axis, and we know the rate of rotation; but we only want to know for our
present purpose how much of the total rotation is about a vertical line in
the room. If the room were at the North Pole, the whole rotation would be
about the vertical line. If the room were at the equator, none of its
rotation would be about a vertical line. In our latitude now, the
horizontal rate of rotation about a vertical axis is about four-fifths of
the whole rate of rotation of the earth on its axis, and this is the amount
that would be measured by our microscope. This experiment would give no
result at a place on the equator, but in our latitude you would have a
laboratory proof of the rotation of the earth. Foucault made the
measurements with great accuracy.
If you now clamp the frame, and allow the spinning axis to have no motion
except in a horizontal plane, the motion which the earth tends to give it
about a vertical axis cannot now affect the gyrostat, but the earth
constrains it to move about an axis due north and south, and consequently
the spinning axis tries to put itself parallel {108} to the north and south
direction (Fig. 47). Hence with such an instrument it is easy to find the
true north. If there were absolutely no friction the instrument would
vibrate about the true north position like the compass needle (Fig. 50),
although with an exceedingly slow swing.
[Illustration: FIG. 47.]
It is with a curious mixture of feelings that one first recognizes the fact
that all rotating bodies, fly-wheels of steam-engines and the like, are
always tending to turn themselves towards the pole star; gently and vainly
tugging at their foundations {109} to get round towards the object of their
adoration all the time they are in motion.
[Illustration: FIG. 48.]
Now we have found the meridian as in Fig. 47, we can begin a third
experiment. Prevent motion horizontally, that is, about a vertical axis,
but give the instrument freedom to move vertically in the meridian, like a
transit instrument in an observatory {110} about its horizontal axis. Its
revolution with the earth will tend to make it change its angular position,
and therefore it places itself parallel to the earth's axis; when in this
position the daily rotation no longer causes any change in its direction in
space, so it continues to point to the pole star (Fig. 48). It would be an
interesting experiment to measure with a delicate chemical balance the
force with which the axis raises itself, and in this way _weigh_ the
rotational motion of the earth.[9]
Now let us turn the frame of the instrument G B round a right angle, so
that the spinning axis can only move in a plane at right angles to the
meridian; obviously it is constrained by the vertical component of the
earth's rotation, and points vertically downwards.
[Illustration: FIG. 49.]
[Illustration: FIG. 50.]
This last as well as the other phenomena of which I have spoken is very
suggestive. Here is a magnetic needle (Fig. 49), sometimes called a dipping
needle from the way in which it is suspended. If I turn its {111} frame so
that it can only move at right angles to the meridian, you see that it
points vertically. You may reflect upon the analogous properties of this
magnetic needle (Fig. 50) and of the gyrostat (Fig. 47); they both, when
only capable of moving horizontally, point to the north; and you see that a
very frictionless gyrostat might be used as a compass, or at all events as
a corrector of compasses.[10] I have just put before you another analogy,
and I want you to understand that, although these are only analogies, they
are not mere chance analogies, for there is undoubtedly a dynamical
connection between the magnetic and the gyrostatic phenomena. Magnetism
depends on rotatory motion. The molecules of matter are in actual rotation,
and a certain allineation of the axes of the rotations produces what we
call magnetism. In a steel bar not magnetized the little axes of rotation
are all in different directions. The process {112} of magnetization is
simply bringing these rotations to be more or less round parallel axes, an
allineation of the axes. A honey-combed mass with a spinning gyrostat in
every cell, with all the spinning axes parallel, and the spins in the same
direction, would--I was about to say, would be a magnet, but it would not
be a magnet in all its properties, and yet it would resemble a magnet in
many ways.[11]
[Illustration: FIG. 51.]
[Illustration: FIG. 52.]
Some of you, seeing electromotors and other electric contrivances near this
table, may think that they have to do with our theories and explanations of
magnetic phenomena. But I must explain that this electromotor which I hold
in my hand (Fig. 51) is used by me merely as the {113} most convenient
means I could find for the spinning of my tops and gyrostats. On the
spindle of the motor is fastened a circular piece of wood; by touching this
key I can supply the motor with electric energy, and the wooden disc is now
rotating very rapidly. I have only to bring its rim in contact with any of
these tops or gyrostats to set them spinning, and you see that I can set
half a dozen gyrostats a-spinning in a few seconds; this chain of
gyrostats, for instance. Again, this larger motor (Fig. 52), too large to
move about in my hand, is fastened to the table, and I have used {114} it
to drive my larger contrivances; but you understand that I use these just
as a barber might use them to brush your hair, or Sarah Jane to clean the
knives, or just as I would use a little steam-engine if it were more
convenient for my purpose. It was more convenient for me to bring from
London this battery of accumulators and these motors than to bring sacks of
coals, and boilers, and steam-engines. But, indeed, all this has the deeper
meaning that we can give to it if we like. Love is as old as the hills, and
every day Love's messages are carried by the latest servant of man, the
telegraph. These spinning tops were known probably to primeval man, and yet
we have not learnt from them more than the most fractional portion of the
lesson that they are always sending out to an unobservant world. Toys like
these were spun probably by the builders of the Pyramids when they were
boys, and here you see them side by side with the very latest of man's
contrivances. I feel almost as Mr. Stanley might feel if, with the help of
the electric light and a magic-lantern, he described his experiences in
that dreadful African forest to the usual company of a London drawing-room.
The phenomena I have been describing to you play such a very important part
in nature, that if time admitted I might go on expounding and {115}
explaining without finding any great reason to stop at one place rather
than another. The time at my disposal allows me to refer to only one other
matter, namely, the connection between light and magnetism and the
behaviour of spinning tops.
You are all aware that sound takes time to travel. This is a matter of
common observation, as one can see a distant woodchopper lift his axe again
before one hears the sound of his last stroke. A destructive sea wave is
produced on the coast of Japan many hours after an earthquake occurs off
the coast of America, the wave motion having taken time to travel across
the Pacific. But although light travels more quickly than sound or wave
motion in the sea, it does not travel with infinite rapidity, and the
appearance of the eclipse of one of Jupiter's satellites is delayed by an
observable number of minutes because light takes time to travel. The
velocity has been measured by means of such observations, and we know that
light travels at the rate of about 187,000 miles per second, or thirty
thousand millions of centimetres per second. There is no doubt about this
figure being nearly correct, for the velocity of light has been measured in
the laboratory by a perfectly independent method.
Now the most interesting physical work done since Newton's time is the
outcome of the experiments of Faraday and the theoretical deductions of
{116} Thomson and Maxwell. It is the theory that light and radiant heat are
simply electro-magnetic disturbances propagated through space. I dare not
do more than just refer to this matter, although it is of enormous
importance. I can only say, that of all the observed facts in the sciences
of light, electricity, and magnetism, we know of none that is in opposition
to Maxwell's theory, and we know of many that support it. The greatest and
earliest support that it had was this. If the theory is correct, then a
certain electro-magnetic measurement ought to result in exactly the same
quantity as the velocity of light. Now I want you to understand that the
electric measurement is one of quantities that seem to have nothing
whatever to do with light, except that one uses one's eyes in making the
measurement; it requires the use of a two-foot rule and a magnetic needle,
and coils of wire and currents of electricity. It seemed to bear a
relationship to the velocity of light, which was not very unlike the fabled
connection between Tenterden Steeple and the Goodwin Sands. It is a
measurement which it is very difficult to make accurately. A number of
skilful experimenters, working independently, and using quite different
methods, arrived at results only one of which is as much as five per cent.
different from the observed velocity of light, and some of them, {117} on
which the best dependence may be placed, agree exactly with the average
value of the measurements of the velocity of light.
There is then a wonderful agreement of the two measurements, but without
more explanation than I can give you now, you cannot perhaps understand the
importance of this agreement between two seemingly unconnected magnitudes.
At all events we now know, from the work of Professor Hertz in the last two
years, that Maxwell's theory is correct, and that light is an
electro-magnetic disturbance; and what is more, we know that
electro-magnetic disturbances, incomparably slower than red-light or heat,
are passing now through our bodies; that this now recognized kind of
radiation may be reflected and refracted, and yet will pass through brick
and stone walls and foggy atmospheres where light cannot pass, and that
possibly all military and marine and lighthouse signalling may be conducted
in the future through the agency of this new and wonderful kind of
radiation, of which what we call light is merely one form. Why at this
moment, for all I know, two citizens of Leeds may be signalling to each
other in this way through half a mile of houses, including this hall in
which we are present.[12]
{118}
I mention this, the greatest modern philosophical discovery, because the
germ of it, which was published by Thomson in 1856, makes direct reference
to the analogy between the behaviour of our spinning-tops and magnetic and
electrical phenomena. It will be easier, however, for us to consider here a
mechanical illustration of the rotation of the plane of polarized light by
magnetism which Thomson elaborated in 1874. This phenomenon may, I think,
be regarded as the most important of all Faraday's discoveries. It was of
enormous scientific importance, because it was made in a direction where a
new phenomenon was not even suspected. Of his discovery of induced currents
of electricity, to which all electric-lighting companies and transmission
of power companies of the present day owe their being, Faraday himself said
that it was a natural consequence of the discoveries of an earlier
experimenter, Oersted. But this magneto-optic discovery was quite
unexpected. I will now describe the phenomenon.
Some of you are aware that when a beam of light is sent through this
implement, called a Nichol's Prism, it becomes polarized, or
one-sided--that is, all the light that comes through is known to be
propagated by vibrations which occur all in one plane. This rope (Fig. 53)
hanging from the ceiling {119} illustrates the nature of plane polarized
light. All points in the rope are vibrating in the same plane. Well, this
prism A, Fig. 54, only lets through it light that is polarized in a
vertical plane. And here at B I have a similar implement, and I place it so
that it also will only allow light to pass through it which is polarized in
a vertical plane. Hence most of the light coming through the polarizer, as
the first prism is called, will pass readily through the analyzer, as the
second is called, and I am now letting this light enter my eye. But when I
turn the analyzer round through a right angle, I find that I see no light;
there was a gradual darkening as I rotated the analyzer. The analyzer will
now only allow light to pass through which is polarized in a horizontal
plane, and it receives no such light.
[Illustration: FIG. 53.]
[Illustration: FIG. 54.]
You will see in this model (Fig. 55) a good illustration of polarized
light. The white, brilliantly illuminated thread M N is {120} pulled by a
weight beyond the pulley M, and its end N is fastened to one limb of a
tuning-fork. Some ragged-looking pieces of thread round the portion N A
prevent its vibrating in any very determinate way, but from A to M the
thread is free from all encumbrance. A vertical slot at A, through which
the thread passes, determines the nature of the vibration of the part A B;
every part of the thread between A and B is vibrating in up and down
directions only. A vertical slot in B allows the vertical vibration to be
communicated through it, and so we see the part B M vibrating in the same
way as A B. I might point out quite a lot of ways in which this is not a
perfect illustration of what occurs with light in Fig. 54. But it is quite
good enough for my present purpose. A is a polarizer of vibration; it only
allows up and down motion to pass through it, and B also allows up and down
motion to pass through. But now, as B is turned round, it lets less and
less of the up and down motion pass through it, until when it is in the
second position shown in the lower part of the figure, it allows no up and
down motion to pass through, and there is no visible motion of the thread
between B and M. You will observe that if we did not know in what plane (in
the present case the plane is vertical) the vibrations of the thread
between A and B occurred, we should only have to turn B round until we
found no vibration {122} passing through, to obtain the information. Hence,
as in the light case, we may call A a polarizer of vibrations, and B an
analyzer.
[Illustration: FIG. 55.]
Now if polarized light is passing from A to B (Fig. 54) through the air,
say, and we have the analyzer placed so that there is darkness, we find
that if we place in the path of the ray some solution of sugar we shall no
longer have darkness at B; we must turn B round to get things dark again;
this is evidence of the sugar solution having twisted round the plane of
polarization of the light. I will now assume that you know something about
what is meant by twisting the plane of polarization of light. You know that
sugar solution will do it, and the longer the path of the ray through the
sugar, the more twist it gets. This phenomenon is taken advantage of in the
sugar industries, to find the strengths of sugar solutions. For the thread
illustration I am indebted to Professor Silvanus Thomson, and the next
piece of apparatus which I shall show also belongs to him.
I have here (_see_ Frontispiece) a powerful armour-clad coil, or
electro-magnet. There is a central hole through it, through which a beam of
light may be passed from an electric lamp, and I have a piece of Faraday's
heavy glass nearly filling this hole. I have a polarizer at one end, and an
analyzer at the other. You see now that the {123} polarized light passes
through the heavy glass and the analyzer, and enters the eye of an
observer. I will now turn B until the light no longer passes. Until now
there has been no magnetism, but I have the means here of producing a most
intense magnetic field in the direction in which the ray passes, and if
your eye were here you would see that there is light passing through the
analyzer. The magnetism has done something to the light, it has made it
capable of passing where it could not pass before. When I turn the analyzer
a little I stop the light again, and now I know that what the magnetism did
was to convert the glass into a medium like the sugar, a medium which
rotates the plane of polarization of light.
In this experiment you have had to rely upon my personal measurement of the
actual rotation produced. But if I insert between the polarizer and
analyzer this disc of Professor Silvanus Thomson's, built up of twenty-four
radial pieces of mica, I shall have a means of showing to this audience the
actual rotation of the plane of polarization of light. You see now on the
screen the light which has passed through the analyzer in the form of a
cross, and if the cross rotates it is a sign of the rotation of the plane
of polarization of the light. By means of this electric key I can create,
destroy, and reverse the magnetic {124} field in the glass. As I create
magnetism you see the twisting of the cross; I destroy the magnetism, and
it returns to its old position; I create the opposite kind of magnetism,
and you see that the cross twists in the opposite way. I hope it is now
known to you that magnetism rotates the plane of polarization of light as
the solution of sugar did.
[Illustration: FIG. 56.]
[Illustration: FIG. 57.]
As an illustration of what occurs between polarizer and analyzer, look
again at this rope (Fig. 53) fastened to the ceiling. I move the bottom end
sharply from east to west, and you see that every part of the rope moves
from east to west. Can you imagine a rope such that when the bottom end was
moved from east to west, a point some yards up moved from east-north-east
to west-sou'-west, that a higher point moved from north-east to south-west,
and so on, the direction gradually changing for higher and higher points?
Some of you, knowing what I have done, may be able to imagine it. We should
have what we want if this rope were a chain of gyrostats such as you see
figured in the diagram; gyrostats all spinning in the same way looked at
from below, with frictionless hinges between them. Here is such a chain
(Fig. 56), one of many that I have tried to use in this way for several
years. But although I have often believed that I saw the phenomenon occur
in {126} such a chain, I must now confess to repeated failures. The
difficulties I have met with are almost altogether mechanical ones. You see
that by touching all the gyrostats in succession with this rapidly
revolving disc driven by the little electromotor, I can get them all to
spin at the same time; but you will notice that what with bad mechanism and
bad calculation on my part, and want of skill, the phenomenon is completely
masked by wild movements of the gyrostats, the causes of which are better
known than capable of rectification. The principle of the action is very
visible in this gyrostat suspended as the bob of a pendulum (Fig. 57). You
may imagine this to represent a particle of the {127} substance which
transmits light in the magnetic field, and you see by the trickling thin
stream of sand which falls from it on the paper that it is continually
changing the plane of polarization. But I am happy to say that I can show
you to-night a really successful illustration of Thomson's principle; it is
the very first time that this most suggestive experiment has been shown to
an audience. I have a number of double gyrostats (Fig. 58) placed on the
same line, joined end to end by short pieces of elastic. Each instrument is
supported at its centre of gravity, and it can rotate both in horizontal
and in vertical planes.
[Illustration: FIG. 58.]
The end of the vibrating lever A can only get a horizontal motion from my
hand, and the motion is transmitted from one gyrostat to the next, until it
has travelled to the very end one. Observe that when the gyrostats are not
spinning, the motion is {128} everywhere horizontal. Now it is very
important not to have any illustration here of a reflected ray of light,
and so I have introduced a good deal of friction at all the supports. I
will now spin all the gyrostats, and you will observe that when A moves
nearly straight horizontally, the next gyrostat moves straight but in a
slightly different plane, the second gyrostat moves in another plane, and
so on, each gyrostat slightly twisting the plane in which the motion
occurs; and you see that the end one does not by any means receive the
horizontal motion of A, but a motion nearly vertical. This is a mechanical
illustration, the first successful one I have made after many trials, of
the effect on light of magnetism. The reason for the action that occurs in
this model must be known to everybody who has tried to follow me from the
beginning of the lecture.
And you can all see that we have only to imagine that many particles of the
glass are rotating like gyrostats, and that magnetism has partially caused
an allineation of their axes, to have a dynamical theory of Faraday's
discovery. The magnet twists the plane of polarization, and so does the
solution of sugar; but it is found by experiment that the magnet does it
indifferently for coming and going, whereas the sugar does it in a way that
corresponds with a spiral structure of molecules. You see that in this
important {129} particular the gyrostat analogue must follow the magnetic
method, and not the sugar method. We must regard this model, then, the
analogue to Faraday's experiment, as giving great support to the idea that
magnetism consists of rotation.
I have already exceeded the limits of time usually allowed to a popular
lecturer, but you see that I am very far from having exhausted our subject.
I am not quite sure that I have accomplished the object with which I set
out. My object was, starting from the very different behaviour of a top
when spinning and when not spinning, to show you that the observation of
that very common phenomenon, and a determination to understand it, might
lead us to understand very much more complex-looking things. There is no
lesson which it is more important to learn than this--That it is in the
study of every-day facts that all the great discoveries of the future lie.
Three thousand years ago spinning tops were common, but people never
studied them. Three thousand years ago people boiled water and made steam,
but the steam-engine was unknown to them. They had charcoal and saltpetre
and sulphur, but they knew nothing of gunpowder. They saw fossils in rocks,
but the wonders of geology were unstudied by them. They had bits of iron
and copper, but not one of them thought of any one of the fifty simple
{130} ways that are now known to us of combining those known things into a
telephone. Why, even the simplest kind of signalling by flags or lanterns
was unknown to them, and yet a knowledge of this might have changed the
fate of the world on one of the great days of battle that we read about. We
look on Nature now in an utterly different way, with a great deal more
knowledge, with a great deal more reverence, and with much less unreasoning
superstitious fear. And what we are to the people of three thousand years
ago, so will be the people of one hundred years hence to us; for indeed the
acceleration of the rate of progress in science is itself accelerating. The
army of scientific workers gets larger and larger every day, and it is my
belief that every unit of the population will be a scientific worker before
long. And so we are gradually making time and space yield to us and obey
us. But just think of it! Of all the discoveries of the next hundred years;
the things that are unknown to us, but which will be so well known to our
descendants that they will sneer at us as utterly ignorant, because these
things will seem to them such self-evident facts; I say, of all these
things, if one of us to-morrow discovered one of them, he would be regarded
as a great discoverer. And yet the children of a hundred years hence will
know it: it will be brought home to {131} them perhaps at every footfall,
at the flapping of every coat-tail.