SCIENTIFIC RESEARCHES: SECOND PERIOD.
With the year 1831 begins the period of the celebrated “Experimental
Researches in Electricity and Magnetism.” During the years which had
elapsed since his discovery of the electromagnetic rotations in 1823,
Faraday, though occupied, as we have seen, with other matters, had
not ceased to ponder the relation between the magnet and the electric
current. The great discoveries of Oersted, Ampère, and Arago had
culminated in England in two results: in Faraday’s discovery that the
wire which carries an electric current tends to revolve around the pole
of a neighbouring magnet; and in Sturgeon’s invention of the soft-iron
electromagnet, a core of iron surrounded by a coil of copper wire,
capable of acting as a magnet at will when the electric current is
transmitted to the coil and so caused to circulate around the iron core.
[Sidenote: FORESHADOWINGS.]
This production of magnetism from electricity, at will, and at a
distance, by the simple device of sending the electricity to circulate
as a current around the central core of iron was then, as now, a cause
of much speculation. The iron core which is to be made temporarily
into a magnet stands alone, isolated. Though surrounded outwardly
by the magnetising coil of copper wire, it does not touch it; nay,
must be screened from contact with it by appropriate insulation. The
electric current entering the copper coil at one end is confined from
leaving the copper wire by any lateral path: it must circulate around
each and every convolution, nor be permitted to flow back by the
return-wire until it has performed the required amount of circulation.
That the mere external circulation of electric current around a totally
disconnected interior core of iron should magnetise that core; that
the magnetisation should be maintained so long as the circulation of
electricity is maintained; and that the magnetising forces should cease
so soon as the current is stopped, are facts, familiar enough to every
beginner in the science, but mysterious enough from the abstract point
of view. Faraday was firmly persuaded that, great as had been these
discoveries of the production of magnetism and magnetic motions from
electricity, there remained other relations of no less importance to
be discovered. Again and again his mind recurred to the subject. If it
were possible to use electricity to produce magnetism, why should not
the converse be true? In 1822 his notebook suggestion was, as we have
seen, “Convert magnetism into electricity.” Yes, but how?
He possessed an intuitive bent of mind to inquire about the relations
of facts to one another. Convinced by sheer converse with nature in
the laboratory, of the correlation of forces and of the conservation
of energy long before either of those doctrines had received distinct
enunciation as principles of natural philosophy, he seems never to
have viewed an action without thinking of the necessary and appropriate
reaction; never to have deemed any physical relation complete in
which discovery had not been made of the converse relations for
which instinctively he sought. So in December, 1824, we find him
experimenting on the passage of a bar magnet through a helix of copper
wire (see _Quarterly Journal_ for July, 1825), but without result. In
November, 1825, he sought for evidence that might prove an electric
current in a wire to exercise an influence upon a neighbouring wire
connected to a galvanometer. But again, and yet again in December
of the same year, the entry stands “No result.” A third failure did
not convince him that the search was hopeless: it showed him that he
had not yet found the right method of experimenting. It is narrated
of him how at this period he used to carry in his waistcoat pocket a
small model of an electromagnetic circuit--a straight iron core about
an inch long, surrounded by a few spiral turns of copper wire--which
model he at spare moments would take out and contemplate, using it thus
objectively to concentrate his thoughts upon the problem to be solved.
A copper coil, an iron core. Given that electricity was flowing through
the one, it evoked magnetism in the other. What was the converse?
At first sight it might seem simple enough. Put magnetism from some
external source into the iron core, and then try whether on connecting
the copper coil to a galvanometer there was any indication of an
electric current. But this was exactly what was found not to result.
[Sidenote: OTHER MEN’S FAILURES.]
And not Faraday alone, but others, too, were foiled in the hope of
observing the expected converse. Not all who tried were as wise or
as frank as Faraday in confessing failure. Fresnel, in the height of
the fever of Oersted’s discovery, had announced to the Academy of
Sciences at Paris, on the 6th of November, 1820, that he had decomposed
water by means of a magnet which was laid motionless within a spiral
of wire. Emboldened by this announcement, Ampère remarked that he
too had noticed something in the way of production of currents from
a magnet. But before the end of the year both these statements were
withdrawn by their authors. Again, in the year 1822, Ampère, being at
Geneva, showed to Professor A. de la Rive in his laboratory a number of
electromagnetic experiments from his classical researches; and amongst
them one[20] which has been almost forgotten, but which, had it been
followed up, would assuredly have led Ampère to the discovery of the
induction of currents. In the experiment in question a thin copper
ring, made of a narrow strip folded into a circle, was hung inside
a circular coil of wire, traversed by a current. To this apparatus
a powerful horse-shoe magnet was presented; and De la Rive states
that, when the magnet was brought up, the suspended ring was observed
sometimes to move between the two limbs of the magnet, and sometimes
to be repelled from between them according to the sense of the current
in the surrounding coil. He and Ampère both attributed the effect to
temporary magnetism conferred upon the copper ring. Ampère himself
was at the time disposed to attribute it to the possible presence of
a little iron as an impurity in the copper. There are, however, some
discrepancies in the three published versions of the story. According
to Becquerel, Ampère had by 1825 satisfied himself of the non-existence
of induction currents.
[Sidenote: A PUZZLING EXPERIMENT.]
Quite independently, the question of the possibility of creating
currents by magnets was raised by another discovery, that of the
so-called “magnetism of rotation.” In 1824 Arago had observed that
a fine magnetic compass constructed for him by Gambey, having the
needle suspended in a cell, the base of which was a plate of pure
copper, was thereby damped in its oscillations, and instead of making
two or three hundred vibrations before it came to rest, as would be
the case in the open air, executed only three or four of rapidly
decreasing amplitude.[21] In vain did Dumas at the request of Arago
analyse the copper, in the supposition that iron might be present.
Inquiry compelled the conclusion that some other explanation must be
sought. And, reasoning from the apparent action of stationary copper
in bringing a moving magnetic needle to rest, he conjectured that a
moving mass of copper might produce motion in a stationary magnetic
needle. Accordingly he set into revolution, beneath a compass needle,
a flat disc of copper, and found that, even when a sheet of card or
glass was interposed to cut off all air-currents, the needle tended to
follow the moving copper disc, turning as if dragged by some invisible
influence. To the suggestion that mere rotation conferred upon copper
a sort of temporary magnetism Arago listened with some impatience. All
theories proposed to account for the phenomenon he discredited, even
though emanating from the great mathematician Poisson. He held his
judgment in absolute suspense. Babbage and Herschel measured the amount
of retarding force exerted on the needle by different materials, and
found the most effective to be silver and copper (which are the two
best conductors of electricity), after them gold and zinc, whilst lead,
mercury, and bismuth were inferior in power. The next year the same
experimenters announced the successful inversion of Arago’s experiment;
for by spinning the magnet underneath a pivoted copper disc they caused
the latter to rotate briskly. They also made the notable observation
that if slits are cut radially in the copper disc they diminish its
tendency to be dragged by the spinning magnet. Sturgeon showed that
the damping effect of a moving copper disc was diminished by the
presence of a second magnet pole of contrary kind placed beside the
first. All these things were most suggestive of the real explanation.
It clearly had something to do with the electric conductivity of
the metal disc, and therefore with electric currents. Sturgeon five
years later came very near to the explanation: after repeating the
experiments he concluded that the effect was an electric disturbance
in the copper disc, “a kind of reaction to that which takes place in
electromagnetism.”
Faraday knew of all the discussions which had arisen respecting
Arago’s rotations. They may have been the cause of his unsuccessful
attempts of 1824 and 1825. In April, 1828, for the fourth time he tried
to discover the currents which he was convinced must be producible
by the magnet, and for the fourth time without result. The cause of
failure was that both magnet and coil were at rest.
[Illustration: FIG. 4.]
The summer of 1831 witnessed him for the fifth time making the attack
on the problem thus persistently before him. In his laboratory
note-book he heads the research “Experiments on the production of
electricity from magnetism.” The following excellent summary of the
laboratory notes is taken from Bence Jones’s “Life and Letters”:--
I have had an iron ring made (soft iron), iron round and ⅞ths
of an inch thick, and ring six inches in external diameter.
Wound many coils of copper round, one half of the coils being
separated by twine and calico; there were three lengths of
wire, each about twenty-four feet long, and they could be
connected as one length, or used as separate lengths. By trials
with a trough each was insulated from the other. Will call
this side of the ring A. On the other side, but separated by
an interval, was wound wire in two pieces, together amounting
to about sixty feet in length, the direction being as with the
former coils. This side call B.[22]
Charged a battery of ten pairs of plates four inches square.
Made the coil on B side one coil, and connected its extremities
by a copper wire passing to a distance, and just over a
magnetic needle (three feet from wire ring), then connected the
ends of one of the pieces on A side with battery: immediately
a sensible effect on needle. It oscillated and settled at last
in original position. On breaking connection of A side with
battery, again a disturbance of the needle.
[Sidenote: SUCCESS IN SIGHT.]
In the seventeenth paragraph, written on the 30th of August, he says,
“May not these transient effects be connected with causes of difference
between power of metals at rest and in motion in Arago’s experiments?”
After this he prepared fresh apparatus.
As was his manner, he wrote off to one of his friends a letter telling
what he was at work upon. On this occasion the recipient of his
confidences was his friend Phillips:--
[_Michael Faraday to Richard Phillips._]
Royal Institution.
Sept. 23, 1831.
MY DEAR PHILLIPS,
I write now, though it may be some time before I send my
letter, but that is of no great consequence. I received your
letter to Dr. Reid and read it on the coach going to Hastings,
where I have been passing a few weeks, and I fancy my fellow
passengers thought I had got something very droll in hand;
they sometimes started at my sudden bursts, especially when I
had the moment before been very grave and serious amongst the
proportions. As you say in the letter there are some new facts
and they are always of value; otherwise I should have thought
you had taken more trouble than the matter deserved. Your
quotation from Boyle has nevertheless great force in it.
I shall send with this a little thing in your own way “On the
Alleged decline of science in England.” It is written by Dr.
Moll of Utrecht, whose name may be mentioned in conversation
though it is not printed in the pamphlet. I understand the
view taken by Moll is not at all agreeable to some. “I do not
know what business Moll had to interfere with our scientific
disputes” is however the strongest observation I have heard of
in reply.
I do not think I thanked you for your last Pharmacopœia. I
do so now very heartily. I shall detain this letter a few
days that I may send a couple of my papers (_i.e._ a paper
and appendix) with it, for though not chemical I think
you will like to have them. I am busy just now again on
Electro-Magnetism, and think I have got hold of a good thing,
but can’t say; it may be a weed instead of a fish that after
all my labour I may at last pull up. I think I know why metals
are magnetic when in motion though not (generally) when at rest.
We think about you all very much at times, and talk over
affairs of Nelson Square, but I think we dwell more upon the
illnesses and nursings and upon the sudden calls and chats
rather than the regular parties. Pray remember us both to Mrs.
Phillips and the damsils--I hope the word is not too familiar.
I am Dear Phillips,
Most Truly Yours,
M. FARADAY.
R. Phillips, Esq.,
&c., &c., &c.
[Sidenote: TEN DAYS OF SPLENDID WORK.]
September 24 was the third day of his experiments. He began (paragraph
21) by trying to find the effect of one helix of wire, carrying the
voltaic current of ten pairs of plates, upon another wire connected
with a galvanometer. “No induction sensible.” Longer and different
metallic helices (paragraph 22) showed no effect; so he gave up those
experiments for that day, and tried the effects of bar magnets instead
of the ring magnet he had used on the first day.
[Illustration: FIG. 5.]
In paragraph 33 he says:--
An iron cylinder had a helix wound on it. The ends of the wires
of the helix were connected with the indicating helix at a
distance by copper wire. Then the iron placed between the poles
of bar magnets as in accompanying figure (Fig. 5). Every time
the magnetic contact at N or S was made or broken, there was
magnetic motion at the indicating helix--the effect being, as
in former cases, not permanent, but a mere momentary push or
pull. But if the electric communication (_i.e_. by the copper
wire) was broken, then the disjunction and contacts produced no
effect whatever. Hence here distinct conversion of magnetism
into electricity.
The fourth day of work was October 1. Paragraphs 36, 37, and 38
describe the discovery of induced voltaic currents:--
36. A battery of ten troughs, each of ten pairs of plates four
inches square, charged with good mixture of sulphuric and
nitric acid, and the following experiments made with it in the
following order.
37. One of the coils (of a helix of copper wire 203 feet long)
was connected with the flat helix, and the other (coil of same
length round same block of wood) with the poles of the battery
(it having been found that there was no metallic contact
between the two); the magnetic needle at the indicating flat
helix was affected, but so little as to be hardly sensible.
38. In place of the indicating helix, our galvanometer was
used, and then a sudden jerk was perceived when the battery
communication was _made_ and _broken_, but it was so slight as
to be scarcely visible. It was one way when made, the other
when broken, and the needle took up its natural position at
intermediate times.
Hence there is an inducing effect without the presence of iron,
but it is either very weak or else so sudden as not to have
time to move the needle. I rather suspect it is the latter.
The fifth day of experiment was October 17. Paragraph 57 describes the
discovery of the production of electricity by the approximation of a
magnet to a wire:--
A cylindrical bar magnet three-quarters of an inch in
diameter, and eight inches and a half in length, had one end
just inserted into the end of the helix cylinder (220 feet
long); then it was quickly thrust in the whole length, and the
_galvanometer_ needle moved; then pulled out, and again the
_needle moved_, but in the opposite direction. This effect was
repeated every time the magnet was put in or out, and therefore
a wave of electricity was so produced from _mere approximation
of a magnet_, and not from its formation _in situ_.
The cause of all the earlier failures was, then, that both magnet and
coil were at rest. The magnet might lie in or near the coil for a
century and cause no effect. But while moving towards the coil, or from
it, or by spinning near it, electric currents were at once induced.
The ninth day of his experiments was October 28, and this day
he “made a copper disc turn round between the poles of the great
horse-shoe magnet of the Royal Society. The axis and edge of the disc
were connected with a galvanometer. The needle moved as the disc
turned.” The next day that he made experiments, November 4, he found
“that a copper wire one-eighth of an inch drawn between the poles and
conductors produced the effect.” In his paper, when describing the
experiment, he speaks of the metal “cutting” the magnetic curves, and
in a note to his paper he says, “By magnetic curves I mean lines of
magnetic forces which would be depicted by iron filings.”
[Sidenote: SUCCESS AND ITS SECRET.]
We here come upon those “lines of force” which played so important a
part in these and many of Faraday’s later investigations. They were
known before Faraday’s time--had, in fact, been known for two hundred
years. Descartes had seen in them evidence for his hypothetical
vortices. Musschenbroek had mapped them. But it was reserved to Faraday
to point out their true significance. To the very end of his life he
continued to speculate and experiment upon them.
All this splendid work had occupied but a brief ten days. Then he
rearranged the facts which he had thus harvested, and wrote them out in
corrected form as the first series of his “Experimental Researches in
Electricity.” The memoir was read to the Royal Society on November 24,
1831, though it did not appear in printed form until January, 1832--a
delay which gave rise to serious misunderstandings. The paper having
been read, he went away to Brighton to take a holiday, and in the
exuberance of his heart penned the following letter[23] to Phillips:--
[_M. Faraday to R. Phillips._]
Brighton: November 29, 1831.
DEAR PHILLIPS,--For once in my life I am able to sit down and
write to you without feeling that my time is so little that my
letter must of necessity be a short one and accordingly I have
taken an extra large sheet of paper intending to fill it with
news and yet as to news I have none for I withdraw more and
more from Society, and all I have to say is about myself.
But how are you getting on? are you comfortable? and how does
Mrs. Phillips do; and the girls? Bad correspondant as I am, I
think you owe me a letter and as in the course of half an hour
you will be doubly in my debt pray write us, and let us know
all about you. Mrs. Faraday wishes me not to forget to put her
kind remembrances to you and Mrs. Phillips in my letter.
To-morrow is St. Andrew’s day,[24] but we shall be here until
Thursday. I have made arrangements to be _out_ of the Council
and care little for the rest although I should as a matter of
curiosity have liked to see the Duke in the chair on such an
occasion.
We are here to refresh. I have been working and writing a
paper and that always knocks me up in health, but now I feel
well again and able to pursue my subject and now I will tell
you what it is about. The title will be, I think, EXPERIMENTAL
RESEARCHES IN ELECTRICITY: §I. _On the induction of electric
currents._ § II. _On the evolution of Electricity from
magnetism._ § III. _On a New electrical condition of matter._
§ IV. _On Arago’s magnetic phenomena._ There is a bill of fare
for you; and what is more I hope it will not disappoint you.
Now the pith of all this I must give you very briefly; the
demonstrations you shall have in the paper when printed--
[Sidenote: THE PITH OF THE DISCOVERY.]
§ I. When an electric current is passed through one of two
parallel wires it causes at first a current in the same
direction[25] through the other, but this induced current does
not last a moment, notwithstanding the inducing current (from
the Voltaic battery) is continued all seems unchanged except
that the principal current continues its course, but when the
current is stopped then a return current occurs in the wire
under induction of about the same intensity and momentary
duration but in the opposite direction to that first found.
Electricity in currents therefore exerts an inductive action
like ordinary electricity but subject to peculiar laws: the
effects are a current in the same direction when the induction
is established: a reverse current when the induction ceases and
a _peculiar state_ in the interim. Common electricity probably
does the same thing but as it is at present impossible to
separate the beginning and the end of a spark or discharge from
each other, all the effects are simultaneous and neutralise
each other--
§ II. Then I found that magnets would induce just like voltaic
currents and by bringing helices and wires and jackets up to
the poles of magnets, electrical currents were produced in
them these currents being able to deflect the galvanometer, or
to make, by means of the helix, magnetic needles, or in one
case even to give a spark. Hence the evolution of _electricity
from magnetism_. The currents were not permanent, they ceased
the moment the wires ceased to approach the magnet because
the new and apparently quiescent state was assumed just as in
the case of the induction of currents. But when the magnet
was removed, and its induction therefore ceased, the return
currents appeared as before. These two kinds of induction
I have distinguished by the terms _Volta-electric_ and
_Magneto-electric_ induction. Their identity of action and
results is, I think, a very powerful proof of the truth of M.
Ampère’s theory of magnetism.
[Sidenote: A JUBILANT EPISTLE.]
§ III. The new electrical condition which intervenes by
induction between the beginning and end of the inducing current
gives rise to some very curious results. It explains why
chemical action or other results of electricity have never
been as yet obtained in trials with the magnet. In fact, the
currents have no sensible duration. I believe it will explain
perfectly the _transference of elements_ between the poles
of the pile in decomposition but this part of the subject I
have reserved until the present experiments are completed
and it is so analogous, in some of its effects to those of
Ritter’s secondary piles, De la Rive and Van Beck’s peculiar
properties of the poles of a voltaic pile, that I should
not wonder if they all proved ultimately to depend on this
state. The condition of matter I have dignified by the term
_Electrotonic_, THE ELECTROTONIC STATE. What do you think of
that? Am I not a bold man, ignorant as I am, to coin words
but I have consulted the scholars,[26] and now for § IV. The
new state has enabled me to make out and explain all Arago’s
phenomena of the rotating magnet or copper plate, I believe,
perfectly; but as great names are concerned Arago, Babbage,
Herschel, &c., and as I have to differ from them, I have spoken
with that modesty which you so well know you and I and John
Frost[27] have in common, and for which the world so justly
commends us. I am even half afraid to tell you what it is.
You will think I am hoaxing you, or else in your compassion
you may conclude I am deceiving myself. However, you need do
neither, but had better laugh, as I did most heartily when I
found that it was neither attraction nor repulsion, but just
one of _my old rotations_ in a new form. I cannot explain to
you all the actions, which are very curious; but in consequence
of the electrotonic state being assumed and lost as the
parts of the plate whirl under the pole, and in consequence
of magneto-electric induction, currents of electricity are
formed in the direction of the radii; continuing, for simple
reasons, as long as the motion continues, but ceasing when
that ceases. Hence the wonder is explained that the metal has
powers on the magnet when moving, but not when at rest. Hence
is also explained the effect which Arago observed, and which
made him contradict Babbage and Herschel, and say the power
was repulsive; but, as a whole, it is really tangential. It
is quite comfortable to me to find that experiment need not
quail before mathematics, but is quite competent to rival
it in discovery; and I am amused to find that what the high
mathematicians have announced as the _essential condition_
to the rotation--namely, that _time is required_--has so
little foundation, that if the time could by possibility be
anticipated instead of being required--_i.e._ if the currents
could be formed _before_ the magnet came over the place instead
of _after_--the effect would equally ensue. Adieu, dear
Phillips.
Excuse this egotistical letter from yours very faithfully,
M. FARADAY.
The second section shows that Faraday had discovered the cause of all
the previous failures to evoke electric currents in wires by means of a
magnet: it required _relative motion_. What the magnet at rest fails to
do, the magnet in motion accomplishes. This crucial point is admirably
commemorated in the following impromptu given by Mr. Herbert Mayo to
Sir Charles Wheatstone:--
Around the magnet Faraday
Was sure that Volta’s lightnings play:
But how to draw them from the wire?
He took a lesson from the heart:
’Tis when we meet, ’tis when we part,
Breaks forth the electric fire.
Faraday’s holiday was brief; by December 5 he was again at work on his
researches. He re-observed the directions of the induced currents about
which, as the slip in his letter to Phillips shows, his mind was in
some doubt. Then on December 14th comes the entry:--“Tried the effects
of terrestrial magnetism in evolving electricity. Obtained beautiful
results.”
“The helix had the soft iron cylinder (freed from magnetism by a full
red heat and cooling slowly) put into it, and it was then connected
with the galvanometer by wires eight foot long; then inverted the bar
and helix, and immediately the needle moved; inverted it again, the
needle moved back; and, by repeating the motion with the oscillations
of the needle, made the latter vibrate 180°, or more.”
The same day he “made Arago’s experiment with the earth magnet, only no
magnet used, but the plate put horizontal and rotated. The effect at
the needle was slight but very distinct.... Hence Arago’s plate a new
electrical machine.”
[Sidenote: POINTS IN THE DISCOVERY.]
When we compare these manuscript notes, recording the experiments in
the order in which they were made with the published account of them
in the “Experimental Researches,” we find many of them transcribed
almost verbatim. But there is a difference in the order of their
arrangement. In point of time the experiments on the evolution of
electricity from magnetism, beginning with the ring (p. 108), preceded
those on the induction of a current by another current. In the printed
“Researches” the experiments on the induction of currents are put
first, with an introductory paragraph on the general phenomenon of
induction.[28] Faraday’s habit of working up an experiment--whether
successful or unsuccessful--by increasing the power to the maximum
available is illustrated in the course of the experiments on the iron
ring. At first he used a battery of ten pairs of plates four inches
square. Then, having been eminently successful in producing deflexions
of his galvanometer, he increased the battery to one hundred pairs of
plates, with the result that when contact was completed or broken in
the primary circuit the impulse on the galvanometer in the secondary
circuit was so great as to make the needle spin round rapidly four
or five times before its motion was reduced to a mere oscillation.
Then he removed the galvanometer and fixed small pencils of charcoal
to the ends of the secondary helix; and to his great joy perceived a
minute _spark_ between the lightly touching charcoal points whenever
the contact of the battery to the primary helix was completed. This
was the first transformer, for the first time set--on a small
scale--to produce a tiny electric light. The spark he regarded as a
precious indication that what he was producing really was an electric
current. Using the great compound steel magnet of the Royal Society
(constructed by Dr. Gowin Knight) at Christie’s house at Woolwich
he had, as narrated above, also obtained a spark from the induced
current. For some time he failed to obtain either physiological or
chemical effects. But upon repeating the experiments more at leisure
at the Royal Institution, with Daniell’s armed loadstone capable of
lifting thirty pounds, a frog was found to be convulsed very strongly
each time magnetic contact between the magnet and the iron core of the
experimental coil was made or broken.
The absence of evidence as to chemical action seemed still to
disquiet him. He wanted to be sure that his induced currents would
do everything that ordinary voltaic currents would do. Failing the
final proof from chemical action, he rested the case on the other
identical properties. “But an agent,” he says, “which is conducted
along metallic wires in the manner described; which, whilst so passing,
possesses the peculiar magnetic actions and force of a current of
electricity; which can agitate and convulse the limbs of a frog;
and which, finally, can produce a spark by its discharge through
charcoal, can only be electricity. As all the effects can be produced
by ferruginous electro-magnets, there is no doubt that arrangements
like the magnets of Professors Moll, Henry, Ten Eyke, and others, in
which as many as two thousand pounds have been lifted, may be used
for these experiments; in which case not only a brighter spark may be
obtained, but wires also ignited, and as the currents can pass liquids,
chemical action be produced. These effects are still more likely to
be obtained when the magneto-electric arrangements, to be explained
in the fourth section, are excited by the powers of such apparatus.”
The apparatus described in the fourth section comprised several forms
of magneto-electric machines, that is to say, primitive kinds of
dynamos. Having in his mind the phenomenon discovered by Arago, and
the experiments of Babbage and Herschel on the so-called magnetism of
rotation, he followed up the idea that these effects might be due to
induced currents eddying round in the copper disc. No sooner had he
obtained electricity from magnets than he attempted to make Arago’s
experiment a new source of electricity, and, as he himself says, “did
not despair” “of being able to construct a new electrical machine.”
[Illustration: FIG. 6. (FACSIMILE OF ORIGINAL SKETCH.)]
[Sidenote: A NEW ELECTRICAL MACHINE.]
The “new electrical machine” was an exceedingly simple contrivance. A
disc of copper, twelve inches in diameter (Fig. 6), and about one-fifth
of an inch in thickness, fixed upon a brass axle, was mounted in
frames, so as to allow of revolution, its edge being at the same time
introduced between the magnetic poles of a large compound permanent
magnet, the poles being about half an inch apart.[29] The magnet first
used was the historical magnet of Gowin Knight. The edge of the plate
was well amalgamated, for the purpose of obtaining a good but movable
contact, and a part round the axle was also prepared in a similar
manner. Conducting strips of copper and lead, to serve as electric
collectors, were prepared, so as to be placed in contact with the edge
of the copper disc; one of these was held by hand to touch the edge
of the disc between the magnet poles. The wires from a galvanometer
were connected, the one to the collecting-strip, the other to the
brass axle; then on revolving the disc a deflexion of the galvanometer
was obtained, which was reversed in direction when the direction of
the rotation was reversed. “Here, therefore, was demonstrated the
production of a permanent current of electricity by ordinary magnets.”
These effects were also obtained from the poles of electro-magnets,
and from copper helices without iron cores. Several other forms of
magneto-electric machines were tried by Faraday.
[Sidenote: NEW FORMS OF APPARATUS.]
[Illustration: FIG. 7.]
In one,[30] a flat ring of twelve inches’ external diameter, and one
inch broad, was cut from a thick copper plate, and mounted to revolve
between the poles of the magnet, two conductors being applied to
make rubbing contact at the inner and outer edge at the part which
passed between the magnetic poles. In another,[31] a disc of copper,
one-fifth of an inch thick and only 1½ inch in diameter (Fig. 7), was
amalgamated at the edge, and mounted on a copper axle. A square piece
of sheet metal had a circular hole cut in it, into which the disc
fitted loosely; a little mercury completed communication between the
disc and its surrounding ring. The latter was connected by wire to a
galvanometer; the other wire being connected from the instrument to the
end of the axle. Upon rotating the disc in a horizontal plane, currents
were obtained, though the earth was the only magnet employed.
[Illustration: FIG. 8.]
Faraday also proposed a multiple machine[32] having several discs,
metallically connected alternately at the edges and centres by means
of mercury, which were then to be revolved alternately in opposite
directions, In another apparatus,[33] a copper cylinder (Fig. 8),
closed at one extremity, was put over a magnet, one half of which
it enclosed like a cap, and to which it was attached without making
metallic contact. The arrangement was then floated upright in a narrow
jar of mercury, so that the lower edge of the copper cap touched the
fluid. On rotating the magnet and its attached cap, a current was sent
through wires from the mercury to the top of the copper cap. In another
apparatus,[34] still preserved at the Royal Institution, a cylindrical
bar magnet, half immersed in mercury, was made to rotate, and generated
a current, its own metal serving as a conductor. In another form,[35]
the cylindrical magnet was rotated horizontally about its own axis, and
was found to generate currents which flowed from the middle to the
ends, or _vice versâ_, according to the rotation. The description of
these new electrical machines is concluded with the following pregnant
words:--
[Sidenote: AN EARTH-INDUCTOR.]
I have rather, however, been desirous of discovering new facts
and relations dependent on magneto-electric induction, than of
exalting the force of those already obtained; being assured
that the latter would find their full development hereafter.
[Illustration: FIG. 9.]
In yet another machine (Fig. 9), constructed by Faraday some time
later,[36] a simple rectangle of copper wire _w_, attached to a frame,
was rotated about a horizontal axis placed east and west, and generated
alternate currents, which could be collected by a simple commutator _c_.
Within a few months machines on the principle of magneto-induction had
been devised by Dal Negro, and by Pixii. In the latter’s apparatus a
steel horseshoe magnet, with its poles upwards, was caused to rotate
about a vertical shaft, inducing alternating currents in a pair of
bobbins fixed above it, and provided with a horseshoe core of soft
iron. Later, in 1832, Pixii produced, at the suggestion of Ampère,[37]
a second machine, provided with mercury cup connections to rectify the
alternations of the current. One of these machines was shown at the
British Association meeting at Oxford in the same year (p. 64).
The idea developed in the third part of this research was intensely
original and suggestive. Faraday’s own statement is as follows:--
[Sidenote: THE ELECTROTONIC STATE.]
Whilst the wire is subject to either volta-electric or
magneto-electric induction, it appears to be in a peculiar
state; for it resists the formation of an electrical current
in it, whereas, if left in its common condition, such a
current would be produced; and when left uninfluenced it has
the power of originating a current, a power which the wire
does not possess under common circumstances. This electrical
condition of matter has not hitherto been recognised, but it
probably exerts a very important influence in many, if not
most, of the phenomena produced by currents of electricity. For
reasons which will immediately appear, I have, after advising
with several learned friends, ventured to designate it as the
_electrotonic_ state.
This peculiar condition shows no known electrical effects
whilst it continues; nor have I yet been able to discover any
peculiar powers exerted or properties possessed by matter
whilst retained in this state.
* * * * *
This state is altogether the effect of the induction exerted,
and ceases as soon as the inductive force is removed.... The
state appears to be instantly assumed, requiring hardly a
sensible portion of time for that purpose.... In all those
cases where the helices or wires are advanced towards or taken
from the magnet, the direct or inverted current of induced
electricity continues for the time occupied in the advance or
recession; for the electro-tonic state is rising to a higher or
falling to a lower degree during that time, and the change is
accompanied by its corresponding evolution of electricity; but
these form no objections to the opinion that the electro-tonic
state is instantly assumed.
This peculiar state appears to be a state of tension, and may
be considered as _equivalent_ to a current of electricity,
at least equal to that produced either when the condition is
induced or destroyed.
Faraday further supposed that the formation of this state in the
neighbourhood of a coil would exert a reaction upon the original
current, giving rise to a retardation of it; but he was unable at
the time to ascertain experimentally whether this was so. He even
looked--though also unsuccessfully--for a self-induced return current
from a conductor of copper through which a strong current was led and
then suddenly interrupted, the expected current of reaction being “due
to the discharge of its supposed electrotonic state.”
If we would understand the rather obscure language in which this idea
of an electrotonic state is couched, we must try to put ourselves
back to the epoch when it was written. At that date the only ideas
which had been formulated to explain magnetic and electric attractions
and repulsions were founded upon the notion of action at a distance.
Michell had propounded the view that the electric and magnetic forces
vary, like gravity, according to a law of the inverse squares of the
distances. Coulomb, in a series of experiments requiring extraordinary
patience as well as delicacy of manipulation, had shown--by an
application of Michell’s torsion balance--that in particular cases
where the electric charges are concentrated on small spheres, or
where the magnetic poles are small, so as to act as mere points,
this law--which is essentially a geometric law of point-action--is
approximately fulfilled. The mathematicians, Laplace and Poisson at
their head, had seized on this demonstration and had elaborated their
mathematical theories. Before them, though the research lay for a
century unpublished, Cavendish had shown that the only law of force
as between one element of an electric charge and another compatible
with a charge being in equilibrium was the law of inverse squares.
But in all these mathematical reasonings one thing had been quite
left out of sight--namely, the possible properties of the intervening
medium. Faraday, to whom the idea of mere action at a distance was
abhorrent, if not unthinkable, conceived of all these forces of
attraction and repulsion as effects taking place by something going
on _in the intervening medium_, as effects propagated from point
to point continuously through space. In his earlier work on the
electromagnetic rotations he had grown to regard the space around
the conducting wire as being affected by the so-called current; and
the space about the poles of a magnet he knew to be traversed by
curved magnetic lines, invisible indeed, but real, needing only the
simplest of expedients--the sprinkling of iron filings--to reveal their
existence and trend. When therefore he found that these new effects of
the induction of one electric current by another could likewise cross
an intervening space, whether empty or filled with material bodies,
he instinctively sought to ascribe this propagation of the effect
to a property or state of the medium. And finding that state to be
different from any state previously known, different from the state
existing between two magnets at rest or between two stationary electric
charges, he followed the entirely philosophical course of exploring its
properties and of denoting it by a name which he deemed appropriate. As
we shall see, this idea of an electrotonic state recurred in his later
researches with new and important connotations.
[Illustration: FIG. 10.]
He was soon at work again, as we have seen.
He experimented, in January, 1832, on the currents produced by the
earth’s rotation--on the 10th at the round pond in Kensington Gardens,
and on the 12th and 13th at Waterloo Bridge.
[Sidenote: A SPARK FROM A MAGNET.]
“This evening,” he writes in his notebook under date February 8, “at
Woolwich, experimenting with magnet,[38] and for the first time got the
magnetic spark myself. Connected ends of a helix into two general ends,
and then crossed the wires in such a way that a blow at _a b_ would
open them a little [Fig. 10]. Then bringing _a b_ against the poles of
a magnet, the ends were disjoined, and bright sparks resulted.”
From succeeding with a steel magnet it was but a short step to
succeed when a natural loadstone was used. The next day we find this
entry:--“At home succeeded beautifully with Mr. Daniell’s magnet.
Amalgamation of wires very needful. This is a natural loadstone, and
perhaps the first used for the spark.”
He sent to the Royal Society an account of these and the earlier
experiments; his paper on terrestrial magneto-electric induction, and
on the force and direction of magneto-electric induction, received the
distinction of being read as the Bakerian lecture of the year.
[Sidenote: TYNDALL’S SUMMARY.]
The following summary of this second paper is from the pen of Professor
Tyndall:--
He placed a bar of iron in a coil of wire, and lifting the
bar into the direction of the dipping needle, he excited by
this action a current in the coil. On reversing the bar, a
current in the opposite direction rushed through the wire.
The same effect was produced, when, on holding the helix in
the line of dip, a bar of iron was thrust into it. Here,
however, the earth acted on the coil through the intermediation
of the bar of iron. He abandoned the bar, and simply set a
copper plate spinning in a horizontal plane; he knew that the
earth’s lines of magnetic force then crossed the plate at an
angle of about 70°. When the plate spun round, the lines of
force were intersected and induced currents generated, which
produced their proper effect when carried from the plate to the
galvanometer. “When the plate was in the magnetic meridian, or
in any other plane coinciding with the magnetic dip, then its
rotation produced no effect upon the galvanometer.”
At the suggestion of a mind fruitful in suggestions of a
profound and philosophic character--I mean that of Sir John
Herschel--Mr. Barlow, of Woolwich, had experimented with
a rotating iron shell. Mr. Christie had also performed an
elaborate series of experiments on a rotating iron disc. Both
of them had found that when in rotation the body exercised a
peculiar action upon the magnetic needle, deflecting it in a
manner which was not observed during quiescence; but neither
of them was aware at the time of the agent which produced this
extraordinary deflection. They ascribed it to some change in
the magnetism of the iron shell and disc.
But Faraday at once saw that his induced currents must come
into play here, and he immediately obtained them from an iron
disc. With a hollow brass ball, moreover, he produced the
effects obtained by Mr. Barlow. Iron was in no way necessary;
the only condition of success was that the rotating body
should be of a character to admit of the formation of currents
in its substance; it must, in other words, be a conductor of
electricity. The higher the conducting power, the more copious
were the currents. He now passes from his little brass globe
to the globe of the earth. He plays like a magician with the
earth’s magnetism. He sees the invisible lines along which
its magnetic action is exerted, and, sweeping his wand across
these lines, he evokes this new power. Placing a simple loop
of wire round a magnetic needle, he bends its upper portion
to the west; the north pole of the needle immediately swerves
to the east; he bends his loop to the east, and the north
pole moves to the west. Suspending a common bar magnet in a
vertical position, he causes it to spin round its own axis.
Its pole being connected with one end of a galvanometer wire,
and its equator with the other end, electricity rushes round
the galvanometer from the rotating magnet. He remarks upon the
“_singular independence_” of the magnetism and the body of
the magnet which carries it. The steel behaves as if it were
isolated from its own magnetism.
And then his thoughts suddenly widen, and he asks himself
whether the rotating earth does not generate induced
currents as it turns round its axis from west to east. In
his experiment with the twirling magnet the galvanometer
wire remained at rest; one portion of the circuit was in
motion _relatively_ to _another portion_. But in the case of
the twirling planet the galvanometer wire would necessarily
be carried along with the earth; there would be no relative
motion. What must be the consequence? Take the case of a
telegraph wire with its two terminal plates dipped into the
earth, and suppose the wire to lie in the magnetic meridian.
The ground underneath the wire is influenced, like the wire
itself, by the earth’s rotation; if a current from south to
north be generated in the wire, a similar current from south
to north would be generated in the earth under the wire; these
currents would run against the same terminal plate, and thus
neutralise each other.
This inference appears inevitable, but his profound vision
perceived its possible invalidity. He saw that it was at least
possible that the difference of conducting power between
the earth and the wire might give one an advantage over the
other, and that thus a residual or differential current might
be obtained. He combined wires of different materials, and
caused them to act in opposition to each other, but found the
combination ineffectual. The more copious flow in the better
conductor was exactly counterbalanced by the resistance of the
worst. Still, though experiment was thus emphatic, he would
clear his mind of all discomfort by operating on the earth
itself. He went to the round lake near Kensington Palace, and
stretched 480 feet of copper wire, north and south, over the
lake, causing plates soldered to the wire at its ends to dip
into the water. The copper wire was severed at the middle,
and the severed ends connected with a galvanometer. No effect
whatever was observed. But though quiescent water gave no
effect, moving water might. He therefore worked at Waterloo
Bridge for three days, during the ebb and flow of the tide, but
without any satisfactory result. Still he urges, “Theoretically
it seems a necessary consequence, that where water is flowing
there electric currents should be formed. If a line be imagined
passing from Dover to Calais through the sea and returning
through the land, beneath the water, to Dover, it traces out
a circuit of conducting matter, one part of which, when the
water moves up or down the Channel, is cutting the magnetic
curves of the earth, whilst the other is relatively at rest....
There is every reason to believe that currents do run in the
general direction of the circuit described, either one way or
the other, according as the passage of the waters is up or down
the Channel.” This was written before the submarine cable was
thought of, and he once informed me that actual observation
upon that cable had been found to be in accordance with his
theoretic deduction.
[Illustration: FIG. 11.]
It may here be apposite to discuss a fundamental question raised in
these researches. In Faraday’s mind there arose the conviction of
a connection between the induction of currents by magnets and the
magnetic lines which invisibly fill all the space in the neighbourhood
of the magnet. That relation he discovered and announced in the
following terms:--
[Sidenote: THE LAW OF INDUCTION.]
“The relation which holds between the magnetic pole, the moving wire
or metal, and the direction of the current evolved--_i.e._ _the
law_ which governs the evolution of electricity by magneto-electric
induction, is very simple, though rather difficult to express. If in
Fig. 11, P N represent a horizontal wire passing by a marked [_i.e._
‘north-seeking’] magnetic pole, so that the direction of its motion
shall coincide with the curved line proceeding from below upwards; or
if its motion parallel to itself be in a line tangential to the curved
line, but in the general direction of the arrows; or if it pass the
pole in other directions, but so as to cut the magnetic curves[39] in
the same general direction, or on the same side as they would be cut by
the wire if moving along the dotted curved line; then the current of
electricity in the wire is from P to N. If it be carried in the reverse
direction, the electric current will be from N to P. Or if the wire be
in the vertical position, figured P´ N´, and it be carried in similar
directions, coinciding with the dotted horizontal curve so far as to
cut the magnetic curves on the same side with it, the current will be
from P´ to N´.”
[Sidenote: CUTTING THE MAGNETIC LINES.]
When resuming the research in December, Faraday investigated the
point whether it was essential or not that the moving wire should,
in “cutting” the magnetic curves, pass into positions of greater or
lesser magnetic force; or whether, always intersecting curves of equal
magnetic intensity, the mere motion sufficed for the production of the
current. He found the latter to be true. This notion of _cutting_ the
invisible magnetic lines as the essential act necessary and sufficient
for induction was entirely original with Faraday. For long it proved
a stumbling-block to the abstract mathematicians, since there was, in
most cases, no direct or easy way in which to express the number of
magnetic lines that were cut. Neither had any convention been adopted
up to that time as to how to reckon numerically the number of magnetic
lines in any given space near a magnet. Later, in 1851, Faraday himself
gave greater precision to these ideas. He found that the current was
proportional to the velocity, when the conductor was moving in a
uniform magnetic field with a uniform motion. Also, that the quantity
of electricity thrown by induction into the circuit was directly
proportional to the “amount of curves intersected.” The following
passage, from Clerk Maxwell’s article on Faraday in the “Encyclopædia
Britannica,” admirably sums up the matter:--
The magnitude and originality of Faraday’s achievement may be
estimated by tracing the subsequent history of his discovery.
As might be expected, it was at once made the subject of
investigation by the whole scientific world, but some of the
most experienced physicists were unable to avoid mistakes in
stating, in what they conceived to be more scientific language
than Faraday’s, the phenomena before them. Up to the present
time the mathematicians who have rejected Faraday’s method of
stating his law as unworthy of the precision of their science,
have never succeeded in devising any essentially different
formula which shall fully express the phenomena without
introducing hypotheses about the mutual action of things which
have no physical existence, such as elements of currents which
flow out of nothing, then along a wire, and finally sink into
nothing again.
After nearly half a century of labour of this kind, we may say
that, though the practical applications of Faraday’s discovery
have increased and are increasing in number and value every
year, no exception to the statement of these laws as given by
Faraday has been discovered, no new law has been added to them,
and Faraday’s original statement remains to this day the only
one which asserts no more than can be verified by experiment,
and the only one by which the theory of the phenomena can
be expressed in a manner which is exactly and numerically
accurate, and at the same time within the range of elementary
methods of exposition.
In the year 1831, which witnessed this masterpiece of scientific
research, Faraday was busy in many other ways. He was still undertaking
chemical analyses and expert work for fees, as witness his letter to
Phillips on p. 62. He was also, until November, on the Council of the
Royal Society. To the “Philosophical Transactions” he contributed
a paper “On Vibrating Surfaces,” in which he solved a problem in
acoustics which had previously gone without explanation. It had long
been known that in the experiments of obtaining the patterns called
“Chladni’s figures,” by strewing powders upon vibrating plates, while
the heavier powders, such as sand, moved into the nodal lines, lighter
substances, such as lycopodium dust, collected in little circular
heaps over the parts where the vibration was most energetic. Faraday’s
explanation was that these lighter powders were caught and whirled
about in little vortices which formed themselves at spots where the
motions were of greatest amplitude.
He also wrote a paper “On a Peculiar Class of Optical Deceptions,”
dealing with the illusions that result from the eye being shown in
successive glimpses, as between the teeth of a revolving wheel,
different views of a moving body. This research was, in effect, the
starting point of a whole line of optical toys, beginning with the
phenakistiscope or stroboscope, which developed through the zoetrope
and praxino-scope into the kinematograph and animatograph of recent
date.
[Sidenote: LECTURES ON PHYSICAL SUBJECTS.]
He gave four afternoon lectures at the Royal Institution and five
Friday evening discourses. These were on optical deceptions, on light
and phosphorescence, being an account of experiments recently made by
Mr. Pearsall, chemical assistant in the Institution; on oxalamide, then
recently discovered by M. Dumas; on Trevelyan’s experiments about the
production of sound by heated bodies; and on the arrangements assumed
by particles upon vibrating surfaces.
In 1832 he gave five Friday evening discourses, four of which related
to his own researches. In August he entered upon the third series
of “Experimental Researches in Electricity,” which was devoted to
the identity of electricities derived from different sources, and on
the relation by measure of common [_i.e._ frictional] and voltaic
electricity. He did not like any doubt to hang about as to whether
the electricity obtained from magnets by induction was really the
same as that obtainable from other sources. Possibly he had in his
mind the difficulties which had arisen thirty years before over the
discoveries of Galvani and Volta, when it was so far doubted whether
the electricity in currents from piles and batteries of cells was
the same as the electricity evoked by friction, that the distinctive
and misleading name of “galvanism” was assigned to the former.
He commented on the circumstance that many philosophers--and he
included Davy by name in an explicit reference--were vainly drawing
distinctions[40] between electricities from different sources, or at
least doubting whether their identity were proven. His first point
was to consider whether “common electricity,” “animal electricity,”
and “magneto-electric currents” could, like “voltaic electricity,”
produce chemical decompositions. He began by demonstrating that an
ordinary electric discharge from a friction machine can affect a
suitably disposed galvanometer. One of his instruments of sufficient
sensitiveness was surrounded by an enclosing cage of double metal
foil and wire-work, duly connected to “earth,” so as to render it
independent of all disturbances by external electric charges in its
neighbourhood. His “earth” for this purpose consisted of a stout metal
wire connected through the pipes in the house to the metallic gas-pipes
belonging to the public gas works of London, and also with the metallic
water-pipes of London--an effectual “discharging train.” He used a
friction electric machine with a glass plate 50 inches in diameter,
and a Leyden-jar battery of fifteen jars, each having about 84 square
inches of coated glass. This battery of jars was first charged from the
machine and then discharged through a wet thread four feet long, and
through the galvanometer to earth _viâ_ the “discharging train.” Having
by this means satisfied himself that these electric discharges could
deflect a galvanometer, whether through the wet thread, a copper wire,
or through water, or rarefied air, or by connection through points in
air, he went on to the question of chemical decomposition. Dipping two
silver wires into a drop of solution of sulphate of copper, he found
that one of them became copper-plated by the electricity that was
evolved by 100 or 200 turns of the disc machine. He bleached indigo,
turned starch purple with iodine liberated from iodide of potassium,
exactly as might have been done by a “volta-electric current” from a
battery of cells. He also decomposed water, giving due recognition to
the antecedent experiments of Van Troostwyk, Pearson, and Wollaston.
[Sidenote: IDENTITY OF ELECTRICITIES.]
In the paper which he drew up he compares these results with others
made with electric discharges from an electric kite and with those of
the torpedo and other electric fishes. He recapitulates the properties
of magneto-electricity and the proofs now accumulating that it can
decompose water. He drew up a schedule of the different effects which
electricity can produce, and of the different sources of electricity,
showing in tabular form how far each so-called kind of electricity
had been found to produce each effect. The conclusion was that there
is no philosophical difference between the different cases; since the
phenomena produced by the different kinds of electricity differ not in
their character but only in degree. “_Electricity, whatever may be its
source, is identical in its nature._” On comparing the effects produced
by different discharges, he concludes that “if the same absolute
quantity[41] of electricity pass through the galvanometer, whatever may
be its intensity, the deflecting force upon the magnetic needle is the
same.” He was then able to go on to a quantitative comparison between
the “quantity” of electricity from different sources, and came to the
conclusion that both in magnetic deflection and in chemical force the
current of electricity given by his standard battery for eight beats of
his watch was equal to that of the friction machine evolved by thirty
revolutions; further, that “the chemical power, like the magnetic
force, is in direct proportion to the absolute quantity of electricity
which passes.”
[Sidenote: ELECTRO-CHEMICAL WORK.]
This series of researches was published in January, 1833. In April
of the same year he sent to the Royal Society another paper--the
fourth series--on electric conduction. It arose from the surprising
observation that, though water conducts, ice acts as a complete
non-conductor. This led to an examination of the conducting power of
fusible solids in general. He found that as a rule--excepting on the
one hand the metals, which conduct whether solid or liquid, and on the
other hand fatty bodies, which are always non-conductors--they assume
conducting power when liquefied, and lose it when congealed. Chloride
of lead, of silver, of potassium, and of sodium, and many chlorates,
nitrates, sulphates, and many other salts and fusible substances were
found to follow this rule. All the substances so found to act were
compound bodies, and capable of decomposition by the current. When
conduction ceased, decomposition ceased also. An apparent exception was
found in sulphide of silver, which, when heated, acquired conducting
powers even before it assumed the liquid state, yet decomposed in the
solid state. This led him on to study electro-chemical decompositions
more closely. Here he was following directly in the footsteps of his
master Davy, whose discovery of the decomposition of potash and soda
by the electric current had been one of the most prominent scientific
advances resulting from the invention of the voltaic cell. The fifth
series of researches, published in June, 1833, embodies the work. He
first combats the prevailing opinion that the presence of water is
necessary for electro-chemical decomposition; then analyses the views
of various philosophers--Grotthuss, Davy, De la Rive, and others--who
had discussed the question whether the decompositions are due to
attractions exercised by the two poles of the electric circuit. This he
contests in the most direct manner. Already he has reason to believe
that for a given quantity of electricity passed through the liquid
the amount of electro-chemical action is a constant quantity, and
depends in no way on the distance of the particles of the decomposable
substance from the poles. He regards the elements as progressing in two
streams in opposite directions parallel to the current, while the poles
“are merely the surfaces or doors by which the electricity enters into
or passes out of the substance suffering decomposition.”
Amongst the laboratory notes of this time are many which were never
published in the “Experimental Researches,” or of which only brief
abstracts appeared. Some of these are of great interest.
Here is one literally transcribed:--
26 Feb. 1833.
_Chloride Magnesium._--When solid and wire fuzed in
non-conductor--When fuzed conducted very well and was
decomposed A and P Pole much action and gas--chlorine? At N
Pole Magnesium separated and no gas. Sometimes Magnesium burnt
flying off in globules burning brilliantly. When wire at that
pole put in water or white M A [muriatic acid] matter round
it acted powerfully evolving hydrogen and forming Magnesia;
and when wire and surrounding matter heated in spirit lamp
_Magnesium_ burnt with intense light into _Magnesia_. VERY GOOD
EXPT.
This recalls the “capital experiment” entry which Sir Humphry Davy
wrote after the account of his decomposition of caustic potash. On the
7th of April we come to a marvellous page of speculations. He has seen
that liquids, both solutions and fused salts, can be decomposed by
the current, and that at least one solid is capable of electrolysis.
But he finds that alloys and metals are not decomposed. He finds
that electrolysis is easiest for those compounds that consist of the
most diverse elements, and is led on to speculate as to the possible
constitution of those conductors that the current does not decompose.
This may involve a recasting of accepted ideas; but from such a step he
does not shrink, as the following extracts show:--
Metals _may_ not be compounds of elements most frequently
combined, but rather of such as are so similar to each other as
to pass out of the limit of voltaic decomposition.
13th April (same page).
If voltaic decomposition of the kind I believe then review
all substances upon the new view to see if they may not be
decomposable, &c. &c. &c.
[Sidenote: ATTRACTION BY POLES DOUBTED.]
He has now found that the facts observed do not admit of being
explained on the supposition that the motion of the ions is due to the
attraction of the poles, and accordingly there follows the entry:--
(Ap. 13, 1833.)
A single element is never attracted by a pole, _i.e._ without
attraction of other element at other pole. Hence doubt Mr.
Brande’s Expts on attraction of gases and vapours. Doubt
attraction by poles altogether.
To this subject he returned in 1834; an intervening memoir--the
sixth--being taken up with the power of metals and solids to bring
about the combination of gaseous bodies. In the seventh series,
published in January, 1834, his first work is to explain the new
terms which he has adopted, on the advice of Whewell, to express the
facts. The so-called poles, being in his view merely doors or ways by
which the current passes, he now terms _electrodes_, distinguishing
the entrance and exit respectively as _anode_ and _cathode_,[42]
while the decomposable liquid is termed an _electrolyte_, and the
decomposing process _electrolysis_. “Finally,” he says, in a passage
(here italicised) worthy to be engraved in gold for the essential
truth it enunciates on a question of terminology, “I require a term to
express those bodies which can pass to the _electrodes_, or, as they
are usually called, the poles. Substances are frequently spoken of as
being _electronegative_, or _electropositive_, according as they go
under the supposed influence of a direct attraction to the positive or
negative pole. But these terms are much too significant for the use to
which I should have to put them; _for though the meanings are perhaps
right, they are only hypothetical, and may be wrong; and then, through
a very imperceptible but still very dangerous, because continual,
influence, they do great injury to science, by contracting and limiting
the habitual views of those engaged in pursuing it_. I propose to
distinguish such bodies by calling those _anions_ which go to the anode
of the decomposing body; and those passing to the _cathode_, _cations_;
and when I shall have occasion to speak of these together, I shall
call them _ions_.[43] Thus, the chloride of lead is an _electrolyte_,
and when _electrolyzed_ evolves the two _ions_, chlorine and lead, the
former being an _anion_ and the latter a _cation_.” In Faraday’s own
bound volume of the “Experimental Researches” he has illustrated these
terms by the sketch here reproduced. (Fig. 12.)
Faraday’s letter to Whewell when he consulted him as to the new words
has not been preserved. He discarded, when the paper was printed, the
terms he had first used. Whewell’s replies of April 25th and May 5th,
1834, have been preserved and are printed in Todhunter’s biography
of Whewell. From the later of the two the following passage is
extracted:--
[Sidenote: NEW NOMENCLATURE.]
[_Whewell to Faraday_], May 5, 1834.
If you take _anode_ and _cathode_, I would propose for the two
elements resulting from _electrolysis_ the terms _anion_ and
_cation_, which are neuter participles signifying _that which
goes up_, and _that which goes down_; and for the two together
you might use the term _ions_.... The word is not a substantive
in Greek, but it may easily be so taken, and I am persuaded
that the brevity and simplicity of the terms you will thus have
will in a fortnight procure their universal acceptation. The
_anion_ is that which goes to the _anode_, the _cation_ is that
which goes to the _cathode_. The _th_ in the latter word arises
from the aspirate in _hodos_ (way), and therefore is not to be
introduced in cases where the second term has not an aspirate,
as _ion_ has not.
[Illustration: FIG. 12.]
On May 15th Faraday replied as follows:--
[_Faraday to Whewell._]
I have taken your advice and the names, and use _anode_,
_cathode_, _anions_, _cations_ and _ions_; the last I shall
have but little occasion for. I had some hot objections made to
them here, and found myself very much in the condition of the
man with his Son and Ass, who tried to please everybody; but
when I held up the shield of your authority it was wonderful
to observe how the tone of objection melted away. I am quite
delighted with the facility of expression which the new terms
give me, and shall ever be your debtor for the kind assistance
you have given me.
As though to prepare the way for a still further cutting of himself
adrift from the slavery of using terms that might be found misleading,
he added the following note:--
It will be well understood that I am giving no opinion
respecting the nature of the electric current now, beyond what
I have done on former occasions; and that though I speak of
the current as proceeding from the parts which are positive to
those which are negative, it is merely in accordance with the
conventional, though in some degree tacit, agreement entered
into by scientific men, that they may have a constant, certain,
and definite means of referring to the direction of the forces
of that current.
The “former occasions” is a reference to an earlier suggestion that
a _current_ might mean anything progressive, whether a flow in one
direction or two fluids moving in opposite directions, or merely
vibrations, or, still more generally, progressive forces. He had
expressly said that what we call the electric current “may perhaps best
be conceived of as _an axis of power having contrary forces, exactly
equal in amount, in contrary directions_.”
[Sidenote: ELECTRO-CHEMICAL LAWS.]
He then suggests as a measurer of current the standard form of
electrolytic cell ever since known as the _voltameter_. He preferred
that kind in which water is decomposed, the quantity of electricity
which had flowed through it being measured by the quantity of the
gas or gases evolved during the operation. Before adopting this he
undertook careful experiments in which his fine manipulative skill, no
less than his chemical experience, was called into service to verify
the fact that the quantity of water decomposed was really proportionate
to the quantity of electricity which has been passed through the
instrument. Having this standard, he investigated numerous other cases
of decomposition by the current, and so arrived at a substantial basis
for the doctrine of _definite electro-chemical_ action. Speaking of
the substances into which electrolytes are divided by the current,
and which he had called ions, he says: “They are combining bodies;
are directly associated with the fundamental parts of the doctrine
of chemical affinity; and have each a definite proportion, in which
they are always evolved during electrolytic action.... I have proposed
to call the numbers representing the proportions in which they are
evolved _electro-chemical equivalents_. Thus hydrogen, oxygen,
chlorine, iodine, lead, tin are _ions_; the three former are _anions_,
the two metals _cations_, and 1, 8, 36, 125, 104, 58, are their
_electro-chemical equivalents_ nearly.”
This fundamental law being set upon an impregnable basis of facts,
he goes on to speculate upon the _absolute quantity_ of electricity
or electric power belonging to different bodies; a notion which only
within the last few years has found general acceptance.
In developing this theory he uses the following language:--
According to it [_i.e._ this theory], the equivalent weights of
bodies are simply those quantities of them which contain equal
quantities of electricity, or have naturally equal electric
powers; it being the ELECTRICITY which _determines_ the
equivalent number, _because_ it determines the combining force.
Or, if we adopt the atomic theory or phraseology, then the
atoms of bodies which are equivalents to each other in their
ordinary chemical action, have equal quantities of electricity
naturally associated with them. But I must confess I am jealous
of the term _atom_....
Here we find the modern doctrine of _electrons_ or unitary atomic
charges, clearly formulated in 1834. In the course of this speculation
he remarks that “if the electrical power which holds the elements of
a grain of water in combination, or which makes a grain of oxygen or
hydrogen in the right proportions unite into water when they are made
to combine, could be thrown into the condition of _a current_, it
would exactly equal the current required for the separation of that
grain of water into its elements again.” And all this years before
there was any doctrine of the conservation of energy to guide the mind
of the philosopher! The passage just cited contains the germs of the
thermodynamic theory of electromotive forces worked out a dozen years
later by Sir William Thomson (now Lord Kelvin), by which theory we can
predict the electromotive forces of any given chemical combination from
a knowledge of the heat evolved by a given mass of the product in the
act of combining.
[Sidenote: ANOTHER UNSUCCESSFUL QUEST.]
The eighth series of the researches, which was read in June, 1834,
deals chiefly with voltaic cells and batteries of cells. He is now
applying to the operations inside the primary cell the electrochemical
principles learned by the study of electrolysis in secondary cells.
His thoughts have been incessantly playing around the problem of
electrolytic conduction. He was convinced that the forces which shear
the anions from combination with the cations and transfer them in
opposite directions must be inherent before the circuit is completed,
and therefore before any actual transfer or movement takes place. “It
seems to me impossible,” he says, “to resist the idea that it [the
“transfer,” or “what is called the voltaic current”] must be preceded
by a _state of tension_ in the fluid. I have sought carefully for
indications of a state of tension in the electrolytic conductor; and
conceiving that it might produce something like structure, either
before or during its discharge, I endeavoured to make this evident by
polarised light.” He used a solution of sulphate of soda, but without
the slightest trace of optical action in any direction of the ray. He
repeated the experiment, using a solid electrolyte, borate of lead, in
its non-conducting state, but equally without result.
During the time of these electrochemical researches in 1833 and 1834,
Faraday’s activities for the Royal Institution were undiminished. In
1833 he gave seven Friday discourses, three of them on the researches
in hand, one on Wheatstone’s investigation of the velocity of the
electric spark, and one on the practical prevention of dry rot in
timber, which was afterwards republished as a pamphlet, and ran to
two editions. In 1834 he gave four Friday discourses; two on his
electrochemical researches, one on Ericsson’s heat-engine, and the
other on caoutchouc.
The ninth series of electrical researches occupied the autumn of 1834.
In it he returns to the study of the magnetic and inductive actions
of the current, investigating the self-induced spark at the break
of the circuit, to which his attention had been directed by Mr. W.
Jenkin. Several points in this research are little known even now to
electricians, the laboratory notes being much more detailed than the
published paper. He describes an exceedingly neat high-speed break
for producing rapid interruptions, using for that purpose stationary
ripples on the surface of a pool of mercury. In a wonderful day’s work
on 13th November, filling thirty-four pages of the laboratory book,
illustrated with numerous unpublished sketches, he tracks out the
properties of self-induction. He proves that the spark (on breaking
circuit) from a wire coiled up in a helix is far brighter than that
from an identical wire laid out straight. He finds that a non-inductive
and, therefore, sparkless coil can be made by winding the wire in two
opposite helices. “Thus the whole [inductive] effect of the length
of wire was neutralised by the reciprocal and contrary action of the
two halves which constituted the helices in contrary directions.” The
next day he writes: “These effects show that every part of an electric
circuit is acting by induction on the neighbouring parts of the same
current, even in the _same wire_ and the _same part_ of the wire.”
[Sidenote: EFFECTS OF SELF-INDUCTION.]
On 22nd November he is trying another set of experiments, also never
fully published. They relate to the diminution of self-induction of a
straight conductor by dividing it into several parallel strands at a
small distance apart from one another. The note in the laboratory book
runs thus:--
Copper wire 1/23 of inch in diameter. Six lengths of five feet
each, soldered at ends to piece of copper plate so as form
terminations, and these amalgamated. When this bundle was used
to connect the electro-motor it gave but very feeble spark on
breaking contact, but the spark was sensibly better when the
wires are held together so as to act laterally than when they
were opened out from each other, thus showing lateral action.
Made a larger bundle of the same fine copper wire. There were
20 lengths of 18 feet 2 inches each and the thick terminal
pieces of copper wire 6 inches long and ⅓ of inch thick.
[Illustration: FIG. 13.]
This bundle he compared with a length of 19 feet 6 inches of a single
copper wire ⅕ inch in diameter, having about equal sectional area. The
latter gave decidedly the largest sparks on breaking circuit.
Faraday did not see fit at this time to accept the idea, suggested
indeed by himself in 1831, that these effects of self-induction were
the analogue of momentum or inertia. That explanation he set aside
on finding that the same wire when coiled had greater self-inductive
action than when straight. Had he at that time grasped this analogy, he
would have seen that the very property which gives rise to the spark
at break of circuit also retards the rapid growth of a current; and
then the experiment described above would have shown him that Sir W.
Snow Harris was right in preferring flat copper ribbon to a round wire
of equivalent section as a material for lightning conductors. He was,
however, disappointed to find so small a difference between round wires
and parallel strands. The memoir as published contains an exceedingly
interesting conclusion:--
Notwithstanding that the effects appear only at the making
and breaking of contact (the current, remaining unaffected,
seemingly, in the interval,) I cannot resist the impression
that there is some connected and correspondent effect produced
by this lateral action of the elements of the electric stream
during the time of its continuance. An action of this kind,
in fact, is evident in the magnetic relations of the parts
of the current. But admitting (as we may do for the moment)
the magnetic forces to constitute the power which produces
such striking and different results at the commencement and
termination of a current, still there appears to be a link in
the chain of effects--a wheel in the physical mechanism of the
action, as yet unrecognised.
The tenth series of researches, on the voltaic battery, though
completed in October, 1834, was not published till June, 1835.
[Sidenote: ACTION IN A MEDIUM.]
The next research, begun in the autumn of 1835, after a lull of
about eight months, lasted over two years. It was not completed till
December, 1837. This investigation took Faraday away from magnetic
and electrochemical matters to the old subject of statical electric
charges, a subject hitherto untouched in his researches. But he had
long brooded over the question as to the nature of an electric charge.
Over and over again, as he had watched the inductive effect of electric
currents acting from wire to wire, his mind turned to the old problem
of the inductive influence--discovered eighty years before, by John
Canton--exerted, apparently at a distance, by electric charges. He
had learned to distrust action at a distance, and now the time was
ripe for a searching inquiry as to whether electric _influence_, or
induction[44] as it was then called, was also an action propagated by
contiguous actions in the intervening medium.
Faraday had done no special electric work during the first nine months
of 1835. He had worked at a chemical investigation of fluorine through
the spring, and in July took a hurried tour in Switzerland, and
returned to work at fluorine. Not till November 3rd does he turn to the
subject over which he had been brooding. On that date, intercalated
between notes of his chemical studies, filling a dozen pages of the
laboratory book, are a magnificent series of speculations as to the
nature of charges, and on the part played by the electric--or, as we
should now say, the dielectric--medium. They begin thus:--
“Have been thinking much lately of the relation of common and
voltaic electricity, of induction by the former and decomposition by
the latter, and am quite convinced that there must be the closest
connection. Will be first needful to make out the true character”--note
the phrase--“of ordinary electrical phenomena.” The following notes
are for experiment and observation.
“Does common electricity reside upon the surface of a conductor or upon
the surface of the [di-]electric in contact with it?”
He goes on to consider the state of a dielectric substance, such as
glass, when situated between a positively charged and a negatively
charged surface, as in a charged Leyden jar, and argues from analogy
thus:--
“Hence the state of the plate [of glass] under induction is the same
as the state of a magnet, and if split or broken would present new
P[ositive] and N[egative] surfaces before not at all evident.” This
speculation was later verified by Matteucci.
“Probable that phenomena of induction prove more decidedly than
anything else that the electricity is in the [di-]electric not in the
conductor.”
He still worked for a week or two on fluorine, interposing some
experiments on the temperature-limit of magnetisation, but on December
4th decides not to go on with fluorine at present. Then, beginning on
December 5th, there follow twenty-nine pages of the laboratory diary,
illustrated with sketches. He had borrowed from a Mr. Kipp a large
deep copper pan thirty-five inches in diameter, and he set to work
electrifying it and exploring the distribution of the charges, inside
and out, and the inductive effect on objects placed within. Everywhere
he is mentally comparing the distribution of the effects with that of
the flow of currents in an electrolyte. Before many days he writes:--
[Sidenote: PREGNANT SUGGESTIONS.]
“It appears to me at present that _ordinary_ and _electrolytic_
induction are identical in their first nature, but that the latter is
followed by an effect which cannot but from the nature and state of
the substances take place with the former.” Then comes this pregnant
suggestion:--
“Try induction through a solid crystalline body as to the consequent
action on polarized light.”
By the end of a week he had begun to suspect that his magnet analogy
went farther than he was at first prepared to hold. The action of a
magnet was along curved lines of force. So he asks:--
“Can induction through air take place in curves or round a corner--can
probably be found experimentally--if so not a radiating effect.”
After ten days more he has made another step.
“Electricity appears to exist only in _polarity_ as in air, glass,
electrolytes, etc. Now metals, being conductors, cannot take up that
polar state of their own power, or rather retain it, and hence probably
cannot retain developed electric forces.
* * * * *
“Metals, however, probably hold it for a moment, as other things do for
a longer time; an end coming at last to all.”
This, it will be observed, is nothing more or less than Clerk Maxwell’s
theory of conduction as being the breaking down of an electrostatic
strain.
In January, 1836, followed the famous experiment of building a
twelve-foot cube, which when electrified exteriorly to the utmost
extent, showed inside no trace of electric forces. The account in the
unpublished MS. of the laboratory book is, as is the case with so
many of these middle-period researches, much fuller than the published
_résumé_ of them in the “Experimental Researches.” All through 1836
he was still at work. Even when on a holiday in the Isle of Wight, in
August, he took his notebook with him, and writes:--
“After much consideration (here at Ryde) of the manner in which the
electric forces are arranged in the various phenomena generally, I have
come to certain conclusions which I will endeavour to note down without
committing myself to any opinion as to the cause of electricity, _i.e._
as to the nature of the power. If electricity exist independently of
matter, then I think that the hypothesis of one fluid will not stand
against that of two fluids. There are, I think, evidently, what I may
call two elements of power of equal force and acting towards each
other. These may conventionally be represented by oxygen and hydrogen,
which represent them in the voltaic battery. But these powers may be
distinguished only _by direction_, and may be no more separate than the
north and south forces in the elements of a magnetic needle. They may
be the polar points of the forces originally placed in the particles of
matter; and the description of the current as an axis of power which I
have formerly given suggests some similar general impression for the
forces of quiescent electricity. Law of electric tension might do, and
though I shall use the terms positive and negative, by them I merely
mean the termini of such lines.”
Right on until November 30th, 1837, this research was continued. The
summary of this and the succeeding researches of 1838 on the same
subject, drawn up by Professor Tyndall,[45] is at once so masterly
and so impartial that it cannot be bettered. It is therefore here
transcribed without alteration.
[Sidenote: ACTION AT A DISTANCE UNTHINKABLE.]
His first great paper on frictional electricity was sent to
the Royal Society on November 30, 1837. We here find him face
to face with an idea which beset his mind throughout his
whole subsequent life--the idea of _action at a distance_. It
perplexed and bewildered him. In his attempts to get rid of
this perplexity he was often unconsciously rebelling against
the limitations of the intellect itself. He loved to quote
Newton upon this point: over and over again he introduces his
memorable words, “That gravity should be innate, inherent, and
essential to matter, so that one body may act upon another
at a distance through a _vacuum_ and without the mediation
of anything else, by and through which this 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.
Gravity must be caused by an agent acting constantly according
to certain laws; but whether this agent be material or
immaterial I have left to the consideration of my readers.”[46]
Faraday does not see the same difficulty in his contiguous
particles. And yet by transferring the conception from masses
to particles we simply lessen size and distance, but we do not
alter the quality of the conception. Whatever difficulty the
mind experiences in conceiving of action at sensible distances,
besets it also when it attempts to conceive of action at
insensible distances. Still the investigation of the point
whether electric and magnetic effects were wrought out through
the intervention of contiguous particles or not, had a physical
interest altogether apart from the metaphysical difficulty.
Faraday grapples with the subject experimentally. By simple
intuition he sees that action at a distance must be exerted in
straight lines. Gravity, he knows, will not turn a corner, but
exerts its pull along a right line; hence his aim and effort to
ascertain whether electric action ever takes place in curved
lines. This once proved, it would follow that the action is
carried on _by means of a medium_ surrounding the electrified
bodies. His experiments in 1837 reduced, in his opinion, this
point to demonstration. He then found that he could electrify
by induction an insulated sphere placed completely in the
shadow of a body which screened it from direct action. He
pictured the lines of electric force bending round the edges
of the screen, and reuniting on the other side of it; and he
proved that in many cases the augmentation of the distance
between his insulated sphere and the inducing body, instead of
lessening, increased the charge of the sphere. This he ascribed
to the coalescence of the lines of electric force at some
distance behind the screen.
[Sidenote: SPECIFIC INDUCTIVE CAPACITY.]
Faraday’s theoretic views on this subject have not received
general acceptance, but they drove him to experiment, and
experiment with him was always prolific of results. By suitable
arrangements he places a metallic sphere in the middle of a
large hollow sphere, leaving a space of something more than
half an inch between them. The interior sphere was insulated,
the external one uninsulated. To the former he communicated a
definite charge of electricity. It acted by induction upon the
concave surface of the latter, and he examined how this act of
induction was affected by placing insulators of various kinds
between the two spheres. He tried gases, liquids, and solids,
but the solids alone gave him positive results. He constructed
two instruments of the foregoing description, equal in size
and similar in form. The interior sphere of each communicated
with the external air by a brass stem ending in a knob. The
apparatus was virtually a Leyden jar, the two coatings of which
were the two spheres, with a thick and variable insulator
between them. The amount of charge in each jar was determined
by bringing a proof-plane into contact with its knob, and
measuring by a torsion balance the charge taken away. He first
charged one of his instruments, and then dividing the charge
with the other, found that when air intervened in both cases,
the charge was equally divided. But when shell-lac, sulphur,
or spermaceti was interposed between the two spheres of one
jar, while air occupied this interval in the other, then he
found that the instrument occupied by the “solid dielectric”
took _more than half_ the original charge. A portion of the
charge was absorbed in the dielectric itself. The electricity
took time to penetrate the dielectric. Immediately after the
discharge of the apparatus no trace of electricity was found
upon its knob. But after a time electricity was found there,
the charge having gradually returned from the dielectric in
which it had been lodged. Different insulators possess this
power of permitting the charge to enter them in different
degrees. Faraday figured their particles as polarised, and
he concluded that the force of induction is propagated from
particle to particle of the dielectric from the inner sphere
to the outer one. This power of propagation possessed by
insulators he calls their “_Specific Inductive Capacity_.”
[Illustration: FIG. 14.]
Faraday visualises with the utmost clearness the state of his
contiguous particles; one after another they become charged,
each succeeding particle depending for its charge upon its
predecessor. And now he seeks to break down the wall of
partition between conductors and insulators. “Can we not,” he
says, “by a gradual chain of association carry up discharge
from its occurrence in air through spermaceti and water to
solutions, and then on to chlorides, oxides, and metals,
without any essential change in its character?” Even copper, he
urges, offers a resistance to the transmission of electricity.
The action of its particles differs from those of an insulator
only in degree. They are charged like the particles of the
insulator, but they discharge with greater ease and rapidity;
and this rapidity of molecular discharge is what we call
conduction. Conduction, then, is always preceded by atomic
induction; and when through some quality of the body, which
Faraday does not define, the atomic discharge is rendered slow
and difficult, conduction passes into insulation.
Though they are often obscure, a fine vein of philosophic
thought runs through these investigations. The mind of the
philosopher dwells amid those agencies which underlie the
visible phenomena of induction and conduction; and he tries by
the strong light of his imagination to see the very molecules
of his dielectrics. It would, however, be easy to criticise
these researches, easy to show the looseness, and sometimes
the inaccuracy, of the phraseology employed; but this critical
spirit will get little good out of Faraday. Rather let those
who ponder his works seek to realise the object he set before
him, not permitting his occasional vagueness to interfere
with their appreciation of his speculations. We may see the
ripples, and eddies, and vortices of a flowing stream, without
being able to resolve all these motions into their constituent
elements; and so it sometimes strikes me that Faraday clearly
saw the play of fluids and ethers and atoms, though his
previous training did not enable him to resolve what he saw
into its constituents, or describe it in a manner satisfactory
to a mind versed in mechanics. And then again occur, I
confess, dark sayings, difficult to be understood, which
disturb my confidence in this conclusion. It must, however,
always be remembered that he works at the very boundaries of
our knowledge, and that his mind habitually dwells in the
“boundless contiguity of shade” by which that knowledge is
surrounded.
[Sidenote: CABLE RETARDATION PREDICTED.]
In the researches now under review the ratio of speculation and
reasoning to experiment is far higher than in any of Faraday’s
previous works. Amid much that is entangled and dark we have
flashes of wondrous insight and utterances which seem less
the product of reasoning than of revelation. I will confine
myself here to one example of this divining power:--By his
most ingenious device of a rapidly rotating mirror, Wheatstone
had proved that electricity required time to pass through
a wire, the current reaching the middle of the wire later
than its two ends. “If,” says Faraday, “the two ends of the
wire in Professor Wheatstone’s experiments were immediately
connected with two large insulated metallic surfaces exposed
to the air, so that the primary act of induction, after
making the contact for discharge, might be in part removed
from the internal portion of the wire at the first instance,
and disposed for the moment on its surface jointly with the
air and surrounding conductors, then I venture to anticipate
that the middle spark would be more retarded than before. And
if those two plates were the inner and outer coatings of a
large jar or Leyden battery, then the retardation of the spark
would be much greater.” This was only a _prediction_, for the
experiment was not made. Sixteen years subsequently, however,
the proper conditions came into play, and Faraday was able to
show that the observations of Werner Siemens and Latimer Clark
on subterraneous and submarine wires were illustrations, on a
grand scale, of the principle which he had enunciated in 1838.
The wires and the surrounding water act as a Leyden jar, and
the retardation of the current predicted by Faraday manifests
itself in every message sent by such cables.
The meaning of Faraday in these memoirs on induction and
conduction is, as I have said, by no means always clear;
and the difficulty will be most felt by those who are best
trained in ordinary theoretic conceptions. He does not know
the reader’s needs, and he therefore does not meet them. For
instance, he speaks over and over again of the impossibility
of charging a body with one electricity, though the
impossibility is by no means evident. The key to the difficulty
is this. He looks upon every insulated conductor as the inner
coating of a Leyden jar. An insulated sphere in the middle of
a room is to his mind such a coating; the walls are the outer
coating, while the air between both is the insulator, across
which the charge acts by induction. Without this reaction of
the walls upon the sphere, you could no more, according to
Faraday, charge it with electricity than you could charge a
Leyden jar, if its outer coating were removed. Distance with
him is immaterial. His strength as a generaliser enables him
to dissolve the idea of magnitude; and if you abolish the
walls of the room--even the earth itself--he would make the
sun and planets the outer coating of his jar. I dare not
contend that Faraday in these memoirs made all these theoretic
positions good. But a pure vein of philosophy runs through
these writings; while his experiments and reasonings on the
forms and phenomena of electrical discharge are of imperishable
importance.
In another part of the twelfth memoir, not included in the above
summary, Faraday deals with the disruptive discharge, and with the
nature of the spark under varying conditions. This is continued on into
the thirteenth memoir, read February, 1838, and is extended to the
cases of “brush” and “glow” discharges. He discovered the existence of
the very remarkable phenomenon of the “dark” discharge near the cathode
in rarefied air. He sought to correlate _all_ the various forms of
discharge, as showing the essential nature of an electric current. “If
a ball be electrified positively,” he says, “in the middle of a room,
and be then moved in any direction, effects will be produced, as if
a _current_ in the same direction (to use the conventional mode of
expression) had existed.” This is the theory of convection currents
later adopted by Maxwell, and verified by experiment by Rowland in 1876.
[Sidenote: COINAGE OF NEW WORDS.]
In the course of this research on induction, Faraday had, as we have
seen, been compelled to adopt new ideas, and therefore to adopt new
names to denote them. The term _dielectric_ for the medium in or across
which the electric forces operate was one of these. As in previous
cases, he consulted with his friends as to suitable terms. In this
instance the following letter from Whewell explains itself. The letter
to which it is a reply has not been preserved, but the reference to
Faraday’s objection to the word _current_ may be elucidated by a
comparison with what Faraday wrote in criticism of that word on pages
146 and 212.
[_Rev. W. Whewell to M. Faraday._]
TRIN. COLL., CAMBRIDGE, _Oct. 14, 1837_.
MY DEAR SIR,--I am always glad to hear of the progress of
your researches, and never the less so because they require
the fabrication of a new word or two. Such a coinage has
always taken place at the great epochs of discovery; like the
medals that are struck at the beginning of a new reign:--or
rather like the change of currency produced by the accession
of a new sovereign; for their value and influence consists in
their coming into common circulation. I am not sure that I
understand the views which you are at present bringing into
shape sufficiently well to suggest any such terms as you think
you want. I think that if I could have a quarter of an hour’s
talk with you I should probably be able to construct terms
that would record your new notions, so far as I could be made
to understand them better than I can by means of letters: for
it is difficult without question and discussion to catch the
precise kind of relation which you want to express. However,
by way of beginning such a discussion, I would ask you whether
you want abstract terms to denote the different and related
conditions of the body which exercises and the body which
suffers induction? For though both are active and both passive
it may still be convenient to suppose a certain ascendancy on
one side. If so would two such words as _inductricity_ and
_inducteity_ answer your purpose? They are not very monstrous
in their form; and are sufficiently distinct. And if you
want the corresponding adjectives you may call the one the
_inductric_, and the other the _inducteous_ body. This last
word is rather a startling one; but if such relations are to
be expressed, terminations are a good artifice, as we see in
chemistry: and I have no doubt if you give the world facts
and laws which are better expressed with than without such
solecisms, they will soon accommodate to the phrases, as they
have often done to worse ones. But I am rather in the dark
as to whether this is the kind of relation which you want to
indicate. If not, the attempt may perhaps serve to shew you
where my dulness lies. I do not see my way any better as to the
other terms, for I do not catch your objection to _current_,
which appears to me to be capable of jogging on very well
from _cathode_ to _anode_, or vice versa. As for positive and
negative, I do not see why _cathodic_ and _anodic_ should not
be used, if they will do the service you want of them.
I expect to be in London at the end of the month, and could
probably see you for half an hour on the 1st of November, say
at 10, 11, or 12. But in the mean time I shall be glad to hear
from you whether you can make anything of such conundrums as I
have mentioned, and am always yours very truly,
W. WHEWELL.
M. FARADAY Esq^{re.}
Royal Institution.
[Sidenote: LATERAL ACTIONS OF CURRENT.]
The concluding part of the thirteenth memoir, in which these new
terms are used, is an exceedingly striking speculation on the lateral
or transverse effects of the current. In calling special attention
to them, he says: “I refer of course to the magnetic action and its
relations; but though this is the only recognised lateral action of
the current, there is great reason for believing that others exist and
would by their discovery reward a close search for them.” He seems to
have had an instinctive perception of something that eluded his grasp.
Not until after Maxwell had given mathematical form to Faraday’s own
suggestions was this vision to be realised. He is dimly aware that
there appears to be a lateral tension or repulsion possessed by the
lines of electric inductive action; and onward runs his thought in free
speculation:--
When current or discharge occurs between two bodies, previously
under inductrical relations to each other, the lines of
inductive force will weaken and fade away, and, as their
lateral repulsive tension diminishes, will contract and
ultimately disappear in the line of discharge. May not this be
an effect identical with the attractions of similar currents?
_i.e._ may not the passage of static electricity into current
electricity, and that of the lateral tension of the lines of
the inductive force into the lateral attraction of lines of
similar discharge, have the same relation and dependences, and
run parallel to each other?
Series fourteen of the memoirs is on the nature of the electric
force and on the relation of the electric and magnetic forces, and
comprises an inconclusive inquiry as to a possible relation between
specific inductive capacity and axes of crystallisation in crystalline
dielectrics--a relation later assumed as true by Maxwell even before
it was demonstrated by Von Boltzmann. In this memoir, too, occurs a
description of a simple but effective induction balance. Then he asks
what happens to insulating substances, such as air or sulphur, when
they are put in a place where the magnetic forces are varying; they
ought, he thinks, to undergo some state or condition corresponding
to the state that causes currents in metals and conductors, and,
further, that state ought to be one of _tension_. “I have,” he says,
“by rotating non-conducting bodies near magnetic poles, and poles near
them, and also by causing powerful electric currents to be suddenly
formed and to cease around and about insulators in various directions,
endeavoured to make some such state sensible, but have not succeeded.”
In short, he was looking for direct evidence of the existence of what
Maxwell called “displacement currents”--evidence which was later found
independently by the author and by Röntgen. And, again, there rises in
his mind a perception of that _electrotonic state_ which had haunted
his earlier researches as a something imposed upon the surrounding
medium during the growth or dying of an electric current.
[Sidenote: INCESSANT ACTIVITIES.]
In these years (1835–1838) Faraday was still indefatigable in his
lecture duties. In 1835 he gave four Friday discourses, and in May
and June eight afternoon lectures at the Royal Institution on the
metals; also a course of fourteen lectures on electricity to the
medical students at St. George’s Hospital. In 1836 he published
in the _Philosophical Magazine_ a paper on the magnetism of the
metals--notable as containing the still unverified speculation that all
metals would become magnetic in the same way as iron if only cooled to
a sufficiently low temperature--and three other papers, including one
on the “passive” state of iron. He gave four Friday discourses and six
afternoon lectures on heat. In 1837 also four Friday night discourses
and six afternoon lectures were delivered. In 1838 three Friday
discourses and eight afternoon lectures on electricity, ending in June
with a distinct enunciation of the doctrine of the transformations of
“force” (_i.e._ energy) and its indestructibility, afforded evidence of
his industry in this respect. At the same time he was giving scientific
advice to the authorities of Trinity House as to their lighthouses.
The laboratory notebook for March to August, 1838, shows a long
research, occupying nearly 100 folio pages, on the relation of specific
inductive capacity to crystalline structure. This is followed by some
experiments upon an electric eel, at the Royal Adelaide Gallery,
with some unpublished sketches of the distribution in the water of
the currents it emits. He proved, with great satisfaction, that the
currents it gave were capable of producing magnetic effects, sparks,
and chemical decomposition. These observations were embodied in the
fifteenth series of memoirs.
One entry in the laboratory book, of date April 5th, 1838, is of great
interest, as showing how his mind ever recurred to the possibility of
finding a connection between optical and electric phenomena: “Must
try polarized light across a crystalline dielectric under charge. Good
reasons perhaps now evident why a non-crystalline dielectric should
have no effect.”
Faraday was now feeling greatly the strain of all these years of work,
and in 1839 did little research until the autumn. Then he returned to
the question of the origin of the electromotive force of the voltaic
cell, and by the end of the year completed two long papers on this
vexed question; they formed the sixteenth and seventeenth series, and
conclude the memoirs of this second period.
[Sidenote: THE CONTACT THEORY OF ELECTRICITY.]
In the eighth series, completed in April, 1834, on the “Electricity of
the Voltaic Pile,” Faraday had dealt with the question--at that time a
topic of excited controversy--of the origin of the electromotive force
in a cell, Volta, who knew nothing of the chemical actions, ascribed
it to the contact of dissimilar metals, whilst Wollaston, Becquerel,
and De la Rive considered it the result of chemical actions. The
controversy has long ceased to interest the scientific world; for,
with the recognition of the principle of the conservation of energy,
it became evident that mere contact cannot provide a continuing supply
of energy. It would now be altogether dead but for the survival of a
belief in the contact theory on the part of one of the most honoured
veterans in science. But in the years 1834 to 1840 it was of absorbing
interest. Faraday’s work quietly removed the props which supported the
older theory, and it crumbled away. He found that the chemical and
electrical effects in the cell were proportional one to the other, and
inseparable. He discovered a way of making a cell without any metallic
contacts. He showed that without chemical action there was no current
produced. But his results were ignored for the time. After six years
Faraday reopened the question. Again the admirable summary of Professor
Tyndall is drawn upon for the following account:--
The memoir on the “Electricity of the Voltaic Pile,” published
in 1834, appears to have produced but little impression upon
the supporters of the contact theory. These indeed were men
of too great intellectual weight and insight lightly to take
up, or lightly to abandon, a theory. Faraday therefore resumed
the attack in two papers communicated to the Royal Society on
February 6 and March 19, 1840. In these papers he hampered
his antagonists by a crowd of adverse experiments. He hung
difficulty after difficulty about the neck of the contact
theory, until in its efforts to escape from his assaults it so
changed its character as to become a thing totally different
from the theory proposed by Volta. The more persistently it was
defended, however, the more clearly did it show itself to be
a congeries of devices, bearing the stamp of dialectic skill
rather than that of natural truth.
In conclusion, Faraday brought to bear upon it an argument
which, had its full weight and purport been understood at
the time, would have instantly decided the controversy. “The
contact theory,” he urged, “assumes that a force which is able
to overcome powerful resistance, as for instance that of the
conductors, good or bad, through which the current passes,
and that again of the electrolytic action where bodies are
decomposed by it, _can arise out of nothing_; that without
any change in the acting matter, or the consumption of any
generating force, a current shall be produced which shall go
on for ever against a constant resistance, or only be stopped,
as in the voltaic trough, by the ruins which its exertion has
heaped up in its own course. This would indeed be _a creation
of power_, and is like no other force in nature. We have many
processes by which the _form_ of the power may be so changed,
that an apparent _conversion_ of one into the other takes
place. So we can change chemical force into the electric
current, or the current into chemical force. The beautiful
experiments of Seebeck and Peltier show the convertibility of
heat and electricity; and others by Oersted and myself show
the convertibility of electricity and magnetism. _But in no
case, not even in those of the gymnotus and torpedo, is there a
pure creation or a production of power without a corresponding
exhaustion of something to supply it._”
In 1839 Faraday gave five Friday discourses and a course of eight
afternoon lectures on the non-metallic elements. In 1840 he gave
three Friday discourses and seven lectures on chemical affinity. But
in the summer came the serious breakdown alluded to on page 75. He
did no experimental work after September 14th, nor indeed for nearly
two years. Even then it was only a temporary return to research to
investigate the source of the electrification produced by steam in the
remarkable experiments of Mr. (afterwards Lord) Armstrong. He proved it
to be due to friction. This done, he continued to rest from research
until the middle of 1844, though he lectured a little for the Royal
Institution. In 1841 he gave the juvenile lectures. In 1842 he gave
two Friday discourses, one of them being on the lateral discharge in
lightning-rods. He also gave the Christmas lectures on electricity.
[Sidenote: END OF SECOND ACTIVE PERIOD.]
In 1843 he gave three Friday discourses, one of which was on the
electricity generated by a jet of steam; and repeated the eight
afternoon lectures he had given in 1838. In 1844 he gave eight lectures
on heat and two Friday discourses. He also resumed research on the
condensation of gases, and vainly tried to liquefy oxygen and hydrogen,
though he succeeded with ammonia and nitrous oxide.
During these years of rest he also did a little work for Trinity House,
chiefly concerning lighthouses and their ventilation.