Chapter IV: Scientific Researches: Second Period (1)

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.

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.

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.”

[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.]

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.

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.

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.

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.

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.