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James Clerk Maxwell






The story of the life of James Clerk Maxwell has been told so recently
by the able pen of his lifelong friend, Professor Lewis Campbell, that
it is unnecessary, in the few pages which now remain to us, to attempt
to give a repetition of the tale which would not only fail to do
justice to its subject, but must of necessity fall far short of the
merits of the (confessedly imperfect) sketch which has recently been
placed within the reach of all. Looking back on the life of Clerk
Maxwell, he seems to have come amongst us as a light from another
world--to have but partly revealed his message to minds too often
incapable of grasping its full meaning, and all too soon to have
returned to the source from whence he came. There was scarcely any
branch of natural philosophy that he did not grapple with, and upon
which his vivid imagination and far-seeing intelligence did not throw
light. He was born a philosopher, and at every step Nature partly drew
aside the veil and revealed that which was hidden from a gaze less
prophetic. A very brief sketch of the principal incidents in his life
may, however, not be out of place.

James Clerk Maxwell was born in Edinburgh, on June 13, 1831. His
father, John Clerk Maxwell, was the second son of James Clerk, of
Penicuik, and took the name of Maxwell on inheriting the estate at
Middlesbie. His mother was the daughter of R. H. Cay, Esq., of North
Charlton, Northumberland. James was the only child who survived
infancy.

Some years before his birth his parents had built a house at Glenlair,
which had been added to their Middlesbie estate, and resided there
during the greater part of the year, though they retained their house
in Edinburgh. Hence it was that James's boyish days were spent almost
entirely in the country, until he entered the Edinburgh Academy in
1841. As a child, he was never content until he had completely
investigated everything which attracted his attention, such as the
hidden courses of bell-wires, water-streams, and the like. His
constant question was "What's the go o' that?" and, if answered in
terms too general for his satisfaction, he would continue, "But what's
the particular go of it?" This desire for the thorough investigation
of every phenomenon was a characteristic of his mind through life.
From a child his knowledge of Scripture was extensive and accurate,
and when eight years old he could repeat the whole of the hundred and
nineteenth psalm. About this time his mother died, and thenceforward
he and his father became constant companions. Together they would
devise all sorts of ingenious mechanical contrivances. Young James was
essentially a child of nature, and free from all conventionality. He
loved every living thing, and took delight in petting young frogs, and
putting them into his mouth to see them jump out. One of his
attainments was to paddle on the duck-pond in a wash-tub, and to make
the vessel go "without spinning"--a recreation which had to be
relinquished on washing-days. He was never without the companionship
of one or two terriers, to whom he taught many tricks, and with whom
he seemed to have complete sympathy.

As a boy, Maxwell was not one to profit much by the ordinary teaching
of the schools, and experience with a private tutor at home did not
lead to very satisfactory results. At the age of ten, therefore, he
was sent to the Edinburgh Academy, under the care of Archdeacon
Williams, who was then rector. On his first appearance in this
fashionable school, he was naturally a source of amusement to his
companions; but he held his ground, and soon gained more respect than
he had previously provoked ridicule. While at school in Edinburgh, he
resided with his father's sister, Mrs. Wedderburn, and devoted a very
considerable share of his time and attention to relieving the solitude
of the old man at Glenlair, by letters written in quaint styles,
sometimes backwards, sometimes in cypher, sometimes in different
colours, so arranged that the characters written in a particular
colour, when placed consecutively, formed another sentence. All the
details of his school and home life, and the special peculiarities of
the masters at the academy, were thus faithfully transmitted to his
father, by whom the letters were religiously preserved. At thirteen he
had evidently made progress in solid geometry, though he had not
commenced Euclid, for he writes to his father, "I have made a
tetrahedron, a dodecahedron, and two other hedrons whose names I don't
know." In these letters to Glenlair he generally signed himself, "Your
most obedient servant." Sometimes his fun found vent even upon the
envelope; for example:--

"Mr. John Clerk Maxwell,
"Postyknowswere,
"Kirkpatrick Durham,
"Dumfries."

Sometimes he would seal his letters with electrotypes of natural
objects (beetles, etc.), of his own making. In July, 1845, he
writes:--

I have got the eleventh prize for scholarship, the first for
English, the prize for English verses, and the mathematical
medal.

When only fifteen a paper on oval curves was contributed by him to the
Proceedings of the Royal Society of Edinburgh. In the spring of 1847
he accompanied his uncle on a visit to Mr. Nicol, the inventor of the
Nicol prism, and on his return he made a polariscope with glass and a
lucifer-match box, and sketched in water-colours the chromatic
appearances presented by pieces of unannealed glass which he himself
prepared. These sketches he sent to Mr. Nicol, who presented him in
return with a pair of prisms of his own construction. The prisms are
now in the Cavendish Laboratory at Cambridge. Maxwell found that, for
unannealed glass, pieces of window-glass placed in bundles of eight or
nine, one on the other, answered the purpose very well. He cut the
figures, triangles, squares, etc., with a diamond, heated the pieces
of glass on an iron plate to redness in the kitchen fire, and then
dropped them into a plate of iron sparks (scales from the smithy) to
cool.

In 1847 Maxwell entered the University of Edinburgh, and during his
course of study there he contributed to the Royal Society of Edinburgh
papers upon rolling curves and on the equilibrium of elastic solids.
His attention was mostly devoted to mathematics, physics, chemistry,
and mental and moral philosophy. In 1850 he went to Cambridge,
entering Peterhouse, but at the end of a year he "migrated" to
Trinity; here he was soon surrounded with a circle of friends who
helped to render his Cambridge life a very happy one. His love of
experiment sometimes extended to his own mode of life, and once he
tried sleeping in the evening and working after midnight, but this was
soon given up at the request of his father. One of his friends writes,
"From 2 to 2.30 a.m. he took exercise by running along the upper
corridor, down the stairs, along the lower corridor, then up the
stairs, and so on until the inhabitants of the rooms along his track
got up and laid perdus behind their sporting-doors, to have shots at
him with boots, hair-brushes, etc., as he passed." His love of fun,
his sharp wit, his extensive knowledge, and above all, his complete
unselfishness, rendered him a universal favourite in spite of the
temporary inconveniences which his experiments may have occasionally
caused to his fellow-students.

An undergraduate friend writes, "Every one who knew him at Trinity can
recall some kindness or some act of his which has left an ineffaceable
impression of his goodness on the memory--for 'good' Maxwell was in
the best sense of the word." The same friend wrote in his diary in
1854, after meeting Maxwell at a social gathering, "Maxwell, as usual,
showing himself acquainted with every subject on which the
conversation turned. I never met a man like him. I do believe there is
not a single subject on which he cannot talk, and talk well too,
displaying always the most curious and out-of-the-way information."
His private tutor, the late well-known Mr. Hopkins, said of him, "It
is not possible for that man to think incorrectly on physical
subjects."

In 1854 Maxwell took his degree at Cambridge as second wrangler, and
was bracketed with the senior wrangler (Mr. E. J. Routh) for the
Smith's prize. During his undergraduate course, he appears to have
done much of the work which formed the basis of his subsequent papers
on electricity, particularly that on Faraday's lines of force. The
colour-top and colour-box appear also to have been gradually
developing during this time, while the principle of the stereoscope
and the "art of squinting" received their due share of attention.
Shortly after his degree, he devoted a considerable amount of time to
the preparation of a manuscript on geometrical optics, which was
intended to form a university text-book, but was never completed. In
the autumn of 1855 he was elected Fellow of Trinity. About this time
the colour-top was in full swing, and he also constructed an
ophthalmoscope. In May, 1855, he writes:--

The colour trick came off on Monday, 7th. I had the proof-sheets
of my paper, and was going to read; but I changed my mind and
talked instead, which was more to the purpose. There were sundry
men who thought that blue and yellow make green, so I had to
undeceive them. I have got Hay's book of colours out of the
University Library, and am working through the specimens,
matching them with the top.

The "colour trick" came off before the Cambridge Philosophical Society.

While a Bachelor Fellow, Maxwell gave lectures to working men in
Barnwell, besides lecturing in college. His father died in April,
1856, and shortly afterwards he was appointed Professor of Natural
Philosophy in Marischal College, Aberdeen. This appointment he held
until the fusion of the college with King's College in 1860. These
four years were very productive of valuable work. During them the
dynamical top was constructed, which illustrates the motion of a rigid
body about its axis of greatest, least, or mean moment of inertia;
for, by the movement of certain screws, the axis of the top may be
made to coincide with any one at will. The Adams Prize Essay on the
stability of Saturn's rings belongs also to this period. In this essay
Maxwell showed that the phenomena presented by Saturn's rings can only
be explained on the supposition that they consist of innumerable small
bodies--"a flight of brickbats"--each independent of all the others,
and revolving round Saturn as a satellite. He compared them to a siege
of Sebastopol from a battery of guns measuring thirty thousand miles
in one direction, and a hundred miles in the other, the shots never
stopping, but revolving round a circle of a hundred and seventy
thousand miles radius. A solid ring of such dimensions would be
completely crushed by its own weight, though made of the strongest
material of which we have any knowledge. If revolving at such a rate
as to balance the attraction of the planet at one part, the stress in
other parts would be more than sufficient to crush or tear the ring.
Laplace had shown that a narrow ring might revolve about the planet
and be stable if so loaded that its centre of gravity was at a
considerable distance from its centre, and thought that Saturn's
rings might consist of a number of such unsymmetrical rings--a theory
to which some support was given by the many small divisions observable
in the bright rings. Maxwell showed that, for stability, the mass
required to load each of Laplace's rings must be four and a half times
that of the rest of the ring; and the system would then be far too
artificially balanced to be proof against the action of one ring on
another. He further showed that, in liquid rings, waves would be
produced by the mutual action of the rings, and that before long some
of these waves would be sure to acquire such an amplitude as would
cause the rings to break up into small portions. Finally, he concluded
that the only admissible theory is that of the independent satellites,
and that the average density of the rings so found cannot be much
greater than that of air at ordinary pressure and temperature.

While he remained at Aberdeen, Maxwell lectured to working men in the
evenings, on the principles of mechanics. On the whole, it is doubtful
whether Aberdeen society was as congenial to him as that of Cambridge
or Edinburgh. He seems not to have been understood even by his
colleagues. On one occasion he wrote:--

Gaiety is just beginning here again.... No jokes of any kind are
understood here. I have not made one for two months, and if I
feel one coming I shall bite my tongue.

But every cloud has its bright side, and, however Maxwell may have
been regarded by his colleagues, he was not long without congenial
companionships. An honoured guest at the home of the Principal, "in
February, 1858, he announced his betrothal to Katherine Mary Dewar,
and they were married early in the following June." Professor Campbell
speaks of his married life as one of unexampled devotion, and those
who enjoyed the great privilege of seeing him at home could more than
endorse the description.

In 1860 Maxwell accepted the chair of Natural Philosophy at King's
College, London. Here he continued his lectures to working men, and
even kept them up for one session after resigning the chair in 1865.
On May 17, 1861, he gave his first lecture at the Royal Institution,
on "The Theory of the Three Primary Colours." This lecture embodies
many of the results of his work with the colour-top and colour-box, to
be again referred to presently. While at King's College, he was placed
on the Electrical Standards Committee of the British Association, and
most of the work of the committee was carried out in his laboratory.
Here, too, he compared the electro-static repulsion between two discs
of brass with the electro-magnetic attraction of two coils of wire
surrounding them, through which a current of electricity was allowed
to flow, and obtained a result which he afterwards applied to the
electro-magnetic theory of light. The colour-box was perfected, and
his experiments on the viscosity of gases were concluded during his
residence in London. These last were described by him in the Bakerian
Lecture for 1866.

After resigning the professorship at King's College, Maxwell spent
most of his time at Glenlair, having enlarged the house, in accordance
with his father's original plans. Here he completed his great work on
"Electricity and Magnetism," as well as his "Theory of Heat," an
elementary text-book which may be said to be without a parallel.

On March 8, 1871, he accepted the chair of Experimental Physics in the
University of Cambridge. This chair was founded in consequence of an
offer made by the Duke of Devonshire, the Chancellor of the
University, to build and equip a physical laboratory for the use of
the university. In this capacity Maxwell's first duty was to prepare
plans for the laboratory. With this view, he inspected the
laboratories of Sir William Thomson at Glasgow, and of Professor
Clifton at Oxford, and endeavoured to embody the best points of both
in the new building. The result was that, in conjunction with Mr. W.
M. Fawcett, the architect, he secured for the university a laboratory
noble in its exterior, and admirably adapted to the purposes for which
it is required. The ground-floor comprises a large battery-room, which
is also used as a storeroom for chemicals; a workshop; a room for
receiving goods, communicating by a lift with the apparatus-room; a
room for experiments on heat; balance-rooms; a room for pendulum
experiments, and other investigations requiring great stability; and a
magnetic observatory. The last two rooms are furnished with stone
supports for instruments, erected on foundations independent of those
of the building, and preserved from contact with the floor. On the
first floor is a handsome lecture-theatre, capable of accommodating
nearly two hundred students. The lecture-table is carried on a wall,
which passes up through the floor without touching it, the joists
being borne by separate brick piers. The lecture-theatre occupies the
height of the first and second floors; its ceiling is of wood, the
panels of which can be removed, thus affording access to the
roof-principals, from which a load of half a ton or more may be safely
suspended over the lecture-table. The panels of the ceiling, adjoining
the wall which is behind the lecturer, can also be readily removed,
and a "window" in this wall communicates with the large
electrical-room on the second floor. Access to the space above the
ceiling of the lecture-theatre is readily obtained from the tower.
Adjoining the lecture-room is the preparation-room, and communicating
with the latter is the apparatus-room. This room is fitted with
mahogany and plate-glass wall and central cases, and at present
contains, besides the more valuable portions of the apparatus
belonging to the laboratory, the marble bust of James Clerk Maxwell,
and many of the home-made pieces of apparatus and other relics of his
early work. The rest of the first floor is occupied by the
professor's private room and the general students' laboratory.
Throughout the building the brick walls have been left bare for
convenience in attaching slats or shelves for the support of
instruments. The second floor contains a large room for electrical
experiments, a dark room for photography, and a number of private
rooms for original work. Water is laid on to every room, including a
small room in the top of the tower, and all the windows are provided
with broad stone ledges without and within the window, the two
portions being in the same horizontal plane, for the support of
heliostats or other instruments. The building is heated with hot
water, but in the magnetic observatory the pipes are all of copper and
the fittings of gun-metal. Open fireplaces for basket fires are also
provided. Over the principal entrance of the laboratory is placed a
stone statue of the present Duke of Devonshire, together with the arms
of the university and of the Cavendish family, and the Cavendish
motto, "Cavendo Tutus." Maxwell presented to the laboratory, in 1874,
all the apparatus in his possession. He usually gave a course of
lectures on heat and the constitution of bodies in the Michaelmas
term; on electricity in the Lent term; and on electro-magnetism in the
Easter term. The following extract from his inaugural lecture,
delivered in October, 1871, is worthy of the attention of all students
of science:--

Science appears to us with a very different aspect after we
have found out that it is not in lecture-rooms only, and by
means of the electric light projected on a screen, that we may
witness physical phenomena, but that we may find illustrations
of the highest doctrines of science in games and gymnastics, in
travelling by land and by water, in storms of the air and of the
sea, and wherever there is matter in motion.

The habit of recognizing principles amid the endless variety of
their action can never degrade our sense of the sublimity of
nature, or mar our enjoyment of its beauty. On the contrary, it
tends to rescue our scientific ideas from that vague condition
in which we too often leave them, buried among the other
products of a lazy credulity, and to raise them into their
proper position among the doctrines in which our faith is so
assured that we are ready at all times to act on them.
Experiments of illustration may be of very different kinds. Some
may be adaptations of the commonest operations of ordinary life;
others may be carefully arranged exhibitions of some phenomenon
which occurs only under peculiar conditions. They all, however,
agree in this, that their aim is to present some phenomenon to
the senses of the student in such a way that he may associate
with it some appropriate scientific idea. When he has grasped
this idea, the experiment which illustrates it has served its
purpose.

In an experiment of research, on the other hand, this is not the
principal aim.... Experiments of this class--those in which
measurement of some kind is involved--are the proper work of a
physical laboratory. In every experiment we have first to make
our senses familiar with the phenomenon; but we must not stop
here--we must find out which of its features are capable of
measurement, and what measurements are required in order to make
a complete specification of the phenomenon. We must then make
these measurements, and deduce from them the result which we
require to find.

This characteristic of modern experiments--that they consist
principally of measurements--is so prominent that the opinion
seems to have got abroad that, in a few years, all the great
physical constants will have been approximately estimated, and
that the only occupation which will then be left to men of
science will be to carry these measurements to another place of
decimals.

If this is really the state of things to which we are
approaching, our laboratory may, perhaps, become celebrated as a
place of conscientious labour and consummate skill; but it will
be out of place in the university, and ought rather to be
classed with the other great workshops of our country, where
equal ability is directed to more useful ends.

But we have no right to think thus of the unsearchable riches of
creation, or of the untried fertility of those fresh minds into
which these riches will continually be poured.... The history
of Science shows that, even during that phase of her progress
in which she devotes herself to improving the accuracy of the
numerical measurement of quantities with which she has long been
familiar, she is preparing the materials for the subjugation of
new regions, which would have remained unknown if she had been
contented with the rough methods of her early pioneers.

Maxwell's "Electricity and Magnetism" was published in 1873. Shortly
afterwards there were placed in his hands, by the Duke of Devonshire,
the Cavendish Manuscripts on Electricity, already alluded to. To these
he devoted much of his spare time for several years, and many of
Cavendish's experiments were repeated in the laboratory by Maxwell
himself, or under his direction by his students. The introductory
matter and notes embodied in "The Electrical Researches of the
Honourable Henry Cavendish, F.R.S.," afford sufficient evidence of the
amount of labour he expended over this work. The volume was published
only a few weeks before his death. Another of Maxwell's publications,
which, as a text-book, is unique and beyond praise, is the little book
on "Matter and Motion," published by the S.P.C.K.

In 1878 Maxwell, at the request of the Vice-Chancellor, delivered the
Rede Lecture in the Senate-House. His subject was the telephone, which
was just then absorbing a considerable amount of public attention.
This was the last lecture which he ever gave to a large public
audience.

It was during his tenure of the Cambridge chair that one of the
cottages on the Glenlair estate was struck by lightning. The discharge
passed down the damp soot and blew out several stones from the base of
the chimney, apparently making its way to some water in a ditch a few
yards distant. The cottage was built on a granite rock, and this event
set Maxwell thinking about the best way to protect, from lightning,
buildings which are erected on granite or other non-conducting
foundations. He decided that the proper course was to place a strip of
metal upon the ground all round the building, to carry another strip
along the ridge-stay, from which one or more pointed rods should
project upwards, and to unite this strip with that upon the ground by
copper strips passing down each corner of the building, which is thus,
as it were, enclosed in a metal cage.

After a brief illness, Maxwell passed away on November 5, 1879. His
intellect and memory remained perfect to the last, and his love of fun
scarcely diminished. During his illness he would frequently repeat
hymns, especially some of George Herbert's, and Richard Baxter's hymn
beginning

"Lord, it belongs not to my care."

"No man ever met his death more consciously or more calmly."

It has been stated that Thomas Young propounded a theory of
colour-vision which assumes that there exist three separate
colour-sensations, corresponding to red, green, and violet, each
having its own special organs, the excitement of which causes the
perception of the corresponding colour, other colours being due to the
excitement of two or more of these simple sensations in different
proportions. Maxwell adopted blue instead of violet for the third
sensation, and showed that if a particular red, green, and blue were
selected and placed at the angular points of an equilateral triangle,
the colours formed by mixing them being arranged as in Young's
diagram, all the shades of the spectrum would be ranged along the
sides of this triangle, the centre being neutral grey. For the mixing
of coloured lights, he at first employed the colour-top, but, instead
of painting circles with coloured sectors, the angles of which could
not be changed, he used circular discs of coloured paper slit along
one radius. Any number of such discs can be combined so that each
shows a sector at the top, and the angle of each sector can be varied
at will by sliding the corresponding disc between the others. Maxwell
used discs of two different sizes, the small discs being placed above
the larger on the same pivot, so that one set formed a central circle,
and the other set a ring surrounding it. He found that, with discs of
five different colours, of which one might be white and another black,
it was always possible to combine them so that the inner circle and
the outer ring exactly matched. From this he showed that there could
be only three conditions to be satisfied in the eye, for two
conditions were necessitated by the nature of the top, since the
smaller sectors must exactly fill the circle and so must the larger.
Maxwell's experiments, therefore, confirmed, in general, Young's
theory. They showed, however, that the relative delicacy of the
several colour-sensations is different in different eyes, for the
arrangement which produced an exact match in the case of one observer,
had to be modified for another; but this difference of delicacy proved
to be very conspicuous in colour-blind persons, for in most of the
cases of colour-blindness examined by Maxwell the red sensation was
completely absent, so that only two conditions were required by
colour-blind eyes, and a match could therefore always be made in such
cases with four discs only. Holmgren has since discovered cases of
colour-blindness in which the violet sensation is absent. He agrees
with Young in making the third sensation correspond to violet rather
than blue. Maxwell explained the fact that persons colour-blind to the
red divide colours into blues and yellows by the consideration that,
although yellow is a complex sensation corresponding to a mixture of
red and green, yet in nature yellow tints are so much brighter than
greens that they excite the green sensation more than green objects
themselves can do, and hence greens and yellows are called yellow by
such colour-blind persons, though their perception of yellow is really
the same as perception of green by normal eyes. Later on, by a
combination of adjustable slits, prisms, and lenses arranged in a
"colour-box," Maxwell succeeded in mixing, in any desired proportions,
the light from any three portions of the spectrum, so that he could
deal with pure spectral colours instead of the complex combinations of
differently coloured lights afforded by coloured papers. From these
experiments it appears that no ray of the solar spectrum can affect
one colour-sensation alone, so that there are no colours in nature so
pure as to correspond to the pure simple sensations, and the colours
occupying the angular points of Maxwell's diagram affect all three
colour-sensations, though they influence two of them to a much smaller
extent than the third. A particular colour in the spectrum corresponds
to light which, according to the undulatory theory, physically
consists of waves all of the same period, but it may affect all three
of the colour-sensations of a normal eye, though in different
proportions. Thus, yellow light of a given wave-length affects the red
and green sensations considerably and the blue (or violet) slightly,
and the same effect may be produced by various mixtures of red or
orange and green. For his researches on the perception of colour, the
Royal Society awarded to Clerk Maxwell the Rumford Medal in 1860.

Another optical contrivance of Maxwell's was a wheel of life, in which
the usual slits were replaced by concave lenses of such focal length
that the picture on the opposite side of the cylinder appeared, when
seen through a lens, at the centre, and thus remained apparently
fixed in position while the cylinder revolved. The same result has
since been secured by a different contrivance in the praxinoscope.

Another ingenious optical apparatus was a real-image stereoscope, in
which two lenses were placed side by side at a distance apart equal to
half the distance between the pictures on the stereoscopic slide.
These lenses were placed in front of the pictures at a distance equal
to twice their focal length. The real images of the two pictures were
then superposed in front of the lenses at the same distance from them
as the pictures, and these combined images were looked at through a
large convex lens.

The great difference in the sensibility to different colours of the
eyes of dark and fair persons when the light fell upon the fovea
centralis, led Maxwell to the discovery of the extreme want of
sensibility of this portion of the retina to blue light. This he made
manifest by looking through a bottle containing solution of chrome
alum, when the central portion of the field of view appears of a light
red colour for the first second or two.

A more important discovery was that of double refraction temporarily
produced in viscous liquids. Maxwell found that a quantity of Canada
balsam, if stirred, acquired double-refracting powers, which it
retained for a short period, until the stress temporarily induced had
disappeared.

But Maxwell's investigations in optics must be regarded as his play;
his real work lay in the domains of electricity and of molecular
physics.

In 1738 Daniel Bernouilli published an explanation of atmospheric
pressure on the hypothesis that air consists of a number of minute
particles moving in all directions, and impinging on any surface
exposed to their action. In 1847 Herapath explained the diffusion of
gases on the hypothesis that they consisted of perfectly hard
molecules impinging on one another and on surfaces exposed to them,
and pointed out the relation between their motion and the temperature
and pressure of a gas. The present condition of the molecular theory
of gases, and of molecular science generally, is due almost entirely
to the work of Joule, Clausius, Boltzmann, and Maxwell. To Maxwell is
due the general method of solving all problems connected with vast
numbers of individuals--a method which he called the statistical
method, and which consists, in the first place, in separating the
individuals into groups, each fulfilling a particular condition, but
paying no attention to the history of any individual, which may pass
from one group to another in any way and as often as it pleases
without attracting attention. Maxwell was the first to estimate the
average distance through which a particle of gas passes without coming
into collision with another particle. He found that, in the case of
hydrogen, at standard pressure and temperature, it is about 1/250000
of an inch; for air, about 1/389000 of an inch. These results he
deduced from his experiments on viscosity, and he gave a complete
explanation of the viscosity of gases, showing it to be due to the
"diffusion of momentum" accompanying the diffusion of material
particles between the passing streams of gas.

One portion of the theory of electricity had been considerably
developed by Cavendish; the application of mathematics to the theory
of attractions, and hence to that of electricity, had been carried to
a great degree of perfection by Laplace, Lagrange, Poisson, Green, and
others. Faraday, however, could not satisfy himself with a
mathematical theory based upon direct action at a distance, and he
filled space, as we have seen, with tubes of force passing from one
body to another whenever there existed any electrical action between
them. These conceptions of Faraday were regarded with suspicion by
mathematicians. Sir William Thomson was the first to look upon them
with favour; and in 1846 he showed that electro-static force might be
treated mathematically in the same way as the flow of heat; so that
there are, at any rate, two methods by which the fundamental formulae
of electro-statics can be deduced. But it is to Maxwell that
mathematicians are indebted for a complete exposition of Faraday's
views in their own language, and this was given in a paper wherein the
phenomena of electro-statics were deduced as results of a stress in a
medium which, as suggested by Newton and believed by Faraday, might
well be that same medium which serves for the propagation of light;
and "the lines of force" were shown to correspond to an actual
condition of the medium when under electrical stress. Maxwell, in
fact, showed, not only that Faraday's lines formed a consistent system
which would bear the most stringent mathematical analysis, but were
more than a conventional system, and might correspond to a state of
stress actually existing in the medium through which they passed, and
that a tension along these lines, accompanied by an equal pressure in
every direction at right angles to them, would be consistent with the
equilibrium of the medium, and explain, on mechanical principles, the
observed phenomena. The greater part of this work he accomplished
while an undergraduate at Cambridge. He showed, too, that Faraday's
conceptions were equally applicable to the case of electro-magnetism,
and that all the laws of the induction of currents might be concisely
expressed in Faraday's language. Defining the positive direction
through a circuit in which a current flows as the direction in which a
right-handed screw would advance if rotating with the current, and the
positive direction around a wire conveying a current as the direction
in which a right-handed screw would rotate if advancing with the
current, Maxwell pointed out that the lines of magnetic force due to
an electric current always pass round it, or through its circuit, in
the positive direction, and that, whenever the number of lines of
magnetic force passing through a closed circuit is changed, there is
an electro-motive force round the circuit represented by the rate of
diminution of the number of lines of force which pass through the
circuit in the positive direction.

The words in italics form a complete statement of the laws regulating
the production of currents by the motion of magnets or of other
currents, or by the variation of other currents in the neighbourhood.
Maxwell showed, too, that Faraday's electro-tonic state, on the
variation of which induced currents depend, corresponds completely
with the number of lines of magnetic force passing through the
circuit.

He also showed that, when a conductor conveying a current is free to
move in a magnetic field, or magnets are free to move in the
neighbourhood of such a conductor, the system will assume that
condition in which the greatest possible number of lines of magnetic
force pass through the circuit in the positive direction.

But Maxwell was not content with showing that Faraday's conceptions
were consistent, and had their mathematical equivalents,--he proceeded
to point out how a medium could be imagined so constituted as to be
able to perform all the various duties which were thus thrown upon it.
Assuming a medium to be made up of spherical, or nearly spherical,
cells, and that, when magnetic force is transmitted, these cells are
made to rotate about diameters coinciding in direction with the lines
of force, the tension along those lines, and the pressure at right
angles to them, are accounted for by the tendency of a rotating
elastic sphere to contract along its polar axis and expand
equatorially so as to form an oblate spheroid. By supposing minute
spherical particles to exist between the rotating cells, the motion of
one may be transmitted in the same direction to the next, and these
particles may be supposed to constitute electricity, and roll as
perfectly rough bodies on the cells in contact with them. Maxwell
further imagined the rotating cells, and therefore, a fortiori, the
electrical particles, to be extremely small compared with molecules of
matter; and that, in conductors, the electrical particles could pass
from molecule to molecule, though opposed by friction, but that in
insulators no such transference was possible. The machinery was then
complete. If the electric particles were made to flow in a conductor
in one direction, passing between the cells, or molecular vortices,
they compelled them to rotate, and the rotation was communicated from
cell to cell in expanding circles by the electric particles, acting as
idle wheels, between them. Thus rings of magnetic force were made to
surround the current, and to continue as long as the current lasted.
If an attempt were made to displace the electric particles in a
dielectric, they would move only within the substance of each
molecule, and not from molecule to molecule, and thus the cells would
be deformed, though no continuous motion would result. The deformation
of the cells would involve elastic stress in the medium. Again, if a
stream of electric particles were started into motion, and if there
were another stream of particles in the neighbourhood free to flow,
though resisted by friction, these particles, instead of at once
transmitting the rotary motion of the cells on one side of them to the
cells on the other side, would at first, on account of the inertia of
the cells, begin to move themselves with a motion of translation
opposite to that of the primary current, and the motion would only
gradually be destroyed by the frictional resistance and the molecular
vortices on the other side made to revolve with their full velocity. A
similar effect, but in the opposite direction, would take place if the
primary current ceased, the vortices not stopping all at once if there
were any possibility of their continuing in motion. The imaginary
medium thus serves for the production of induced currents.

The mechanical forces between currents and magnets and between
currents and currents, as well as between magnets and currents, were
accounted for by the tension and pressure produced by the molecular
vortices. When currents are flowing in the same direction in
neighbouring conductors, the vortices in the space between them are
urged in opposite directions by the two currents, and remain almost at
rest; the lateral pressure exerted by those on the outside of the
conductors is thus unbalanced, and the conductors are pushed together
as though they attracted each other. When the currents flow in
opposite directions in parallel conductors, they conspire to give a
greater velocity to the vortices in the space between them, than to
those outside them, and are thus pushed apart by the pressure due to
the rotation of the vortices, as though they repelled each other. In a
similar way, the actions of magnets on conductors conveying currents
may be explained. The motion of a conductor across a series of lines
of magnetic force may squeeze together and lengthen the threads of
vortices in front, and thus increase their speed of rotation, while
the vortices behind will move more slowly because allowed to contract
axially and expand transversely. The velocity of the vortices thus
being greater on one side of the wire than the other, a current must
be induced in the wire. Thus the current induced by the motion of a
conductor in a magnetic field may be accounted for.

This conception of a medium was given by Maxwell, not as a theory, but
to show that it was possible to devise a mechanism capable, in
imagination at least, of producing all the phenomena of electricity
and magnetism. "According to our theory, the particles which form the
partitions between the cells constitute the matter of electricity. The
motion of these particles constitutes an electric current; the
tangential force with which the particles are pressed by the matter of
the cells is electro-motive force; and the pressure of the particles
on each other corresponds to the tension or potential of the
electricity."

When a current is maintained in a wire, the molecular vortices in the
surrounding space are kept in uniform motion; but if an attempt be
made to stop the current, since this would necessitate the stoppage of
the vortices, it is clear that it cannot take place suddenly, but the
energy of the vortices must be in some way used up. For the same
reason it is impossible for a current to be suddenly started by a
finite force. Thus the phenomena of self-induction are accounted for
by the supposed medium.

The magnetic permeability of a medium Maxwell identified with the
density of the substance composing the rotating cells, and the
specific inductive capacity he showed to be inversely proportional to
its elasticity. He then proved that the ratio of the electro-magnetic
unit to the electro-static unit must be equal to the velocity of
transmission of a transverse vibration in the medium, and consequently
proportional to the square root of the elasticity, and inversely
proportional to the square root of the density. If the medium is the
same as that engaged in the propagation of light, then this ratio
ought to be equal to the velocity of light, and, moreover, in
non-magnetic media, the refractive index should be proportional to the
square root of the specific inductive capacity. The different
measurements which had been made of the ratio of the electrical units
gave a mean very nearly coinciding with the best determinations of the
velocity of light, and thus the truth underlying Maxwell's speculation
was strikingly confirmed, for the velocity of light was determined by
purely electrical measurements. In the case also of bodies whose
chemical structure was not very complicated, the refractive index was
found to agree fairly well with the square root of the specific
inductive capacity; but the phenomenon of "residual charge" rendered
the accurate measurement of the latter quantity a matter of great
difficulty. It therefore appeared highly probable that light is an
electro-magnetic disturbance due to a motion of the electric particles
in an insulating medium producing a strain in the medium, which
becomes propagated from particle to particle to an indefinite
distance. In the case of a conductor, the electric particles so
displaced would pass from molecule to molecule against a frictional
resistance, and thus dissipate the energy of the disturbance, so that
true (i.e. metallic) conductors must be nearly impervious to light;
and this also agrees with experience.

Maxwell thus furnished a complete theory of electrical and
electro-magnetic action in which all the effects are due to actions
propagated in a medium, and direct action at a distance is dispensed
with, and exposed his theory successfully to most severe tests. In his
great work on electricity and magnetism, he gives the mathematical
theory of all the above actions, without, however, committing himself
to any particular form of mechanism to represent the constitution of
the medium. "This part of that book," Professor Tait says, "is one of
the most splendid monuments ever raised by the genius of a single
individual.... There seems to be no longer any possibility of doubt
that Maxwell has taken the first grand step towards the discovery of
the true nature of electrical phenomena. Had he done nothing but this,
his fame would have been secured for all time. But, striking as it is,
this forms only one small part of the contents of this marvellous
work."









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