[Footnote: A lecture by Dr. O. J. Lodge, delivered at the London
Institution on December 16, 1880.]
Ever since the subject on which I have the honor to speak to you to-night
was arranged, I have been astonished at my own audacity in proposing to
deal in the course of sixty minutes with a subject so gigantic and so
profound that a course of sixty lectures would be quite inadequate for its
thorough and exhaustive treatment.
I must indeed confine myself carefully to some few of the typical and most
salient points in the relation between electricity and light, and I must
economize time by plunging at once into the middle of the matter without
further preliminaries.
Now, when a person is setting off to discuss the relation between
electricity and light, it is very natural and very proper to pull him up
short with the two questions: What do you mean by electricity? and What do
you mean by light? These two questions I intend to try briefly to answer.
And here let me observe that in answering these fundamental questions, I do
not necessarily assume a fundamental ignorance on your part of these two
agents, but rather the contrary; and must beg you to remember that if I
repeat well-known and simple experiments before you, it is for the purpose
of directing attention to their real meaning and significance, not to their
obvious and superficial characteristics; in the same way that I might
repeat the exceedingly familiar experiment of dropping a stone to the earth
if we were going to define what we meant by gravitation.
Now, then, we will ask first, What is electricity? and the simple answer
must be, We don't know. Well, but this need not necessarily be depressing.
If the same question were asked about matter, or about energy, we should
have likewise to reply, No one knows.
But then the term Matter is a very general one, and so is the term Energy.
They are heads, in fact, under which we classify more special phenomena.
Thus, if we were asked, What is sulphur? or what is selenium? we should at
least be able to reply, A form of matter; and then proceed to describe its
properties, _i. e._, how it affected our bodies and other bodies.
Again, to the question, What is heat? we can reply, A form of energy; and
proceed to describe the peculiarities which distinguish it from other forms
of energy.
But to the question. What is electricity? we have no answer pat like this.
We can not assert that it is a form of matter, neither can we deny it; on
the other hand, we certainly can not assert that it is a form of energy,
and I should be disposed to deny it. It may be that electricity is an
entity _per se_, just as matter is an entity _per se_.
Nevertheless, I can tell you what I mean by electricity by appealing to its
known behavior.
Here is a battery, that is, an electricity pump; it will drive electricity
along. Prof. Ayrtou is going, I am afraid, to tell you, on the 20th of
January next, that it _produces_ electricity; but if he does, I hope you
will remember that that is exactly what neither it nor anything else can
do. It is as impossible to generate electricity in the sense I am trying to
give the word, as it is to produce matter. Of course I need hardly say that
Prof. Ayrton knows this perfectly well; it is merely a question of words,
_i. e._, of what you understand by the word electricity.
I want you, then, to regard this battery and all electrical machines and
batteries as kinds of electricity pumps, which drive the electricity along
through the wire very much as a water-pump can drive water along pipes.
While this is going on the wire manifests a whole series of properties,
which are called the properties of the current.
[Here were shown an ignited platinum wire, the electric arc between two
carbons, an electric machine spark, an induction coil spark, and a vacuum
tube glow. Also a large nail was magnetized by being wrapped in the
current, and two helices were suspended and seen to direct and attract each
other.]
To make a magnet, then, we only need a current of electricity flowing round
and round in a whirl. A vortex or whirlpool of electricity is in fact a
magnet; and _vice versa_. And these whirls have the power of directing and
attracting other previously existing whirls according to certain laws,
called the laws of magnetism. And, moreover, they have the power of
exciting fresh whirls in neighboring conductors, and of repelling them
according to the laws of diamagnetism. The theory of the actions is known,
though the nature of the whirls, as of the simple stream of electricity, is
at present unknown.
[Here was shown a large electro-magnet and an induction-coil vacuum
discharge spinning round and round when placed in its field.]
So much for what happens when electricity is made to travel along
conductors, _i. e._, when it travels along like a stream of water in a
pipe, or spins round and round like a whirlpool.
But there is another set of phenomena, usually regarded as distinct and of
another order, but which are not so distinct as they appear, which
manifest themselves when you join the pump to a piece of glass, or any
non-conductor, and try to force the electricity through that. You succeed
in driving some through, but the flow is no longer like that of water in an
open pipe; it is as if the pipe were completely obstructed by a number of
elastic partitions or diaphragms. The water can not move without straining
and bending these diaphragms, and if you allow it, these strained
partitions will recover themselves, and drive the water back again. [Here
was explained the process of charging a Leyden jar.] The essential thing to
remember is that we may have electrical energy in two forms, the static
and the kinetic; and it is, therefore, also possible to have the rapid
alternation from one of these forms to the other, called vibration.
Now we will pass to the second question: What do you mean by light? And the
first and obvious answer is, Everybody knows. And everybody that is not
blind does know to a certain extent. We have a special sense organ for
appreciating light, whereas we have none for electricity. Nevertheless, we
must admit that we really know very little about the intimate nature of
light--very little more than about electricity. But we do know this,
that light is a form of energy, and, moreover, that it is energy rapidly
alternating between the static and the kinetic forms--that it is, in fact,
a special kind of energy of vibration. We are absolutely certain that light
is a periodic disturbance in some medium, periodic both in space and time;
that is to say, the same appearances regularly recur at certain equal
intervals of distance at the same time, and also present themselves at
equal intervals of time at the same place; that in fact it belongs to the
class of motions called by mathematicians undulatory or wave motions. The
wave motion in this model (Powell's wave apparatus) results from the simple
up and down motion popularly associated with the term wave. But when
a mathematician calls a thing a wave he means that the disturbance is
represented by a certain general type of formula, not that it is an
up-and-down motion, or that it looks at all like those things on the top of
the sea. The motion of the surface of the sea falls within that formula,
and hence is a special variety of wave motion, and the term wave has
acquired in popular use this signification and nothing else. So that when
one speaks ordinarily of a wave or undulatory motion, one immediately
thinks of something heaving up and down, or even perhaps of something
breaking on the shore. But when we assert that the form of energy called
light is undulatory, we by no means intend to assert that anything whatever
is moving up and down, or that the motion, if we could see it, would be
anything at all like what we are accustomed to in the ocean. The kind of
motion is unknown; we are not even sure that there is anything like motion
in the ordinary sense of the word at all.
Now, how much connection between electricity and light have we perceived in
this glance into their natures? Not much, truly. It amounts to about
this: That on the one hand electrical energy may exist in either of two
forms--the static form, when insulators are electrically strained by having
had electricity driven partially through them (as in the Leyden jar), which
strain is a form of energy because of the tendency to discharge and do
work; and the kinetic form, where electricity is moving bodily along
through conductors or whirling round and round inside them, which motion
of electricity is a form of energy, because the conductors and whirls can
attract or repel each other and thereby do work.
And, on the other hand, that light is the rapid alternation of energy
from one of these forms to the other--the static form where the medium is
strained, to the kinetic form when it moves. It is just conceivable, then,
that the static form of the energy of light is _electro_ static, that is,
that the medium is _electrically_ strained, and that the kinetic form of
the energy of light is _electro_-kinetic, that is, that the motion is
not ordinary motion, but electrical motion--in fact, that light is an
electrical vibration, not a material one.
On November 5, last year, there died at Cambridge a man in the full
vigor of his faculties--such faculties as do not appear many times in a
century--whose chief work has been the establishment of this very fact, the
discovery of the link connecting light and electricity; and the proof--for
I believe it amounts to a proof--that they are different manifestations
of one and the same class of phenomena--that light is, in fact, an
electro-magnetic disturbance. The premature death of James Clerk-Maxwell is
a loss to science which appears at present utterly irreparable, for he was
engaged in researches that no other man can hope as yet adequately to grasp
and follow out; but fortunately it did not occur till he had published his
book on "Electricity and Magnetism," one of those immortal productions
which exalt one's idea of the mind of man, and which has been mentioned by
competent critics in the same breath as the "Principia" itself.
But it is not perfect like the "Principia;" much of it is rough-hewn, and
requires to be thoroughly worked out. It contains numerous misprints and
errata, and part of the second volume is so difficult as to be almost
unintelligible. Some, in fact, consists of notes written for private use
and not intended for publication. It seems next to impossible now to mature
a work silently for twenty or thirty years, as was done by Newton two and a
half centuries ago. But a second edition was preparing, and much might have
been improved in form if life had been spared to the illustrious author.
The main proof of the electro-magnetic theory of light is this: The rate at
which light travels has been measured many times, and is pretty well known.
The rate at which an electro-magnetic wave disturbance would travel if such
could be generated (and Mr. Fitzgerald, of Dublin, thinks he has proved
that it can not be generated directly by any known electrical means) can
be also determined by calculation from electrical measurements. The two
velocities agree exactly. This is the great physical constant known as the
ratio V, which so many physicists have been measuring, and are likely to be
measuring for some time to come.
Many and brilliant as were Maxwell's discoveries, not only in electricity,
but also in the theory of the nature of gases, and in molecular science
generally, I can not help thinking that if one of them is more striking and
more full of future significance than the rest, it is the one I have just
mentioned--the theory that light is an electrical phenomenon.
The first glimpse of this splendid generalization was caught in 1845, five
and thirty years ago, by that prince of pure experimentalists, Michael
Faraday. His reasons for suspecting some connection between electricity and
light are not clear to us--in fact, they could not have been clear to him;
but he seems to have felt a conviction that if he only tried long enough
and sent all kinds of rays of light in all possible directions across
electric and magnetic fields in all sorts of media, he must ultimately
hit upon something. Well, this is very nearly what he did. With a sublime
patience and perseverance which remind one of the way Kepler hunted down
guess after guess in a different field of research, Faraday combined
electricity, or magnetism, and light in all manner of ways, and at last he
was rewarded with a result. And a most out-of-the-way result it seemed.
First, you have to get a most powerful magnet and very strongly excite it;
then you have to pierce its two poles with holes, in order that a beam of
light may travel from one to the other along the lines of force; then, as
ordinary light is no good, you must get a beam of plane polarized light,
and send it between the poles. But still no result is obtained until,
finally, you interpose a piece of a rare and out-of-the-way material, which
Faraday had himself discovered and made--a kind of glass which contains
borate of lead, and which is very heavy, or dense, and which must be
perfectly annealed.
And now, when all these arrangements are completed, what is seen is simply
this, that if an analyzer is arranged to stop the light and make the field
quite dark before the magnet is excited, then directly the battery is
connected and the magnet called into action, a faint and barely perceptible
brightening of the field occurs, which will disappear if the analyzer be
slightly rotated. [The experiment was then shown.] Now, no wonder that no
one understood this result. Faraday himself did not understand it at all.
He seems to have thought that the magnetic lines of force were rendered
luminous, or that the light was magnetized; in fact, he was in a fog,
and had no idea of its real significance. Nor had any one. Continental
philosophers experienced some difficulty and several failures before they
were able to repeat the experiment. It was, in fact, discovered too soon,
and before the scientific world was ready to receive it, and it was
reserved for Sir William Thomson briefly, but very clearly, to point
out, and for Clerk-Maxwell more fully to develop, its most important
consequences. [The principle of the experiment was then illustrated by the
aid of a mechanical model.]
This is the fundamental experiment on which Clerk-Maxwell's theory of
light is based; but of late years many fresh facts and relations between
electricity and light have been discovered, and at the present time they
are tumbling in in great numbers.
It was found by Faraday that many other transparent media besides heavy
glass would show the phenomenon if placed between the poles, only in a less
degree; and the very important observation that air itself exhibits the
same phenomenon, though to an exceedingly small extent, has just been made
by Kundt and Rontgen in Germany.
Dr. Kerr, of Glasgow, has extended the result to opaque bodies, and has
shown that if light be passed through magnetized _iron_ its plane is
rotated. The film of iron must be exceedingly thin, because of its opacity,
and hence, though the intrinsic rotating power of iron is undoubtedly very
great, the observed rotation is exceedingly small and difficult to observe;
and it is only by a very remarkable patience and care and ingenuity that
Dr. Kerr has obtained his result. Mr. Fitzgerald, of Dublin, has examined
the question mathematically, and has shown that Maxwell's theory would have
enabled Dr. Kerr's result to be predicted.
Another requirement of the theory is that bodies which are transparent
to light must be insulators or non-conductors of electricity, and that
conductors of electricity are necessarily opaque to light. Simple
observation amply confirms this; metals are the best conductors, and are
the most opaque bodies known. Insulators such as glass and crystals are
transparent whenever they are sufficiently homogeneous, and the very
remarkable researches of Prof. Graham Bell in the last few months have
shown that even _ebonite_, one of the most opaque insulators to ordinary
vision, is certainly transparent to some kinds of radiation, and
transparent to no small degree.
[The reason why transparent bodies must insulate, and why conductors must
be opaque, was here illustrated by mechanical models.]
A further consequence of the theory is that the velocity of light in a
transparent medium will be affected by its electrical strain constant; in
other words, that its refractive index will bear some close but not yet
quite ascertained relation to its specific inductive capacity. Experiment
has partially confirmed this, but the confirmation is as yet very
incomplete. But there are a number of results not predicted by theory, and
whose connection with the theory is not clearly made out. We have the fact
that light falling on the platinum electrode of a voltameter generates a
current, first observed, I think, by Sir W. R. Grove--at any rate, it is
mentioned in his "Correlation of Forces"--extended by Becquerel and Robert
Sabine to other substances, and now being extended to fluorescent and other
bodies by Prof. Minchin. And finally--for I must be brief--we have
the remarkable action of light on selenium. This fact was discovered
accidentally by an assistant in the laboratory of Mr. Willoughby Smith, who
noticed that a piece of selenium conducted electricity very much better
when light was falling upon it than when it was in the dark. The light of
a candle is sufficient, and instantaneously brings down the resistance to
something like one-fifth of its original value.
I could show you these effects, but there is not much to see; it is an
intensely interesting phenomenon, but its external manifestation is not
striking--any more than Faraday's heavy glass experiment was.
This is the phenomenon which, as you know, has been utilized by Prof.
Graham Bell in that most ingenious and striking invention, the photophone.
By the kindness of Prof. Silvanus Thompson, I have a few slides to show the
principle of the invention, and Mr. Shelford Bidwell has been kind enough
to lend me his home-made photophone, which answers exceedingly well for
short distances.
I have now trespassed long enough upon your patience, but I must just
allude to what may very likely be the next striking popular discovery; and
that is the transmission of light by electricity; I mean the transmission
of such things as views and pictures by means of the electric wire. It has
not yet been done, but it seems already theoretically possible, and it may
very soon be practically accomplished.
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