(First published in 1952)
I have no new discovery to bring before you this evening.
I must ask you to go over very old ground and to turn your attention to
a question which has been raised again and again ever since men began to
The question is that of the transmission of force. We
see that two bodies at a distance from each other exert a mutual influence
on each other’s motion. Does this mutual action depend on the existence
of some third thing, some medium of communication, occupying the space
between the bodies, or do the bodies act on each other immediately, without
the intervention of anything else?
The mode in which Faraday was accustomed to look at phenomena
of this kind differs from that adopted by many other modern inquirers,
and my special aim will be to enable you to place yourselves at Faraday’s
point of view, and to point out the scientific value of that conception
of lines of force which, in his hands, became the key to the science
When we observe on body acting on another at a distance,
before we assume that this action is direct and immediate, we generally
inquire whether there is any material connection between the two bodies;
and if we find strings, or rods, or mechanism of any kind, capable of accounting
for the observed action between the bodies, we prefer to explain the action
by means of these intermediate connections, rather than to admit the notion
of direct action at a distance.
Thus, when we ring a bell by means of a wire, the successive
parts of the wire are first tightened and then moved, till at last the
bell is rung at a distance by a process in which all the intermediate particles
of the wire have taken part one after the other. We may ring a bell at
a distance in other ways, as by forcing air into a long tube, at the other
end of which is a cylinder with a piston which is made to fly out and strike
the bell. We may also use a wire; but instead of pulling it, we may connect
it at one end with a voltaic battery, and at the other with an electromagnet,
and thus ring the bell by electricity.
Here are three different ways of ringing a bell. They
all agree, however, in the circumstance that between the ringer and the
bell there is an unbroken line of communication, and that at every point
of this line some physical process goes on by which the action is transmitted
from one end to the other. The process of transmission is not instantaneous
but gradual; so that there is an interval of time after the impulse has
been given to one extremity of the line of communication, during which
the impulse is on its way but has not reached the other end.
It is clear, therefore, that in many cases the action
between bodies at a distance may be accounted for by a series of actions
between each successive pair of a series of bodies which occupy the intermediate
space; and it is asked, by the advocates of mediate action, whether, in
those cases in which we cannot perceive the intermediate agency, it is
not more philosophical to admit the existence of a medium which we cannot
at present perceive than to assert that a body can act at a place where
it is not.
To a person ignorant of the properties of air, the transmission
of force by means of that invisible medium would appear as unaccountable
as any other example of action at a distance, and yet in this case we can
explain the whole process and determine the rate at which the action is
passed on from one portion to another of the medium.
Why then should we not admit that the familiar mode of
communicating motion by pushing and pulling with our hands is the type
and exemplification of all action between bodies, even in cases in which
we can observe nothing between the bodies which appears to take part in
Here for instance is a kind of attraction with which
Professor Guthrie has made us familiar. A disk is set in vibration and
is then brought near a light suspended body, which immediately begins to
move toward the disk, as if drawn toward it by an invisible cord. What
is this cord? Sir W. Thomson has pointed out that in a moving fluid the
pressure is least where the velocity is the greatest. The velocity of the
vibratory motion of the air is greatest nearest the disk. Hence the pressure
of the air on the suspended body is less on the side nearest the disk than
on the opposite side, the body yields to the greater pressure and moves
toward the disk.
The disk, therefore, does not act where it is not. It
sets the air next it in motion by pushing it, this motion is communicated
to more and more distant portions of the air in turn, and thus the pressures
on opposite sides of the suspended body are rendered unequal, and it moves
toward the disk in consequence of the excess of pressure. The force is
therefore a force of the old school—a case of vis a tergo—a shove
The advocates of the doctrine of action at a distance,
however, have not been put to silence by such arguments. What right, say
they, have we to assert that a body cannot act where it is not? Do we not
see an instance of action at a distance in the case of a magnet, which
acts on another magnet not only at a distance but with the most complete
indifference to the nature of the matter which occupies the intervening
space? If the action depends on something occupying the space between the
two magnets, it cannot surely be a matter of indifference whether this
space is filled with air or not, or whether wood, glass, or copper be placed
between the magnets.
Besides this, Newton’s law of gravitation, which every
astronomical observation only tends to establish more firmly, asserts not
only that the heavenly bodies act on one another across immense intervals
of space but that two portions of matter, the one buried a thousand miles
deep in the interior of the earth, and the other hundred thousand miles
deep in the body of the sun, act on one another with precisely the same
force as if the strata beneath which each is buried had been nonexistent.
If any medium takes part in transmitting this action, it must surely make
some difference whether the space between the bodies contains nothing but
this medium, or whether it is occupied by the dense matter of the earth
or of the sun.
But the advocates of direct action at a distance are
not content with instances of this kind, in which the phenomena, even at
first sight, appear to favour their doctrine. They push their operations
into the enemy’s camp and maintain that even when the action is apparently
the pressure of contiguous portions of matter, the contiguity is only apparent—that
a space always intervenes between the bodies which act on each other.
They assert, in short, that so far from action at a distance being impossible,
it is the only kind of action which ever occurs, and that the favourite
old vis a tergo of the schools has no existence in nature and exists
only in the imagination of schoolmen.
The best way to prove that when one body pushes another
it does not touch it is to measure the distance between them. Here are
two glass lenses, one of which is pressed against the other by means of
a weight. By means of the electric light we may obtain on the screen an
image of the place where the one lens presses against the other. A series
of coloured rings is formed on the screen. These rings were first observed
and first explained by Newton. The particular colour of any ring depends
on the distance between the surfaces of the pieces of glass. Newton formed
a table of the colours corresponding to different distances, so that by
comparing the colour of any ring with Newton’s table, we may ascertain
the distance between the surfaces at that ring. The colours are arranged
in rings because the surfaces are spherical, and therefore the interval
between the surfaces depends on the distance from the line joining the
centres of the spheres. The central spot of the rings indicates the place
where the lenses are nearest together, and each successive ring corresponds
to an increase of about the 4,000th parts of a millimetre in the distance
of the surfaces.
The lenses are now pressed together with a force equal
to the weight of an ounce; but there is still a measurable interval between
them, even at the place where they are nearest together. They are not in
optical contact. To prove this, I apply a greater weight. A new colour
appears at the central spot, and the diameters of all the rings increase.
This shows that the surfaces are now nearer than at first, but they are
not yet in optical contact, for if they were, the central spot would be
black. I therefore increase the weights, so as to press the lenses into
But what we call optical contact is not real contact.
Optical contact indicates only that the distance between the surfaces is
much less than a wavelength of light. To show that the surfaces are not
in real contact, I remove the weights. The rings contract, and several
of them vanish at the centre. Now it is possible to bring two pieces of
glass so close together that they will not tend to separate at all but
adhere together so firmly that when torn asunder the glass will break,
not at the surface of contact, but at some other place. The glasses must
then be many degrees nearer than when in mere optical contact.
Thus we have shown that bodies begin to press against
each other while still at a measurable distance, and that even when pressed
together with great force they are not in absolute contact but may be brought
nearer still, and that by many degrees.
Why, then, say the advocates to direct action, should
we continue to maintain the doctrine, founded only on the rough experience
of a prescientific age, that matter cannot act where it is not, instead
of admitting that all the facts from which our ancestors concluded that
contact is essential to action were in reality cases of action at a distance,
the distance being too small to be measured by their imperfect means of
If we are ever to discover the laws of nature, we must
do so by obtaining the most accurate acquaintance with the facts of nature,
and not by dressing up in philosophical language the loose opinions of
men who had no knowledge of the facts which throw most light on these laws.
And as for those who introduce aethereal, or other media, to account for
these actions, without any direct evidence of the existence of such media,
or any clear understanding of how the media do their work, and who fill
all space three and four times over with aethers of different sorts, why
the less these men talk about their philosophical scruples about admitting
action at a distance the better.
If the progress of science were regulated by Newton’s
first law of motion, it would be easy to cultivate opinions in advance
of the age. We should only have to compare the science of today with that
of fifty years ago; and by producing, in the geometrical sense, the line
of progress, we should obtain the science of fifty years hence.
The progress of science in Newton’s time consisted in
getting rid of the celestial machinery with which generations of astronomers
had encumbered the heavens, and thus ‘‘sweeping cobwebs off the sky.’’
Though the planets had already got rid of their crystal
spheres, they were still swimming in the vortices of Descartes. Magnets
were surrounded by effluvia, and electrified bodies by atmospheres, the
properties of which resembled in no respect those of ordinary effluvia
When Newton demonstrated that the force which acts on
each of the heavenly bodies depends on its relative position with respect
to the other bodies, the new theory met with violent opposition from the
advanced philosophers of the day, who described the doctrine of gravitation
as a return to the exploded method of explaining everything by occult causes,
attractive virtues, and the like.
Newton himself, with that wise moderation which is characteristic
of all his speculations, answered that he made no pretense of explaining
the mechanism by which the heavenly bodies act on each other. To determine
the mode in which their mutual action depends on their relative position
was a great step in science, and this step Newton asserted that he had
made. To explain the process by which this action is effected was a quite
distinct step, and this step Newton, in his Principia, does not
attempt to make.
But so far was Newton from asserting that bodies really
do act on one another at a distance, independently of anything between
them, that in a letter to Bentley, which has been quoted by Faraday, he
It is inconceivable that inanimate brute matter should,
without the mediation of something else, which is not material, operate
upon and affect other matter without mutual contact, as it must do if gravitation,
in the sense of Epicurus, be essential and inherent in it. ... That gravity
should be innate, inherent, and essential to matter, so that one body can
act upon another at a distance, through a vacuum, without the mediation
of anything else, by and through which their action and force may be conveyed
from one to another, is to me so great an absurdity, that I believe no
man who has in philosophical matters a competent faculty of thinking can
ever fall into it.
Accordingly, we find in his Optical Queries, and
in his letters to Boyle, that Newton had very early made the attempt to
account for gravitation by means of the pressure of a medium, and that
the reason he did not publish these investigations ‘‘proceeded from hence
only, that he found he was not able, from experiment and observation, to
give a satisfactory account of this medium, and the manner of its operation
in producing the chief phenomena of nature.’’
The doctrine of direct action at a distance cannot claim
for its author the discoverer of universal gravitation. It was first asserted
by Roger Cotes, in his preface to the Principia, which he edited
during Newton’s life. According to Cotes, it is by experience that we learn
that all bodies gravitate. We do not learn in any other way that they are
extended, movable, or solid. Gravitation, therefore, has as much right
to be considered an essential property of matter as extension, mobility,
And when the Newtonian philosophy gained ground in Europe,
it was the opinion of Cotes rather than that of Newton that became most
prevalent, till at last Boscovich propounded his theory, that matter is
a congeries of mathematical points, each endowed with the power of attracting
or repelling the others according to fixed laws. In his world, matter is
unextended, and contact is impossible. He did not forget, however, to endow
his mathematical points with inertia. In this some of the modern representatives
of his school have thought that he ‘‘had not quite got so far as the strict
modern view of ‘matter’ as being but an expression for modes or manifestations
But if we leave out of account for the present the development
of the ideas of science and confine our attention to the extension of its
boundaries, we shall see that it was most essential that Newton’s method
should be extended to every branch of science to which it was applicable—that
we should investigate the forces with which bodies act on each other in
the first place, before attempting to explain
how that force is
transmitted. No men could be better fitted to apply themselves exclusively
to the first part of the problem than those who considered the second part
Accordingly Cavendish, Coulomb, and Poisson, the founders
of the exact sciences of electricity and magnetism, paid no regard to those
old notions of ‘‘magnetic effluvia’’ and ‘‘electric atmospheres,’’ which
had been put forth in the previous century, but turned their undivided
attention to the determination of the law of force, according to which
electrified and magnetized bodies attract or repel each other. In this
way the true laws of these actions were discovered, and this was done by
men who never doubted that the action took place at a distance, without
the intervention of any medium, and who would have regarded the discovery
of such a medium as complicating rather than as explaining the undoubted
phenomena of attraction.
We have now arrived at the great discovery by Ørsted
of the connection between electricity and magnetism. Ørsted found
that an electric current acts on a magnetic pole, but that it neither attracts
it nor repels it but causes it to move round the current. He expressed
this by saying that ‘‘the electric conflict acts in a revolving manner.’’
The most obvious deduction from this new fact was that
the action of the current on the magnet is not a push-and-pull force but
a rotary force, and accordingly many minds were set a-speculating on vortices
and streams of aether whirling round the current.
But Ampère, by a combination of mathematical skill
with experimental ingenuity, first proved that two electric currents act
on one another, and then analyzed this action into the resultant of a system
of push-and-pull forces between the elementary parts of these currents.
The formula of Ampère, however, is of extreme
complexity, as compared with Newton’s law of gravitation, and many attempts
have been made to resolve it into something of greater apparent simplicity.
I have no wish to lead you into a discussion of any of
these attempts to improve a mathematical formula. Let us turn to the independent
method of investigation employed by Faraday in those researches in electricity
and magnetism which have made this Institution one of the most venerable
shrines of science.
No man ever more conscientiously and systematically laboured
to improve all his powers of mind than did Faraday from the very beginning
of his scientific career. But whereas the general course of scientific
method then consisted in the application of the ideas of mathematics and
astronomy to each new investigation in turn, Faraday seems to have had
no opportunity of acquiring a technical knowledge of mathematics, and his
knowledge of astronomy was mainly derived from books.
Hence, though he had a profound respect for the great
discovery of Newton, he regarded the attraction of gravitation as a sort
of sacred mystery, which, as he was not an astronomer, he had no right
to gainsay or to doubt, his duty being to believe it in the exact form
in which it was delivered to him. Such a dead faith was not likely to lead
him to explain new phenomena by means of direct attractions.
Besides this, the treatises of Poisson and Ampère
are of so technical a form that to derive any assistance from them the
student must have been thoroughly trained in mathematics, and it is very
if such a training can be begun with advantage in mature years.
Thus Faraday, with his penetrating intellect, his devotion
to science, and his opportunities for experiments, was debarred from following
the course of thought which had led to the achievements of the French philosophers
and was obliged to explain the phenomena to himself by means of a symbolism
which he could understand, instead of adopting what had hitherto been the
only tongue of the learned.
This new symbolism consisted of those lines of force
extending themselves in every direction from electrified and magnetic bodies,
which Faraday in his mind’s eye saw as distinctly as the solid bodies from
which they emanated.
The idea of lines of force and their exhibition by means
of iron filings was nothing new. They had been observed repeatedly, and
investigated mathematically, as an interesting curiosity of science. But
let us hear Faraday himself, as he introduces to his reader the method
which in his hands became so powerful.
It would be a voluntary and unnecessary abandonment
of most valuable aid if an experimentalist, who chooses to consider magnetic
power as represented by lines of magnetic force, were to deny himself the
use of iron filings. By their employment he may make many conditions of
the power, even in complicated cases, visible to the eye at once; may trace
the varying direction of the lines of force and determine the relative
polarity; may observe in which direction the power is increasing or diminishing;
and in complex systems may determine the neutral points or places where
there is neither polarity nor power, even they occur in the midst of powerful
magnets. By their use probable results may be seen at once, and many a
valuable suggestion gained for future leading experiments.
lines of force
In this experiment each filing becomes a little
magnet. The poles of opposite names belonging to different filings attract
each other and stick together, and more fillings attach themselves to the
exposed poles, that is, to the ends of the row of fillings. In this way
the fillings, instead of forming a confused system of dots over the paper,
draw together, filing to filing, till long fibres of filings are formed,
which indicate by their direction the lines of force in every part of the
The mathematicians saw in this experiment nothing but
a method of exhibiting at one view the direction in different places of
the resultant of two forces, one directed to each pole of the magnet; a
somewhat complicated result of the simple law of force.
But Faraday, by a series of steps as remarkable for their
geometrical definiteness as for their speculative ingenuity, imparted to
his conception of these lines of force a cleaners and precision far in
advance of that with which the mathematicians could then invest their own
In the first place, Faraday’s lines of force are not
to be considered merely as individuals but as forming a system, drawn in
space in a definite manner so that the number of the lines which pass through
any area, say of one square inch, indicates the intensity of the force
acting through the area. Thus the lines of force become definite in number.
The strength of a magnetic pole is measured by the number of lines which
proceed from it; the electronic state of a circuit is measured by the number
of lines which pass through it.
In the second place, each individual line has a continuous
existence in space and time. When a piece of steel becomes a magnet, or
when an electric current begins to flow, the lines of force do not start
into existence each in its own place, but as the strength increases new
lines are developed within the magnet or current and gradually grow outward,
so that the whole system expands from within, like Newton’s rings in our
former experiment. Thus every line of force preserves its identity during
the whole course of its existence, though its shape and size may be altered
to any extent.
I have no time to describe the methods by which every
question relating to the forces acting on magnets or on currents, or to
the induction of currents in conduction circuits, may be solved by the
consideration of Faraday’s lines of force. In this place they can never
be forgotten. By means of this new symbolism, Faraday defined with mathematical
precision the whole theory of electromagnetism, in language free from mathematical
technicalities, and applicable to the most complicated as well as the simplest
cases. But Faraday did not stop here. He went on from the conception of
geometrical lines of force to that of physical lines of force. He observed
that the motion which the magnetic or electric force tends to produce is
invariably such as to shorten the lines of force and to allow them to spread
out laterally from each other. He thus perceived in the medium a state
of stress, consisting of a tension, like that of a rope, in the direction
of the lines of force, combined with a pressure in all directions at right
angles to them.
This is quite a new conception of action at a distance,
reducing it to a phenomenon of the same kind as that action at a distance
which is exerted by means of the tension of ropes and the pressure of rods.
When the muscles of our bodies are excited by that stimulus which we are
able in some unknown way to apply to them, the fibres tend to shorten themselves
and at the same time to expand laterally. A state of stress is produced
in the muscle, and the limb moves. This explanation of muscular action
is by no means complete. It gives no account of the cause of the excitement
of the state of stress, nor does it even investigate those forces of cohesion
which enable the muscles to support this stress. Nevertheless, the simple
fact that it substitutes a kind of action, which extends continuously along
a material substance, for one which we know only a cause and an effect
at a distance from each other induces us to accept it as a real addition
to our knowledge of animal mechanics.
For similar reasons we may regard Faraday’s conception
of a state of stress in the electromagnetic field as a method of explaining
action at a distance by means of the continuous transmission of force,
even though we do not know how the state of stress is produced.
But one of Faraday’s most pregnant discoveries, that
of the magnetic rotation of polarized light, enables us to proceed a step
farther. The phenomenon, when analyzed into its simplest elements, may
be described thus: Of two circularly polarized rays of light, precisely
similar in configuration, but rotating in opposite directions, that ray
is propagated with the greater velocity which rotates in the same direction
as the electricity of the magnetizing current.
It follows from this, as Sir W. Thomson has shown by
strict dynamical reasoning, that the medium when under the action of magnetic
force must be in a state of rotation—that is to say, that small portions
of the medium, which we may call molecular vortices, are rotating, each
on its own axis, the direction of this axis being that of the magnetic
Here, then, we have an explanation of the tendency of
the lines of magnetic force to spread out laterally and to shorten themselves.
It arises from the centrifugal force of the molecular vortices.
The mode in which electromotive force acts in starting
and stopping the vortices is more abstruse, though it is of course consistent
with dynamical principles.
We have thus found that there are several different kinds
of work to be done by the electromagnetic medium if it exists. We have
also seen that magnetism has an intimate relation to light, and we know
that there is a theory of light which supposes it to consist of the vibrations
of a medium. How is this luminiferous medium related to our electromagnetic
It fortunately happens that electromagnetic measurements
have been made from which we can calculate by dynamical principles the
velocity of propagations of small magnetic disturbances in the supposed
This velocity is very great, from 288 to 314 millions
of metres per second, according to different experiments. Now the velocity
of light, according to Foucault’s experiments, is 298 millions of metres
per second. In fact, the different determinations of either velocity differ
from each other more than the estimated velocity of light does from the
estimated velocity of propagation of small electromagnetic disturbance.
But if the luminiferous and the electromagnetic media occupy the same place,
and transmit disturbances with the same velocity, what reason have we to
distinguish the one from the other? By considering them as the same, we
avoid at least the reproach of filling space twice over with different
kinds of aether.
Besides this, the only kind of electromagnetic disturbances
which can be propagated through a nonconducting medium is a disturbance
transverse to the direction of propagation, agreeing in this respect with
what we know of that disturbance which we call light. Hence, for all we
know, light also may be an electromagnetic disturbance in a nonconduction
medium. If we admit this, the electromagnetic theory of light will agree
in every respect with the undulatory theory, and the work of Thomas Young
and Fresnel will be establish on a firmer basis than ever, when joined
with that of Cavendish and Coulomb by the keystone of the combined sciences
of light and electricity—Faraday’s great discovery of the electromagnetic
rotation of light.
The vast interplanetary and interstellar regions will
no longer be regarded as waste places in the universe, which the Creator
has not seen fit to fill with the symbols of the manifold order of His
kingdom. We shall find them to be already full of this wonderful medium;
so full, that no human power can remove it from the smallest portion of
space, or produce the slightest flaw in its infinite continuity. It extends
unbroken from star to star; and when a molecule of hydrogen vibrates in
the Dog Star, the medium receives the impulses of these vibrations and,
after carrying them in its immense bosom for three years, delivers them
in due course, regular order, and full tale into the spectroscope of Mr.
Huggins, at Tulse Hill.
But the medium has other functions and operations besides
bearing light from man to man, and from world to world, and giving evidence
of the absolute unity of the metric system in the universe. Its minute
parts may have rotatory as well as vibratory motions, and the axes of rotation
form those lines of magnetic force which extend in unbroken continuity
into regions which no eye has seen, and which, by their action on our magnets,
are telling us in language not yet interpreted what is going on in the
hidden underworld from minute to minute and from century to century.
And these lines must not be regarded as mere mathematical
abstractions. They are the directions in which the medium is exerting a
tension like that of a robe, or rather, like that of our own muscles. The
tension of the medium in the direction of the earth’s magnetic force is
in this country one grain weight on eight square feet. In some of Dr. Joule’s
experiments, the medium has exerted a tension of 200 pounds weight per
But the medium, in virtue of the very same elasticity
by which it is able to transmit the undulations of light, is also able
to act as a spring. When properly wound up, it exerts a tension, different
from the magnetic tension, by which it draws oppositely electrified bodies
together, produces effects through the length of telegraph wires, and,
when of sufficient intensity, leads to the rupture and explosion called
These are some of the already discovered properties of
that which has often been called vacuum, or nothing at all. They enable
us to resolve several kinds of action at a distance into actions between
contiguous parts of a contiguous substance. Whether this resolution is
of the nature of explication or complication or complication, I must leave
to the metaphysicians.