Chapter IV: Scientific Researches: Second Period (2)

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.

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.

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_.”

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.]