes a mountain of water 1¹⁄₄ miles high between
the places. The electric waves used in wireless telegraphy have a
wave-length of about 600 to 1000 feet, or say five or six to the
mile. Hence the interposition of an object, the height of which is
one-fortieth of the distance, is not sufficient to make a complete
_electric shadow_. If we were, for instance, blowing a trumpet creating
air waves 5 feet long, the interposition of a cliff between two places
a mile apart, but so situated that the cliff protruded to the extent
of 40 yards across the line joining them, would not cut off all sound.
There would be diffraction or diffusion enough of the air waves to
enable the sound to be heard round the corner. In the same manner
the electric waves are, so to speak, propagated round the corner
of the earth. More remarkable still, they have been detected, when
sufficiently powerful, at a distance of 6000 miles from the generating
station, and in this case they must have travelled a quarter of the way
round the earth.
A good conception of the relative speeds of water waves, air waves, and
æther waves can be gained by considering the time each of these would
take to cross the Atlantic Ocean, travelling in its own medium. Suppose
we could, at the same moment, create a splash in the sea near England
sufficiently great to cause a wave which would travel over the surface
of the Atlantic at the speed of many ocean waves, say at 30 miles an
hour. To cover a distance of 3000 miles this water wave would then
require 100 hours. Imagine that we could, at the same moment, make a
sound loud enough to be heard across the same ocean, travelling at the
rate of 1100 feet a second, or about 700 miles an hour, the sound wave
would cross from England to the coast of the United States in about
four hours. If, however, we were to make an æther wave it would flit
across the same distance in about the sixtieth part of a second.
If you have been able to follow me in these descriptions, you will see
that the progress of scientific investigation has led us from simple
beginnings to a wonderful conclusion. It is that all space is filled
with what we may call an ocean of æther, which can be tossed into waves
and ripples just as the air we breathe is traversed in all directions
by aerial vibrations, and the restless sea by waves and ripples on the
water-surface. We cannot feel or handle this imponderable æther, but
we have indubitable proof that we can create waves in it by suddenly
applying or reversing something we call electric force, just as we
are able to produce air or water waves by the very sudden application
of mechanical force or pressure. These æther waves, when started, not
only travel through the ocean of æther with astonishing speed, but they
are the means by which enormous quantities of energy are transferred
through space.
From every square yard of the sun’s surface energy is cast forth at
a rate equal to that produced by the combustion of eleven tons of
best Welsh coal per hour, and conveyed away into surrounding space by
æther ripples, to warm and light the sun’s family of planets. Every
plant that grows upon the earth’s surface is nourished into maturity
by the energy delivered to it in this way. Every animal that basks in
the sunlight is kept warm by the impact of these æther waves upon the
earth. All the coal we possess buried in the earth’s crust, and in this
age of steam forming the life-blood of the world, has been manufactured
originally by æther ripples beating in their millions, in long-past
ages, upon the vegetation of the primeval world.
But in another way the æther serves as a vehicle of energy—in the form
of an electric current. Every electric lamp that is lighted, every
electric tram-car that glides along, is drawing its supply of energy
through the æther. The wire or conductor, as we call it, serves to
guide and direct the path of the energy transferred; but the energy is
not in but around the wire. We have lately learnt to make what we may
best describe as billows in the æther, and these are the long waves
we employ in wireless telegraphy. But in telegraphy, whether with
wires or without, we are merely manipulating the æther as a medium of
communication, just as in speech or hearing we use the air.
We therefore find our physical investigations lead us to three great
final inquiries, when we ask—What is the nature of electricity, æther,
and energy? Already, it seems possible, we may obtain some clue to an
answer to the first question, and find it in a study of the electrons,
or tiny corpuscles which build up atoms. Concerning the structure of
æther, physical investigation, which has revealed its existence, may be
able to analyze a little more deeply its operations. But the question,
What is Energy? seems to take us to the very confines of physical
inquiry, where problems concerning the structure of the material
universe seem to merge into questions concerning its origin and
mystery. In its ultimate essence, energy may be incomprehensible by us,
except as an exhibition of the direct operation of that which we call
Mind and Will. In these final inquiries into the nature of things, the
wisest of us can merely speculate, and the majority but dimly apprehend.
We must not, however, travel beyond the limits of thought proper for
these elementary lectures. Their chief object has been to show you
that the swiftly moving ocean waves, which dash and roll unceasingly
against the coast-lines of our island home, are only instances of one
form of wave-motion, of which we find other varieties in other media,
giving rise to all the entrancing effects of sound and light. In these
expositions we have been able to do no more than touch the fringe of
a great subject. Their object will have been fulfilled if they have
stimulated in you a desire to know more about these interesting things.
Every star and flower, every wave or bird that hovers over it, can
tell us a marvellous story, if only we have eyes to see, and ears to
hear. We may find in the commonest of surrounding things a limitless
opportunity for intelligent study and delight. When, therefore, you
next sail your boat upon a pond, or watch ducks or swans swimming, or
throw stones into a pool, or visit the seaside, may I hope that some
of the matters here discussed will recur to your minds, and that you
will find a fresh meaning and new interest in these everyday objects.
Yon may thus, perhaps, receive an impulse attracting you to the study
of some chapters in the “Fairy Tale of Science,” more wonderful than
any romance woven by the imaginations of men, and open to yourselves
a source of elevating pleasure, which time will neither diminish nor
destroy.
APPENDIX.
—⋄—
NOTE A (see p. 21).
The distinction between the individual wave-velocity and a wave-group
velocity, to which, as stated in the text, attention was first called
by Sir G. G. Stokes in an Examination question set at Cambridge in
1876, is closely connected with the phenomena of _beats_ in music.
If two infinitely long sets of deep-sea waves, having slightly
different wave-lengths, and therefore slightly different velocities,
are superimposed, we obtain a resultant wave-train which exhibits a
variation in wave-amplitude along its course periodically. If we were
to look along the train, we should see the wave-amplitude at intervals
waxing to a maximum and then waning again to nothing. These points of
maximum amplitude regularly arranged in space constitute, as it were,
waves on waves. They are spaced at equal distances, and separated by
intervals of more or less waveless or smooth water. These maximum
points move forward with a uniform velocity, which we may call the
_velocity of the wave-train_, and the distance between maximum and
maximum surface-disturbances may be called the _wave-train length_.
Let _v_ and _v′_ be the velocities, and _n_ and _n′_ the frequencies,
of the two constituent wave-motions. Let λ and λ′ be the corresponding
wave-lengths. Let V be the wave-train velocity, N the wave-train
frequency, and L the wave-train length. Then N is the number of times
per second which a place of maximum wave-amplitude passes a given fixed
point.
Then we have the following obvious relations:—
_v_ = _n_λ, _v′_ = _n′_λ′, N = _n_ - _n′_ = _v_/λ - _v′_/λ′
Also a little consideration will show that—
L/λ′ = λ/(λ - λ′)
since λ is nearly equal, by assumption, to λ′. Hence we have—
1/L = 1/λ - 1/λ′; and also V = NL
Accordingly—
V = N/(1/L) = (_v_/λ - _v′_/λ′)/(1/λ - 1/λ′)
Let us write 2π/_k_ instead of λ, and 2π/_k′_ instead of λ′; then we
have—
V = (_vk_ - _v′k′_)/(_k_ - _k′_) (i.)
And since _k_ and _k′_, _v_ and _v′_ are nearly equal, we may write the
above expression as a differential coefficient; thus—
V = _d_(_vk_)/_d_(_k_) (ii.)
Suppose, then, that, as in the case of deep-sea waves, the
wave-velocity varies as the square root of the wave-length.
Then if C is a constant, which in the case of gravitation
waves is equal to _g_/2π, where _g_ is the acceleration due
to gravity, we have—
_v_^2 = Cλ, or _v_^2 = (_g_/2π) × λ
But λ = 2π/_k, hence—
_vk_ = 2πC/_v_
Hence if we differentiate with respect to _v_, we have—
_d_(_vk_)/_dv_ = -2πC/_v_^2
Again, _k_ = 2π/λ = 2πC/_v_^2; therefore—
_d_(_k_)/_dv_ = -2(2πC/_v_^3)
Hence, dividing the expression for _d_(_vk_)/_dv_ by that
for _d_(_k_)/_dv_, we have—
V = _d_(_vk_)/_d_(_k_) = _v_/2
In other words, the wave-train velocity is equal to half the
wave-velocity. This is the case with deep-sea waves. Suppose, however,
that, as in the case of air waves, the wave-velocity is independent
of the wave-length. Then if two trains of waves of slightly different
wave-length are superposed, we have _k_ and _k′_ different in value but
nearly equal, and _v_ and _v′_ equal. Hence the equation (i.) takes the
form—
V = _v_
In other words, the _beats_ travel forward with the same speed as the
constituent waves. And in this case there is no difference between the
velocity of the wave-train and the velocity of the individual wave. The
above proof may be generalized as follows:—
Let the wave-velocity vary as the _n_th root of the wave-length, or let
_v_^_n_ = Cλ; and let λ = 2π/_k_ as before.
Then—
_v_^_n_ = 2πC/_k_, and _vk_ = 2πC/_v_^{_n_-1} = 2πC_v_^{-(_n_-1)}
also _k_ = 2π/λ = 2πC/_v_^_n_ = 2πC_v_^{-_n_}
Hence _d_(_vk_)/_d_(_k_) = (_n_-1_v_^{-(_n_-1)-1})/(_nv_^{-_n_-1})
= ((_n_-1)/_n_)_v_
or V = ((_n_-1)/_n_)_v_
That is, the wave-train velocity is equal to (_n_-1)/_n_ times the
wave-velocity.
In the case of sea waves _n_ = 2, and in the case of air waves _n_ =
infinity.
If _n_ were 3, then V = (2/3)_v_, or the group-velocity would be
two-thirds the wave-velocity.
NOTE B (see p. 273).
Every electric circuit comprising a coil of wire and a condenser has
a definite time-period in which an electric charge given to it will
oscillate if a state of electric strain in it is suddenly released.
Thus the Leyden jar L and associated coil P shown in Fig. 82, p. 271,
constitutes an electric circuit, having a certain _capacity_ measured
in units, called a microfarad, and a certain _inductance_, or electric
inertia measured in centimetres. The capacity of the circuit is the
quality of it in virtue of which an electric strain or displacement
can be made by an electromotive force acting on it. The inductance is
the inertia quality of the circuit, in virtue of which an electric
current created in it tends to persist. In the case of mechanical
oscillations such as those made by vibrating a pendulum, the time of
one complete oscillation, T, is connected with the _moment of inertia_,
I, and the mechanical force brought into play by a small displacement
as follows: Suppose we give the pendulum a small angular displacement,
denoted by θ. Then this displacement brings into existence a restoring
force or torque which brings the pendulum back, when released, to
its original position of rest. In the case of a simple pendulum
consisting of a small ball attached to a string, the restoring torque
created by displacing the pendulum through a small angle, θ, is equal
to the product _mgl_θ, where _m_ is the mass of the bob, _g_ is the
acceleration of gravity, and _l_ is the length of the string. The ratio
of displacement (θ) to the restoring torque _mgl_θ is 1/_mgl_. This may
be called the displacement per unit torque, and may otherwise be called
the _pliability_ of the system, and denoted generally by P. Let I
denote the moment of inertia. This quantity, in the case of a simple
pendulum, is the product of the mass of the bob and the square of the
length of the string, or I = _ml_^2.
In the case of a body of any shape which can vibrate round any centre
or axis, the moment of inertia round this axis of rotation is the sum
of the products of each element of its mass and the square of their
respective distances from this axis. The periodic time T of any small
vibration of this body is then obtained by the following rule:—
T = 2π√((moment of inertia round the axis of rotation)
× (displacement per unit of torque, or pliability))
or T = 2π√(IP).
In the case of an electric circuit the inductance corresponds to the
moment of inertia of a body in mechanical vibration; and the capacity
to its pliability as above defined. Hence the time of vibration, or the
electrical time-period of an electric circuit, is given by the equation—
T = 2π√(LC)
where L is the inductance, and C is the capacity.
It can be shown easily that the frequency _n_, or number of electrical
vibrations per second, is given by the rule—
_n_ = 5000000/(√((capacity in microfarads)
× (inductance in centimetres)))
For instance, if we discharge a Leyden jar having a capacity of ¹⁄₃₀₀
of a microfarad through a stout piece of copper wire about 4 feet in
length and one-sixth of an inch in diameter, having an inductance of
about 1200 centimetres, the electrical oscillations ensuing would be at
the rate of 2¹⁄₂ millions per second.
Any two electrical circuits which have the same time-period are said
to be “in tune” with each other, and the process of adjusting the
inductance and capacity of the circuits to bring about this result is
called electrical tuning. In the case of a vertical aerial wire as used
in wireless telegraphy, in which the oscillations are created by the
inductive action of an oscillation-transformer as shown in Fig. 82,
page 271, the capacity of the Leyden jar in the condenser circuit must
be adjusted so that the time-period of the nearly closed or primary
oscillation P agrees with that of the open or secondary circuit S. When
this is the case, the electrical oscillations set up in the closed
circuit have a far greater effect in producing others in the open
circuit than if the two circuits were not in tune. The length of the
wave given off from the open circuit is approximately equal to four
times the length of the aerial wire, including the length of the coil
forming the secondary circuit of the oscillation-transformer in series
with it.
FOOTNOTES
[1] The wave-velocity in the case of waves on deep water varies as
√(_g_λ/2π), where λ is the wave-length. The rule in the text is deduced
from this formula.
[2] If V is the velocity of the wave in feet per minute, and V′ is the
velocity in miles per hour, then (V′ × 5280)/60 = V. But V′ = √(2¹⁄₄λ),
and V = _n_λ, where λ is the wave-length in feet and _n_ the frequency
per minute; from which we have V′ = 198/_n_, or the rule given in the
text.
[3] The amplitude of disturbance of a particle of water at a depth
equal to one wave-length is equal to 1/ϵ^{2π} of its amplitude at the
surface. (See Lamb’s “Hydrodynamics,” p. 189.)
[4] This can easily be shown to an audience by projecting the apparatus
on a screen by the aid of an optical lantern.
[5] See “The Splash of a Drop,” by Professor A. M. Worthington, F.R.S.,
Romance of Science Series, published by the Society for Promoting
Christian Knowledge.
[6] See Osborne Reynolds, _Nature_, vol. 16, 1877, p. 343, a paper read
before the British Association at Plymouth; see also Appendix, Note A.
[7] A very interesting article on “Kumatology, or the Science of
Waves,” appeared in a number of _Pearson’s Magazine_ for July, 1901. In
this article, by Mr. Marcus Tindal, many interesting facts about, and
pictures of, sea waves are given.
[8] Lord Kelvin (see lecture on “Ship Waves,” Popular Lectures, vol.
iii. p. 468) says the wave-length must be at least fifty times the
depth of the canal.
[9] See article “Tides,” by G. H. Darwin, “Encyclopædia Britannica,”
9th edit., vol. 23, p. 353.
[10] The progress of the Severn “bore” has been photographed and
reproduced by a kinematograph by Dr. Vaughan Cornish. For a series
of papers bearing on this sort of wave, by Lord Kelvin, see the
_Philosophical Magazine_ for 1886 and 1887.
[11] See Lord Kelvin, “Hydrokinetic Solutions and Observations,”
_Philosophical Magazine_, November, 1871.
[12] “On the Photography of Ripples,” by J. H. Vincent, _Philosophical
Magazine_, vol. 43, 1897, p. 411, and also vol. 48, 1899. These
photographs of ripples have been reproduced as lantern slides by
Messrs. Newton and Co., of Fleet Street, London.
[13] Some smokers can blow these smoke rings from their mouth, and they
may sometimes be seen when a gun is fired with black old-fashioned
gunpowder, or from engine-funnels.
[14] For details and illustrations of these researches, the reader
is referred to papers by Professor H. S. Hele-Shaw, entitled,
“Investigation of the Nature of Surface-resistance of Water, and of
Stream-line Motion under Experimental Conditions,” _Proceedings of
the Institution of Naval Architects_, July, 1897, and March, 1898.
A convenient apparatus for exhibiting these experiments in lectures
has been designed by Professor Hele-Shaw, and is manufactured by the
Imperial Engineering Company, Pembroke Place, Liverpool.
[15] The French word _échelon_ means a step-ladder-like arrangement;
but it is usually applied to an arrangement of rows of objects when
each row extends a little beyond its neighbour. Soldiers are said to
march in echelon when the ranks of men are so ordered.
[16] See Lord Kelvin on “Ship Waves,” Popular Lectures, vol. iii. p.
482.
[17] More accurately, as the 1·83 power of the speed.
[18] This figure is taken by permission from an article by Mr. R. W.
Dana, which appeared in _Nature_ for June 5, 1902, the diagram being
borrowed from a paper by Naval Const. D. W. Taylor, U.S., read before
the (U.S.) Society of Naval Architects and Marine Engineers (1900).
[19] “Practical Applications of Model Experiments to Merchant Ship
Design,” by Mr. Archibald Denny, Engineering Conference, Institution of
Civil Engineers, May 25, 1897.
[20] Reproduced here by the kind permission of the editor of
_Harmsworth’s Magazine_.
[21] See Lord Kelvin’s Popular Lectures, vol. iii., “Navigation,”
Lecture on “Ship Waves.”
[22] See Professor W. F. Barrett, _Nature_, 1877, vol. 16, p. 12.
[23] This follows from the ordinary formula for the focal length _f_
of a biconvex lens, each surface having a radius of curvature equal to
_r_. For then it can be shown that
_f_ = (r_/2) · (1/(μ - 1))
where μ is the index of refracture of the lens material. As shown later
on, the acoustic index of refraction of carbonic acid, when that of
air is taken as unity, is 1·273. Hence, μ - 1 = 0·273, and 1/(μ - 1) =
3²⁄₃. Hence, _f_ = 2_r_(¹¹⁄₁₂), or _f_ is slightly less than twice the
radius of curvature of the spherical segment forming the sound-lens.
[24] We can, in fact, discover the ratio of the velocities from the
amount of bending the ray experiences and the angle BAC of the prism,
called its refracting angle. It can be shown that if we denote this
refracting angle by the letter A, and the deflection or total bending
of the ray by the letter D, then the ratio of the velocity of the wave
in air to its velocity in carbonic acid gas (called the _acoustic
refractive index_), being denoted by the Greek letter μ; we have—
μ = sin((A + D)/2)/sin(A/2)
[25] On the occasion when this lecture was given at the Royal
Institution, a large phonograph, kindly lent by the Edison-Bell
Phonograph Company, Ltd., of Charing Cross Road, London, was employed
to reproduce a short address on Natural History to the young people
present which had been spoken to the instrument ten days previously
by Lord Avebury, at the request of the author. The address was heard
perfectly by the five or six hundred persons comprising the audience.
[26] In the case of the paraffin prism the refracting angle (_i_) was
60°, and the deviation of the ray (_d_) was 50°. Hence, by the known
optical formula for the index of refraction (_r_), we have—
_r_ = sin((_i_ + _d_)/2)/sin(_i_/2) = sin(55°)/sin(30°) = 1·64
For the ice prism the refracting angle was 50°, and the deviation 50°;
accordingly for ice we have—
_r_ = sin((50 + 50)/2)/sin(50/2) = sin(50°)/sin(25°) = 1·88
See “Cantor Lectures,” Society of Arts, December 17, 1900. J. A Fleming
on “Electric Oscillations and Electric Waves.”
[27] See Appendix, Note B.
INDEX.
A
Actinic rays, 254
Æther, the, 191
——, properties of, 192
—— wave radiation, range of, 262
—— waves, various kinds of, 234
Air, movement of, in a concert-room, 183
—— necessary for production of sound, 103
—— particles, mode of motion of, in case of sound wave, 112
—— waves, 103
—— ——, interference of, 139
—— ——, length of, 114
—— ——, nature of, 114
—— ——, speed of, 115
Alphabet used in telegraphy, 274
America Cup race, pictures of yachts entered for the, 94, 95
_America_ yacht, the, 93
Amplitude, 8
Anti-node, 159
Apparatus for detecting electric waves, 237
—— for exhibiting motion of air in case of sound wave, 109
—— for investigating the laws of falling bodies, 35
Atlantic waves, height of, 8
—— ——, length of, 9
Atomic theory, 226
B
Beam of sound focused by collodion lens, 131
Beats, Helmholtz’s theory of, 165
——, musical, 163
Billows, 1
Bore, 38
Branly, Professor, electric wave detector, 211
Breaking wave, 29
C
Canal-boat waves, 100
Canal wave, 32
—— —— velocity, 34
Capillary ripples, 44
Chromatic scale, 162
Clef, musical, 160
Closed organ-pipe, 171
Cloud waves, 30
Coherer, Lodge, 212
——, Marconi, 276
Column of air set in vibration by tuning-fork, 158
Conclusion, 285
Concords and discords in music, nature of difference between, 162
——, musical, 162
Conditions necessary for production of true wave in a medium, 15
Conductor, electric, 197
Conservation of energy, law of, 23
Conservation of matter, law of, 22
Convergence of an electric ray by a paraffin lens, 243
Cornish, Dr. Vaughan, 31
Corresponding speeds, Froude’s law of, 81
Cup Races, 93
Current, electric, 193
D
Dark heat, 250
Deep-sea waves, rule for determining speed of, 11
—— ——, velocity of, 11
Definition of a ripple, 42
—— of wave-frequency, 8
—— of wave-length, 7
Depth of water, effect of, on speed of canal wave, 34
Difference between electric conductors and non-conductors, 198
—— between velocity of a wave and of a wave-train, 20
Discords in music, 162
Dispersion of æther waves, 256
Distances at which sound can be heard, 120
E
Ear, power of, to analyze sound, 182, 183
Echelon waves made by a duck, 74
Eddies in liquids, 60
Eddy, 60
—— motion, 60
—— resistance, 68
Edison phonograph, 142
Elasticity of the air, 111
Electric circuit, open, 217
—— conductor and non-conductor, 197
—— corpuscles, 227
—— current, 193
—— ——, alternating, 194
—— ——, continuous, 194
—— —— energy, 215
—— ——, nature of, 193
—— displacement, 221
—— energy, mechanical analogue of, 216
—— force, 228
Electric index of refraction, 245
—— inductance, 214
—— inertia, 214
—— oscillations, 185
—— ——, apparatus for producing, 203
—— —— in open circuit, 217, 218
—— —— produced by discharge of Leyden jar, 200
—— radiation, 238
—— —— and light, identity in nature of, 251
—— radiation-detector (Fleming), 224
—— —— (Miller), 235
—— radiation, velocity of, 249
—— ray, reflection of, 241
—— ——, refraction of, 243
—— strain, 197
—— transparency and opacity, 239, 240
—— wave and air wave compared, 223
—— —— detector, 210
—— ——, nature of, 230
—— ——, production of, 209
—— waves, 185
Electrical or Hertz rays, 254
Electrodeless discharge, 205
—— ——, apparatus for producing, 206
Electro-magnet, 195
Electro-magnetic medium, 220
—— theory of light, 262
Electromotive force, 196
Electronic theory of electricity, 229
Electrons, 227
Energy, 21
——, kinetic, 25
—— of electro-static strain, 215
—— of motion, 25
—— of moving water, 32
——, potential, 25
——, two forms of electric, 215
Ether, the, 191
Experimental tank, uses of, in ship-design, 86
—— tanks, 85, 86
Experiments illustrating surface tension, 40, 41
Explosion of guns heard at great distances, 120
F
Falling bodies, laws of, 35
Fish, motion of a, 67
Flame, sensitive, 127
Flow of liquid in non-uniform tubes, 66
—— —— in tubes, 65
—— —— in uniform tubes, 65
Fog-signals, 123
——, influence of wind upon distance at which they are heard, 124
——, power absorbed in making, 125
Free period of vibration, influence upon force required to move a
body, 151, 152
Froude, Mr. William, 81
Froude’s experimental tank, 81
—— experiments at Torquay, 81
G
Gamut, 160
—— of æther waves, 260
Gravitation wave, 40
Ground swell, 31
H
Harmonic curve, 108
—— motion, 107
Harmonics, 156
Hele-Shaw, Professor, 62
—— ——, discovery of means of producing irrotational motion in
liquids, 63
—— ——, investigations on liquid motion, 63
Helmholtz’s investigation into nature of musical tones, 167
—— theory of discords and concords, 165
Hertz oscillation, 207
Hertz’s researches. Experiments with electric waves, apparatus for,
234
Hughes, Professor, investigations on electric waves, made by, 210
Human ear, the, 181
Hydraulic gradient, 65
I
Illustration of difference between wave-velocity and wave-group
velocity, 28
Inaudible sound, 140
Index of refraction, 54
Inductance, 214
Induction coil for wireless telegraphy, 267
Inefficiency of present methods of manufacturing light, 265
Inertia of the air, 111
Interference of air waves, 139
—— of electric rays, 248
—— of ripples and waves, 48
Irrotational motion, 59
K
Kelvin, Lord, investigations on ship waves, 77
Kinetic energy, 25
Krakatoa, eruption of, 116
——, sound produced by the eruption of, 116
L
Laplace, calculation of, concerning sound-velocity, 69
Law connecting velocity and pressure in liquid motion, 67
Length of wave, definition of the term, 7
Light, velocity of, 187, 189
Liquid flow in constricted tube, 66
Lodge, Sir Oliver, coherer invented by, 211
Long wave, 7
Longitudinal waves, 7
Luminous efficiency, 264
—— rays, 254
M
Magnetic force, 228
Major third in music, 161
Marconi coherer, 276
—— aerial wire, 266
——, experiments with wireless telegraphy across the Atlantic by, 280
—— system of wireless telegraphy, 276, 279
Matter, 22
Maxwell’s electro-magnetic theory of light, 262
Mechanical explanations of electrical phenomena, 222
Methods of manufacturing light, 265
Minor third in music, 161
Model illustrating longitudinal wave, 114
—— —— nature of an air wave, 113
Models, illustrating wave-motion, 5, 6
Morse alphabet, 274
Motion, harmonic, 107
——, irrotational, 59
—— of water in sea waves, 3
——, periodic, 107
——, rotational, 59
——, vortex, 59
Movement of the air in the case of a sound wave, 112
Music, theory of, 159
Musical beats, 163
—— scale, notes of the, 160
—— tones and noises contrasted, 110
—— ——, sharp and flat, 161
N
Natural period of vibration of a body, 148, 150
Node, 159
Non-conductor, electric, 197
Non-resistance to body moving through perfect fluid, 72
O
Octave, 160
Open electric circuit, 217
—— organ-pipe, 171
Optical proof that a sounding body is in vibration, 105, 106
Organ-pipes, construction of, 169, 171
——, distribution of air-pressure in, 172
——, overtones of, 173
——, relation between length of pipe and length of wave, 173, 174
Oscillations, electric, 185
—— of a stretched string, 154
Oscillator, Hertz, 207
Oscillatory electric sparks, photographs of, 202
Overtones, 156
P
Pendulum, isochronism of the, 149
Perfect fluid, 59
Periodic motion, 107
—— time, 9
Phonograph, action of the, 142
Photographic study of the production of waves, 16
Photographs of ripples on a mercury surface, 51
Plane wave, 55
Potential energy, 25
Power required to propel ships, 90, 91
Prism for refracting a beam of sound, 136
Production of a sound wave, 111
Q
Quality of sound, 115
R
Radiation, electric, 238
——, nature of, 263
—— of energy from the sun, 284
Rayleigh, Lord, an acoustic experiment with an open pipe by, 174
Receiver for wireless telegraphy (Marconi), 275
Reflection of a beam of sound, 132, 133
—— of an electric ray, 241
—— of a wave, 55
—— of ripples, 46, 47
Refraction, explanation of, 53, 54
—— of a beam of sound, 133, 134
—— of an electric ray, 243
—— —— by an ice prism, 242
—— of ripples, 52
Refractive index, 54
Relation of wave-velocity and wave-length, 9
Relay, telegraphic, 236
Resistance curves of ships, 92
—— to a body moving through a fluid, causes of the, 68
Resonance, 148
——, an experiment on, 158
Ripple and wave, distinction between, 40
—— mark, 30
——, reflection of a, 45, 46
——, scientific definition of a, 42
Ripples, 1
——, apparatus for producing, 43
——, interference of, 48
——, intersecting, 49
—— in the air, 103
—— on a lake, photographs of, 19
——, photography of, 51
——, by J. H. Vincent, 54
—— produced by stone thrown into water, 19
——, velocity of, 41, 42, 43
Rotational motion, 59
Russell, Scott, Mr., 81
—— experiments of, on canal-boat waves, 101
S
Scale, musical, notes of the, 160
—— of equal temperament, 162
Sea waves, 2
—— ——, motion of, 3
—— ——, relation of velocity and length in case of, 10
Semitone, 161
Sensitive flame, influence of sound upon a, 129
Severn bore, 38
_Shamrock II._, trials of, 97
Ship bow wave, mode of production, 75, 76
—— design, 87, 88
—— ——, the problem of, 73
—— models, the testing of, 85, 86
—— resistance, Froude’s law of, 82, 83
—— waves, 57
—— ——, complete system of, 78
—— ——, various systems of, 73
Short wave, 7
Sine curve, mode of drawing a, 108
Singing flame, 175
Siren, 123
Skin friction, 59, 68
—— —— for various classes of ships, 91
Soap film thrown into vibration by air waves, 145
—— solution for making bubbles, 144
Solitary wave, 26
—— —— and wave-train, difference between, 26
Sound, and music, 147
——, causes in variation in quality of, 142
—— due to air waves, 103, 104
—— lens, method of making a, 130
—— prism, 136
——, quality of, 115
—— signals, 123
——, velocity of, in various cases, 126
Sounding body is in vibration, 104, 105
Speed of a falling body, 35
—— of sound, 115
Stationary waves, 155, 156
Stone falling into water, photographs of a, 17
Stream-lines, 64
—— —— round an ovoid, 71
Stroh violin, 179
Structure of the human ear, 181
Surface tension of liquid, 40
T
Temperature, effect of, on sound-velocity, 118
Tesla coil, 205
Tidal wave, 38
—— ——, speed of, 39
Tides, 39
Time of vibration of a stretched string, 154
Tone, 161
Torpedo, motion of, in water, 67
Transference of wave-motion, 27
Transverse ship wave, 79
—— wave, 7
True wave, conditions for producing a, 15
—— ——, definition of a, 12
Tubes of flow, 64
—— —— in a liquid, 71
U
Utilization of the æther, 284
V
Various kinds of resistance to a body moving through a fluid, 68
Velocity of electric radiation, 249
—— of light, 189
—— of sea waves, rule for calculating the, 10
—— of sound, how affected by temperature, 117
—— —— in different gases, 126
—— ——, influence of specific heats upon the, 119
—— ——, measurements of the, 116
—— ——, theoretical determination of the, 118
—— of sound wave, 115
—— of waves in water, air, and æther compared, 283
Vernon Boys, Professor, instructions by, for making soap solution, 144
Vibration rates of musical tones, 160
—— —— of various æther waves, 252
Vibrations, forced, 149
——, free, 149
—— giving rise to musical tones, 160
Vincent, Mr. J. H., 51
Violin, improved by Mr. Augustus Stroh, 179
Violin, structure of, 177, 178
Viscosity of liquids, 58
Vortex motion, 59
—— ring in air, 61
—— ——, production of, in air, 61
W
Wave amplitude, definition of, 8
——, causes of breaking, 29
——, electric, nature of an, 185
—— frequency, 8
——, gravitation, 40
—— group, velocity of a, 20
—— length, 7
—— lengths of various kinds of æther waves, 257
——, longitudinal, 7
—— motion, 2
—— ——, definition of, 4
—— —— model, 5
—— ——, model for illustrating, 13, 14
—— ——, various kinds of, 4
—— produced in a canal, 33
——, reflection of a, 55, 56
—— resistance, 68
Waves, 1
—— and ripples in the æther, 232
——, conditions of, for interference of, 49, 50
——, electric, 185
——, interference of, 48
—— in the air, 103
—— made by canal-boats, experiments on, 101, 102
—— made by ships, 57, 58
—— on a snow surface, 31
—— on clouds, 30
—— produced by high-speed ships, 79
——, refraction of, 52, 53
——, sea, 2
——, stationary, 155
—— train, 20
—— ——, velocity of, 20
——, transverse, 7
—— velocity, a rule for determining, 9
Wind, influence of, upon distances at which sounds are heard, 120,
121, 122
Wireless telegraphy across the Atlantic, 280
—— ——, apparatus for, 267
—— ——, explanation of, 273
—— ——, Marconi’s system of, 269
—— ——, method of conducting, 274
—— ——, transmitter for, 271
—— ——, utility of, 281
Worthington, Professor, photographs of splash of drop, 16, 17
Y
Yacht-design, object of, 96
Yachts entered for the America Cup race, pictures of, 94, 95
Young, Dr. Thomas, investigations of, on interference of light, 190
THE END.
PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, LONDON AND BECCLES.
————————————————— End of Book —————————————————
Transcriber’s Note (continued)
Errors in punctuation and simple typos have been corrected without
note. Variations in spelling, hyphenation, accents, etc., have been
left as they appear in the original publication unless as stated in
the following:
Page 56 – “sea-side” changed to “seaside” (At the seaside)
Page 56 – “sea-side” changed to “seaside” (study of seaside pools)
Page 136 – “sound ray” changed to “sound-ray” in Fig. 51 caption
Page 145 – “limelight” changed to “lime-light” (lime-light lantern)
Page 156 – “over-tones” changed to “overtones” (accompanied by the
harmonics or overtones)
Page 162 – “key-board” changed to “keyboard” (keyboard of a piano)
Page 176 – “aërial” changed to “aerial” (stationary aerial oscillation)
Page 177 – “horse-hair” changed to “horsehair” (bow made of horsehair)
Page 274 - “Full Stop — — — — — —” changed to
“Full Stop — ——— — ——— — ———”
————
Footnotes have been re-indexed using numbers and placed before the Index.
————
Large bracketing is not available to surround built-up fractions in
displayed equations and expressions so ordinary bracketing and the
solidus (/) is used to represent them instead. Thus the built-up
x + 1
fraction ————— is rendered in this text in the conventional manner
2
as (x + 1)/2. If the sin of that expression is required it would be
rendered unambiguously as sin((x + 1)/2). For consistency this also
means that a simple sin function that would normally be displayed as
‘sin x’ will instead be rendered as ‘sin(x)’.
A large radical (√) symbol is not available either so in order to make
the radicand expression unambiguous a similar approach to the above is
used.
x + 1
Thus the square root of the fraction ————— is rendered in this text as
2
√((x + 1)/2). Again this means that, for consistency, the simple square
root of, say, the natural number 5, which would normally be displayed
as ‘√5’ will instead be rendered as ‘√(5)’.
In other expressions and equations some additional bracketing may have
been added to make the meaning clear and unambiguous or consistent with
bracketing as used in immediately surrounding equations and expressions.