§ 18. _On Induction (continued)._ ¶ vii. _Conduction, or conductive
discharge._ ¶ viii. _Electrolytic discharge._ ¶ ix. _Disruptive
discharge--Insulation--Spark--Brush--Difference of discharge at the
positive and negative surfaces of conductors._
Received January 11,--Read February 8, 1838.
1318. I Proceed now, according to my promise, to examine, by the great
facts of electrical science, that theory of induction which I have ventured
to put forth (1165. 1295. &c.). The principle of induction is so universal
that it pervades all electrical phenomena; but the general case which I
purpose at present to go into consists of insulation traced into and
terminating with discharge, with the accompanying effects. This case
includes the various _modes_ of discharge, and also the condition and
characters of a current; the elements of magnetic action being amongst the
latter. I shall necessarily have occasion to speak theoretically, and even
hypothetically; and though these papers profess to be experimental
researches, I hope that, considering the facts and investigations contained
in the last series in support of the particular view advanced, I shall not
be considered as taking too much liberty on the present occasion, or as
departing too far from the character which they ought to have, especially
as I shall use every opportunity which presents itself of returning to that
strong test of truth, experiment.
1319. Induction has as yet been considered in these papers only in cases of
insulation; opposed to insulation is _discharge_. The action or effect
which may be expressed by the general term _discharge_, may take place, as
far as we are aware at present, in several modes. Thus, that which is
called simply _conduction_ involves no chemical action, and apparently no
displacement of the particles concerned. A second mode may be called
_electrolytic discharge_; in it chemical action does occur, and particles
must, to a certain degree, be displaced. A third mode, namely, that by
sparks or brushes, may, because of its violent displacement of the
particles of the _dielectric_ in its course, be called the _disruptive
discharge_; and a fourth may, perhaps, be conveniently distinguished for a
time by the words _convection_, or _carrying discharge_, being that in
which discharge is effected either by the carrying power of solid
particles, or those of gases and liquids. Hereafter, perhaps, all these
modes may appear as the result of one common principle, but at present they
require to be considered apart; and I will now speak of the _first_ mode,
for amongst all the forms of discharge, that which we express by the term
conduction appears the most simple and the most directly in contrast with
insulation.
¶ vii. _Conduction, or conductive discharge._
1320. Though assumed to be essentially different, yet neither Cavendish nor
Poisson attempt to explain by, or even state in, their theories, what the
essential difference between insulation and conduction is. Nor have I
anything, perhaps, to offer in this respect, _except_ that, according to my
view of induction, insulation and conduction depend upon the same molecular
action of the dielectrics concerned; are only extreme degrees of _one
common condition_ or effect; and in any sufficient mathematical theory of
electricity must be taken as cases of the same kind. Hence the importance
of the endeavour to show the connection between them under my theory of the
electrical relations of contiguous particles.
1321. Though the action of the insulating dielectric in the charged Leyden
jar, and that of the wire in discharging it, may seem very different, they
may be associated by numerous intermediate links, which carry us on from
one to the other, leaving, I think, no necessary connection unsupplied. We
may observe some of these in succession for information respecting the
whole case.
1322. Spermnceti has been examined and found to be a dielectric, through
which induction can take place (1240. 1246.), its specific inductive
capacity being about or above 1.8 (1279.), and the inductive action has
been considered in it, as in all other substances, an action of contiguous
particles.
1323. But spermaceti is also a _conductor_, though in so low a degree that
we can trace the process of conduction, as it were, step by step through
the mass (1247.); and even when the electric force has travelled through it
to a certain distance, we can, by removing the coercitive (which is at the
same time the inductive) force, cause it to return upon its path and
reappear in its first place (1245. 1246.). Here induction appears to be a
necessary preliminary to conduction. It of itself brings the contiguous
particles of the dielectric into a certain condition, which, if retained by
them, constitutes _insulation_, but if lowered by the communication of
power from one particle to another, constitutes _conduction_.
1324. If _glass_ or _shell-lac_ be the substances under consideration, the
same capabilities of suffering either induction or conduction through them
appear (1233. 1239. 1247.), but not in the same degree. The conduction
almost disappears (1239. 1242.); the induction therefore is sustained, i.e.
the polarized state into which the inductive force has brought the
contiguous particles is retained, there being little discharge action
between them, and therefore the _insulation_ continues. But, what discharge
there is, appears to be consequent upon that condition of the particles
into which the induction throws them; and thus it is that ordinary
insulation and conduction are closely associated together or rather are
extreme cases of one common condition.
1325. In ice or water we have a better conductor than spermaceti, and the
phenomena of induction and insulation therefore rapidly disappear, because
conduction quickly follows upon the assumption of the inductive state. But
let a plate of cold ice have metallic coatings on its sides, and connect
one of these with a good electrical machine in work, and the other with the
ground, and it then becomes easy to observe the phenomena of induction
through the ice, by the electrical tension which can be obtained and
continued on both the coatings (419. 426.). For although that portion of
power which at one moment gave the inductive condition to the particles is
at the next lowered by the consequent discharge due to the conductive act,
it is succeeded by another portion of force from the machine to restore the
inductive state. If the ice be converted into water the same succession of
actions can be just as easily proved, provided the water be distilled, and
(if the machine be not powerful enough) a voltaic battery be employed.
1326. All these considerations impress my mind strongly with the
conviction, that insulation and ordinary conduction cannot be properly
separated when we are examining into their nature; that is, into the
general law or laws under which their phenomena are produced. They appear
to me to consist in an action of contiguous particles dependent on the
forces developed in electrical excitement; these forces bring the particles
into a state of tension or polarity, which constitutes both _induction_ and
_insulation_; and being in this state, the continuous particles have a
power or capability of communicating their forces one to the other, by
which they are lowered, and discharge occurs. Every body appears to
discharge (444. 987.); but the possession of this capability in a _greater
or smaller degree_ in different bodies, makes them better or worse
conductors, worse or better insulators; and both _induction_ and
_conduction_ appear to be the same in their principle and action (1320.),
except that in the latter an effect common to both is raised to the highest
degree, whereas in the former it occurs in the best cases, in only an
almost insensible quantity.
1327. That in our attempts to penetrate into the nature of electrical
action, and to deduce laws more general than those we are at present
acquainted with, we should endeavour to bring apparently opposite effects
to stand side by side in harmonious arrangement, is an opinion of long
standing, and sanctioned by the ablest philosophers. I hope, therefore, I
may be excused the attempt to look at the highest cases of conduction as
analogous to, or even the same in kind with, those of induction and
insulation.
1328. If we consider the slight penetration of sulphur (1241. 1242.) or
shell-lac (1234.) by electricity, or the feebler insulation sustained by
spermaceti (1279. 1240.), as essential consequences and indications of
their _conducting_ power, then may we look on the resistance of metallic
wires to the passage of electricity through them as _insulating_ power. Of
the numerous well-known cases fitted to show this resistance in what are
called the perfect conductors, the experiments of Professor Wheatstone best
serve my present purpose, since they were carried to such an extent as to
show that _time_ entered as an element into the conditions of conduction[A]
even in metals. When discharge was made through a copper wire 2640 feet in
length, and 1/15th of an inch in diameter, so that the luminous sparks at
each end of the wire, and at the middle, could be observed in the same
place, the latter was found to be sensibly behind the two former in time,
they being by the conditions of the experiment simultaneous. Hence a proof
of retardation; and what reason can be given why this retardation should
not be of the same kind as that in spermaceti, or in lac, or sulphur? But
as, in them, retardation is insulation, and insulation is induction, why
should we refuse the same relation to the same exhibitions of force in the
metals?
[A] Philosophical Transactions, 1834, p. 583.
1329. We learn from the experiment, that if _time_ be allowed the
retardation is gradually overcome; and the same thing obtains for the
spermaceti, the lac, and glass (1248.); give but time in proportion to the
retardation, and the latter is at last vanquished. But if that be the case,
and all the results are alike in kind, the only difference being in the
length of time, why should we refuse to metals the previous inductive
action, which is admitted to occur in the other bodies? The diminution of
_time_ is no negation of the action; nor is the lower degree of tension
requisite to cause the forces to traverse the metal, as compared to that
necessary in the cases of water, spermaceti, or lac. These differences
would only point to the conclusion, that in metals the particles under
induction can transfer their forces when at a lower degree of tension or
polarity, and with greater facility than in the instances of the other
bodies.
1330. Let us look at Mr. Wheatstone's beautiful experiment in another point
of view, If, leaving the arrangement at the middle and two ends of the long
copper wire unaltered, we remove the two intervening portions and replace
them by wires of iron or platina, we shall have a much greater retardation
of the middle spark than before. If, removing the iron, we were to
substitute for it only five or six feet of water in a cylinder of the same
diameter as the metal, we should have still greater retardation. If from
water we passed to spermaceti, either directly or by gradual steps through
other bodies, (even though we might vastly enlarge the bulk, for the
purpose of evading the occurrence of a spark elsewhere (1331.) than at the
three proper intervals,) we should have still greater retardation, until at
last we might arrive, by degrees so small as to be inseparable from each
other, at actual and permanent insulation. What, then, is to separate the
principle of these two extremes, perfect conduction and perfect insulation,
from each other; since the moment we leave in the smallest degree
perfection at either extremity, we involve the element of perfection at the
opposite end? Especially too, as we have not in nature the case of
perfection either at one extremity or the other, either of insulation or
conduction.
1331. Again, to return to this beautiful experiment in the various forms
which may be given to it: the forces are not all in the wire (after they
have left the Leyden jar) during the whole time (1328.) occupied by the
discharge; they are disposed in part through the surrounding dielectric
under the well-known form of induction; and if that dielectric be air,
induction takes place from the wire through the air to surrounding
conductors, until the ends of the wire are electrically related through its
length, and discharge has occurred, i.e. for the _time_ during which the
middle spark is retarded beyond the others. This is well shown by the old
experiment, in which a long wire is so bent that two parts (Plate VIII.
fig. 115.), _a, b_, near its extremities shall approach within a short
distance, as a quarter of an inch, of each other in the air. If the
discharge of a Leyden jar, charged to a sufficient degree, be sent through
such a wire, by far the largest portion of the electricity will pass as a
spark across the air at the interval, and not by the metal. Does not the
middle part of the wire, therefore, act here as an insulating medium,
though it be of metal? and is not the spark through the air an indication
of the tension (simultaneous with _induction_) of the electricity in the
ends of this single wire? Why should not the wire and the air both be
regarded as dielectrics; and the action at its commencement, and whilst
there is tension, as an inductive action? If it acts through the contorted
lines of the wire, so it also does in curved and contorted lines through
air (1219, 1224, 1231.), and other insulating dielectrics (1228); and we
can apparently go so far in the analogy, whilst limiting the case to the
inductive action only, as to show that amongst insulating dielectrics some
lead away the lines of force from others (1229.), as the wire will do from
worse conductors, though in it the principal effect is no doubt due to the
ready discharge between the particles whilst in a low state of tension. The
retardation is for the time insulation; and it seems to me we may just as
fairly compare the air at the interval _a, b_ (fig. 115.) and the wire in
the circuit, as two bodies of the same kind and acting upon the same
principles, as far as the first inductive phenomena are concerned,
notwithstanding the different forms of discharge which ultimately
follow[A], as we may compare, according to Coulomb's investigations[B]
_different lengths_ of different insulating bodies required to produce the
same amount of insulating effect.
[A] These will be examined hereafter (1348. &c.).
[B] Mémoires de l'Académie, 1785, p. 612. or Ency. Britann. First
Supp. vol. i. p. 614.
1332. This comparison is still more striking when we take into
consideration the experiment of Mr. Harris, in which he stretched a fine
wire across a glass globe, the air within being rarefied[A]. On sending a
charge through the joint arrangement of metal and rare air, as much, if not
more, electricity passed by the latter as by the former. In the air,
rarefied as it was, there can be no doubt the discharge was preceded by
induction (1284.); and to my mind all the circumstances indicate that the
same was the case with the metal; that, in fact, both substances are
dielectrics, exhibiting the same effects in consequence of the action of
the same causes, the only variation being one of degree in the different
substances employed.
[A] Philosophical Transactions, 1834, p, 212.
1333. Judging on these principles, velocity of discharge through the _same
wire_ may be varied greatly by attending to the circumstances which cause
variations of discharge through spermaceti or sulphur. Thus, for instance,
it must vary with the tension or intensity of the first urging force (1234.
1240.), which tension is charge and induction. So if the two ends of the
wire, in Professor Wheatstone's experiment, 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 instant,
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 these two plates were the inner
and outer coating of a large jar or a Leyden battery, then the retardation
of that spark would be still greater.
1334. Cavendish was perhaps the first to show distinctly that discharge was
not always by one channel[A], but, if several are present, by many at once.
We may make these different channels of different bodies, and by
proportioning their thicknesses and lengths, may include such substances as
air, lac, spermaceti, water, protoxide of iron, iron and silver, and by
_one_ discharge make each convey its proportion of the electric force.
Perhaps the air ought to be excepted, as its discharge by conduction is
questionable at present (1336.); but the others may all be limited in their
mode of discharge to pure conduction. Yet several of them suffer previous
induction, precisely like the induction through the air, it being a
necessary preliminary to their discharging action. How can we therefore
separate any one of these bodies from the others, as to the _principles and
mode_ of insulating and conducting, except by mere degree? All seem to me
to be dielectrics acting alike, and under the same common laws.
[A] _Philosophical Transactions_, 1776, p. 197.
1335. I might draw another argument in favour of the general sameness, in
nature and action, of good and bad conductors (and all the bodies I refer
to are conductors more or less), from the perfect equipoise in action of
very different bodies when opposed to each other in magneto-electric
inductive action, as formerly described (213.), but am anxious to be as
brief as is consistent with the clear examination of the probable truth of
my views.
1336. With regard to the possession by the gases of any conducting power
of the simple kind now under consideration, the question is a very
difficult one to determine at present. Experiments seem to indicate that
they do insulate certain low degrees of tension perfectly, and that the
effects which may have appeared to be occasioned by _conduction_ have been
the result of the carrying power of the charged particles, either of the
air or of dust, in it. It is equally certain, however, that with higher
degrees of tension or charge the particles discharge to one another, and
that is conduction. If the gases possess the power of insulating a certain
low degree of tension continuously and perfectly, such a result may be due
to their peculiar physical state, and the condition of separation under
which their particles are placed. But in that, or in any case, we must not
forget the fine experiments of Cagniard de la Tour[A], in which he has
shown that liquids and their vapours can be made to pass gradually into
each other, to the entire removal of any marked distinction of the two
states. Thus, hot dry steam and cold water pass by insensible gradations
into each other; yet the one is amongst the gases as an insulator, and the
other a comparatively good conductor. As to conducting power, therefore,
the transition from metals even up to gases is gradual; substances make but
one series in this respect, and the various cases must come under one
condition and law (444.). The specific differences of bodies as to
conducting power only serves to strengthen the general argument, that
conduction, like insulation, is a result of induction, and is an action of
contiguous particles.
[A] Annales de Chimie, xxi. pp. 127, 178, or Quarterly Journal of
Science, xv. 145.
1337. I might go on now to consider induction and its concomitant,
_conduction_, through mixed dielectrics, as, for instance, when a charged
body, instead of acting across air to a distant uninsulated conductor, acts
jointly through it and an interposed insulated conductor. In such a case,
the air and the conducting body are the mixed dielectrics; and the latter
assumes a polarized condition as a mass, like that which my theory assumes
_each particle_ of the air to possess at the same time (1679). But I fear
to be tedious in the present condition of the subject, and hasten to the
consideration of other matter.
1338. To sum up, in some degree, what has been said, I look upon the first
effect of an excited body upon neighbouring matters to be the production of
a polarized state of their particles, which constitutes _induction_; and
this arises from its action upon the particles in immediate contact with
it, which again act upon those contiguous to them, and thus the forces are
transferred to a distance. If the induction remain undiminished, then
perfect insulation is the consequence; and the higher the polarized
condition which the particles can acquire or maintain, the higher is the
intensity which may be given to the acting forces. If, on the contrary, the
contiguous particles, upon acquiring the polarized state, have the power to
communicate their forces, then conduction occurs, and the tension is
lowered, conduction being a distinct act of discharge between neighbouring
particles. The lower the state of tension at which this discharge between
the particles of a body takes place, the better conductor is that body. In
this view, insulators may be said to be bodies whose particles can retain
the polarized state; whilst conductors are those whose particles cannot be
permanently polarized. If I be right in my view of induction, then I
consider the reduction of these two effects (which have been so long held
distinct) to an action of contiguous particles obedient to one common law,
as a very important result; and, on the other hand, the identity of
character which the two acquire when viewed by the theory (1326.), is
additional presumptive proof in favour of the correctness of the latter.
* * * * *
1339. That heat has great influence over simple conduction is well known
(445.), its effect being, in some cases, almost an entire change of the
characters of the body (432. 1340.). Harris has, however, shown that it in
no respect affects gaseous bodies, or at least air[A]; and Davy has taught
us that, as a class, metals have their conducting power _diminished_ by
it[B].
[A] _Philosophical Transactions_, 1834, p. 230
[B] Ibid. 1821, p. 431.
1340. I formerly described a substance, sulphuret of silver, whose
conducting power was increased by heat (433. 437. 438.); and I have since
then met with another as strongly affected in the same way: this is
fluoride of lead. When a piece of that substance, which had been fused and
cooled, was introduced into the circuit of a voltaic battery, it stopped
the current. Being heated, it acquired conducting powers before it was
visibly red-hot in daylight; and even sparks could be taken against it
whilst still solid. The current alone then raised its temperature (as in
the case of sulphuret of silver) until it fused, after which it seemed to
conduct as well as the metallic vessel containing it; for whether the wire
used to complete the circuit touched the fused fluoride only, or was in
contact with the platina on which it was supported, no sensible difference
in the force of the current was observed. During all the time there was
scarcely a trace of decomposing action of the fluoride, and what did occur,
seemed referable to the air and moisture of the atmosphere, and not to
electrolytic action.
1341. I have now very little doubt that periodide of mercury (414. 448.
691.) is a case of the same kind, and also corrosive sublimate (692.). I am
also inclined to think, since making the above experiments, that the
anomalous action of the protoxide of antimony, formerly observed and
described (693. 801.), may be referred in part to the same cause.
1342. I have no intention at present of going into the particular relation
of heat and electricity, but we may hope hereafter to discover by
experiment the law which probably holds together all the above effects with
those of the _evolution_ and the _disappearance_ of heat by the current,
and the striking and beautiful results of thermo-electricity, in one common
bond.
¶ viii. _Electrolytic discharge._
1343. I have already expressed in a former paper (1164.), the view by which
I hope to associate ordinary induction and electrolyzation. Under that
view, the discharge of electric forces by electrolyzation is rather an
effect superadded, in a certain class of bodies, to those already described
as constituting induction and insulation, than one independent of and
distinct from these phenomena.
1344. Electrolytes, as respects their insulating and conducting forces,
belong to the general category of bodies (1320. 1334.); and if they are in
the solid state (as nearly all can assume that state), they retain their
place, presenting then no new phenomenon (426. &c.); or if one occur, being
in so small a proportion as to be almost unimportant. When liquefied, they
also belong to the same list whilst the electric intensity is below a
certain degree; but at a given intensity (910. 912. 1007.), fixed for each,
and very low in all known cases, they play a new part, causing discharge in
proportion (783.) to the development of certain chemical effects of
combination and decomposition; and at this point, move out from the general
class of insulators and conductors, to form a distinct one by themselves.
The former phenomena have been considered (1320. 1338.); it is the latter
which have now to be revised, and used as a test of the proposed theory of
induction.
1345. The theory assumes, that the particles of the dielectric (now an
electrolyte) are in the first instance brought, by ordinary inductive
action, into a polarized state, and raised to a certain degree of tension
or intensity before discharge commences; the inductive state being, in
fact, a _necessary preliminary_ to discharge. By taking advantage of those
circumstances which bear upon the point, it is not difficult to increase
the tension indicative of this state of induction, and so make the state
itself more evident. Thus, if distilled water be employed, and a long
narrow portion of it placed between the electrodes of a powerful voltaic
battery, we have at once indications of the intensity which can be
sustained at these electrodes by the inductive action through the water as
a dielectric, for sparks may be obtained, gold leaves diverged, and Leyden
bottles charged at their wires. The water is in the condition of the
spermaceti (1322. 1323.) a bad conductor and a bad insulator; but what it
does insulate is by virtue of inductive action, and that induction is the
preparation for and precursor of discharge (1338.).
1346. The induction and tension which appear at the limits of the portion
of water in the direction of the current, are only the sums of the
induction and tension of the contiguous particles between those limits; and
the limitation of the inductive tension, to a certain degree shows (time
entering in each case as an important element of the result), that when the
particles have acquired a certain relative state, _discharge_, or a
transfer of forces equivalent to ordinary conduction, takes place.
1347. In the inductive condition assumed by water before discharge comes
on, the particles polarized are the particles of the _water_ that being the
dielectric used[A]; but the discharge between particle and particle is not,
as before, a mere interchange of their powers or forces at the polar parts,
but an actual separation of them into their two elementary particles, the
oxygen travelling in one direction, and carrying with it its amount of the
force it had acquired during the polarization, and the hydrogen doing the
same thing in the other direction, until they each meet the next
approaching particle, which is in the same electrical state with that they
have left, and by association of their forces with it, produce what
constitutes discharge. This part of the action may be regarded as a
carrying one (1319. 1572. 1622.), performed by the constituent particles of
the dielectric. The latter is always a compound body (664. 823.); and by
those who have considered the subject and are acquainted with the
philosophical view of transfer which was first put forth by Grotthuss[B],
its particles may easily be compared to a series of metallic conductors
under inductive action, which, whilst in that state, are divisible into
these elementary moveable halves.
[A] See 1699-1708.--_Dec. 1838_
[B] Annales de Chimie, lviii. 60. and lxiii, 20.
1348. Electrolytic discharge depends, of necessity, upon the non-conduction
of the dielectric as a whole, and there are two steps or acts in the
process: first a polarization of the molecules of the substance and then a
lowering of the forces by the separation, advance in opposite directions,
and recombination of the elements of the molecules, these being, as it
were, the halves of the originally polarized conductors or particles.
1349. These views of the decomposition of electrolytes and the consequent
effect of discharge, which, as to the particular case, are the same with
those of Grotthuss (481.) and Davy (482.), though they differ from those of
Biot (487.), De la Rive (490.), and others, seem to me to be fully in
accordance not merely with the theory I have given of induction generally
(1165.), but with all the known _facts_ of common induction, conduction,
and electrolytic discharge; and in that respect help to confirm in my mind
the truth of the theory set forth. The new mode of discharge which
electrolyzation presents must surely be an evidence of the _action of
contiguous particles_; and as this appears to depend directly upon a
previous inductive state, which is the same with common induction, it
greatly strengthens the argument which refers induction in all cases to an
action of contiguous particles also (1295, &c.).
1350. As an illustration of the condition of the polarized particles in a
dielectric under induction, I may describe an experiment. Put into a glass
vessel some clear rectified oil of turpentine, and introduce two wires
passing through glass tubes where they coincide with the surface of the
fluid, and terminating either in balls or points. Cut some very clean dry
white silk into small particles, and put these also into the liquid: then
electrify one of the wires by an ordinary machine and discharge by the
other. The silk will immediately gather from all parts of the liquid, and
form a band of particles reaching from wire to wire, and if touched by a
glass rod will show considerable tenacity; yet the moment the supply of
electricity ceases, the band will fall away and disappear by the dispersion
of its parts. The _conduction_ by the silk is in this case very small; and
after the best examination I could give to the effects, the impression on
my mind is, that the adhesion of the whole is due to the polarity which
each filament acquires, exactly as the particles of iron between the poles
of a horse-shoe magnet are held together in one mass by a similar
disposition of forces. The particles of silk therefore represent to me the
condition of the molecules of the dielectric itself, which I assume to be
polar, just as that of the silk is. In all cases of conductive discharge
the contiguous polarized particles of the body are able to effect a
neutralization of their forces with greater or less facility, as the silk
does also in a very slight degree. Further we are not able to carry the
parallel, except in imagination; but if we could divide each particle of
silk into two halves, and let each half travel until it met and united with
the next half in an opposite state, it would then exert its carrying power
(1347.), and so far represent electrolytic discharge.
1351. Admitting that electrolytic discharge is a consequence of previous
induction, then how evidently do its numerous cases point to induction in
curved lines (521. 1216.), and to the divergence or lateral action of the
lines of inductive force (1231.), and so strengthen that part of the
general argument in the former paper! If two balls of platina, forming the
electrodes of a voltaic battery, are put into a large vessel of dilute
sulphuric acid, the whole of the surfaces are covered with the respective
gases in beautifully regulated proportions, and the mind has no difficulty
in conceiving the direction of the curved lines of discharge, and even the
intensity of force of the different lines, by the quantity of gas evolved
upon the different parts of the surface. From this condition of the lines
of inductive force arise the general effects of diffusion; the appearance
of the anions or cathions round the edges and on the further side of the
electrodes when in the form of plates; and the manner in which the current
or discharge will follow all the forms of the electrolyte, however
contorted. Hence, also, the effects which Nobili has so well examined and
described[A] in his papers on the distribution of currents in conducting
masses. All these effects indicate the curved direction of the currents or
discharges which occur in and through the dielectrics, and these are in
every case _preceded_ by equivalent inductive actions of the contiguous
particles.
[A] Bibliothèque Universelle, 1835, lix. 263. 416.
1352. Hence also the advantage, when the exciting forces are weak or
require assistance, of enlarging the mass of the electrolyte; of increasing
the size of the electrodes; of making the coppers surround the zincs:--all
is in harmony with the view of induction which I am endeavouring to
examine; I do not perceive as yet one fact against it.
1353. There are many points of _electrolytic discharge_ which ultimately
will require to be very closely considered, though I can but slightly touch
upon them. It is not that, as far as I have investigated them, they present
any contradiction to the view taken (for I have carefully, though
unsuccessfully, sought for such cases), but simply want of time as yet to
pursue the inquiry, which prevents me from entering upon them here.
1354. One point is, that different electrolytes or dielectrics require
different initial intensities for their decomposition (912.). This may
depend upon the degree of polarization which the particles require before
electrolytic discharge commences. It is in direct relation to the chemical
affinity of the substances concerned; and will probably be found to have a
relation or analogy to the specific inductive capacity of different bodies
(1252. 1296.). It thus promises to assist in causing the great truths of
those extensive sciences, which are occupied in considering the forces of
the particles of matter, to fall into much closer order and arrangement
than they have heretofore presented.
1355. Another point is the facilitation of electrolytic conducting power or
discharge by the addition of substances to the dielectric employed. This
effect is strikingly shown where water is the body whose qualities are
improved, but, as yet, no general law governing all the phenomena has been
detected. Thus some acids, as the sulphuric, phosphoric, oxalic, and
nitric, increase the power of water enormously; whilst others, as the
tartaric and citric acids, give but little power; and others, again, as the
acetic and boracic acids, do not produce a change sensible to the
voltameter (739.). Ammonia produces no effect, but its carbonate does. The
caustic alkalies and their carbonates produce a fair effect. Sulphate of
soda, nitre (753.), and many soluble salts produce much effect. Percyanide
of mercury and corrosive sublimate produce no effect; nor does iodine, gum,
or sugar, the test being a voltameter. In many cases the added substance is
acted on either directly or indirectly, and then the phenomena are more
complicated; such substances are muriatic acid (758.), the soluble
protochlorides (766.), and iodides (769.), nitric acid (752.), &c. In other
cases the substance added is not, when alone, subject to or a conductor of
the powers of the voltaic battery, and yet both gives and receives power
when associated with water. M. de la Rive has pointed this result out in
sulphurous acid[A], iodine and bromine[B]; the chloride of arsenic produces
the same effect. A far more striking case, however, is presented by that
very influential body sulphuric acid (681.): and probably phosphoric acid
also is in the same peculiar relation.
[A] Quarterly Journal, xxvii. 407. or Bibliothèque Universelle, xl.
205. Kemp says sulphurous acid is a very good conductor, Quarterly
Journal, 1831, p. 613.
[B] Quarterly Journal, xxiv, 465. or Annales de Chimie, xxxv. 161.
1356. It would seem in the cases of those bodies which suffer no change
themselves, as sulphuric acid (and perhaps in all), that they affect water
in its conducting power only as an electrolyte; for whether little or much
improved, the decomposition is proportionate to the quantity of electricity
passing (727. 730.), and the transfer is therefore due to electrolytic
discharge. This is in accordance with the fact already stated as regards
water (984.), that the conducting power is not improved for electricity of
force below the electrolytic intensity of the substance acting as the
dielectric; but both facts (and some others) are against the opinion which
I formerly gave, that the power of salts, &c. might depend upon their
assumption of the liquid state by solution in the water employed (410.). It
occurs to me that the effect may perhaps be related to, and have its
explanation in differences of specific inductive capacities.
1357. I have described in the last paper, cases, where shell-lac was
rendered a conductor by absorption of ammonia (1294.). The same effect
happens with muriatic acid; yet both these substances, when gaseous, are
non-conductors; and the ammonia, also when in strong solution (718.). Mr.
Harris has mentioned instances[A] in which the conducting power of metals
is seriously altered by a very little alloy. These may have no relation to
the former cases, but nevertheless should not be overlooked in the general
investigation which the whole question requires.
[A] Philosophical Transactions, 1827, p. 22.
1358. Nothing is perhaps more striking in that class of dielectrics which
we call electrolytes, than the extraordinary and almost complete suspension
of their peculiar mode of effecting discharge when they are rendered
_solid_ (380, &c.), even though the intensity of the induction acting
through them may be increased a hundredfold or more (419.). It not only
establishes a very general relation between the physical properties of
these bodies and electricity acting by induction through them, but draws
both their physical and chemical relations so near together, as to make us
hope we shall shortly arrive at the full comprehension of the influence
they mutually possess over each other.
¶ ix. _Disruptive discharge and insulation._
1359. The next form of discharge has been distinguished by the adjective
_disruptive_ (1319.), as it in every case displaces more or less the
particles amongst and across which it suddenly breaks. I include under it,
discharge in the form of sparks, brushes, and glow (1405.), but exclude the
cases of currents of air, fluids, &c., which, though frequently
accompanying the former, are essentially distinct in their nature.
1360. The conditions requisite for the production of an electric spark in
its simplest form are well-known. An insulating dielectric must be
interposed between two conducting surfaces in opposite states of
electricity, and then if the actions be continually increased in strength,
or otherwise favoured, either by exalting the electric state of the two
conductors, or bringing them nearer to each other, or diminishing the
density of the dielectric, a _spark_ at last appears, and the two forces
are for the time annihilated, for _discharge_ has occurred.
1361. The conductors (which may be considered as the termini of the
inductive action) are in ordinary cases most generally metals, whilst the
dielectrics usually employed are common air and glass. In my view of
induction, however, every dielectric becomes of importance, for as the
results are considered essentially dependent on these bodies, it was to be
expected that differences of action never before suspected would be evident
upon close examination, and so at once give fresh confirmation of the
theory, and open new doors of discovery into the extensive and varied
fields of our science. This hope was especially entertained with respect to
the gases, because of their high degree of insulation, their uniformity in
physical condition, and great difference in chemical properties.
1362. All the effects prior to the discharge are inductive; and the degree
of tension which it is necessary to attain before the spark passes is
therefore, in the examination I am now making of the new view of induction,
a very important point. It is the limit of the influence which the
dielectric exerts in resisting discharge; it is a measure, consequently, of
the conservative power of the dielectric, which in its turn may be
considered as becoming a measure, and therefore a representative of the
intensity of the electric forces in activity.
1363. Many philosophers have examined the circumstances of this limiting
action in air, but, as far as I know, none have come near Mr. Harris as to
the accuracy with, and the extent to, which he has carried on his
investigations[A]. Some of his results I must very briefly notice,
premising that they are all obtained with the use of air as the
_dielectric_ between the conducting surfaces.
[A] Philosophical Transactions, 1834, p. 225.
1364. First as to the _distance_ between the two balls used, or in other
words, the _thickness_ of the dielectric across which the induction was
sustained. The quantity of electricity, measured by a unit jar, or
otherwise on the same principle with the unit jar, in the charged or
inductive ball, necessary to produce spark discharge, was found to vary
exactly with the distance between the balls, or between the discharging
points, and that under very varied and exact forms of experiment[A].
[A] Philosophical Transactions, 1834, p. 225.
1365. Then with respect to variation in the _pressure_ or _density_ of the
air. The quantities of electricity required to produce discharge across a
_constant_ interval varied exactly with variations of the density; the
quantity of electricity and density of the air being in the same simple
ratio. Or, if the quantity was retained the same, whilst the interval and
density of the air were varied, then these were found in the inverse simple
ratio of each other, the same quantity passing across twice the distance
with air rarefied to one-half[A].
[A] Philosophical Transactions, 1834, p.229.
1366. It must be remembered that these effects take place without any
variation of the _inductive_ force by condensation or rarefaction of the
air. That force remains the same in air[A], and in all gases (1284. 1292.),
whatever their rarefaction may be.
[A] Philosophical Transactions, 1834, p. 237, 244.
1367. Variation of the _temperature_ of the air produced no variation of
the quantity of electricity required to cause discharge across a given
interval[A].
[A] Philosophical Transactions, 1834, p. 230
Such are the general results, which I have occasion for at present,
obtained by Mr. Harris, and they appear to me to be unexceptionable.
1368. In the theory of induction founded upon a molecular action of the
dielectric, we have to look to the state of that body principally for the
cause and determination of the above effects. Whilst the induction
continues, it is assumed that the particles of the dielectric are in a
certain polarized state, the tension of this state rising higher in each
particle as the induction is raised to a higher degree, either by
approximation of the inducing surfaces, variation of form, increase of the
original force, or other means; until at last, the tension of the particles
having reached the utmost degree which they can sustain without subversion
of the whole arrangement, discharge immediately after takes place.
1369. The theory does not assume, however, that _all_ the particles of the
dielectric subject to the inductive action are affected to the same amount,
or acquire the same tension. What has been called the lateral action of the
lines of inductive force (1231. 1297.), and the diverging and occasionally
curved form of these lines, is against such a notion. The idea is, that any
section taken through the dielectric across the lines of inductive force,
and including _all of them,_ would be equal, in the sum of the forces, to
the sum of the forces in any other section; and that, therefore, the whole
amount of tension for each such section would be the same.
1370. Discharge probably occurs, not when all the particles have attained
to a certain degree of tension, but when that particle which is most
affected has been exalted to the subverting or turning point (1410.). For
though _all_ the particles in the line of induction resist charge, and are
associated in their actions so as to give a sum of resisting force, yet
when any one is brought up to the overturning point, _all_ must give way in
the case of a spark between ball and ball. The breaking down of that one
must of necessity cause the whole barrier to be overturned, for it was at
its utmost degree of resistance when it possessed the aiding power of that
one particle, in addition to the power of the rest, and the power of that
one is now lost. Hence _tension_ or _intensity_[A] may, according to the
theory, be considered as represented by the particular condition of the
particles, or the amount in them of forced variation from their normal
state (1298. 1368.).
[A] See Harris on proposed particular meaning of these terms,
Philosophical Transactions, 1834, p. 222.
1371. The whole effect produced by a charged conductor on a distant
conductor, insulated or not, is by my theory assumed to be due to an action
propagated from particle to particle of the intervening and insulating
dielectric, all the particles being considered as thrown for the time into
a forced condition, from which they endeavour to return to their normal or
natural state. The theory, therefore, seems to supply an easy explanation
of the influence of _distance_ in affecting induction (1303. 1364.). As the
distance is diminished induction increases; for there are then fewer
particles in the line of inductive force to oppose their united resistance
to the assumption of the forced or polarized state, and _vice versa._
Again, as the distance diminishes, discharge across happens with a lower
charge of electricity; for if, as in Harris's experiments (1364), the
interval be diminished to one-half, then half the electricity required to
discharge across the first interval is sufficient to strike across the
second; and it is evident, also, that at that time there are only half the
number of interposed molecules uniting their forces to resist the
discharge.
1372. The effect of enlarging the conducting surfaces which are opposed to
each other in the act of induction, is, if the electricity be limited in
its supply, to lower the intensity of action; and this follows as a very
natural consequence from the increased area of the dielectric across which
the induction is effected. For by diffusing the inductive action, which at
first was exerted through one square inch of sectional area of the
dielectric, over two or three square inches of such area, twice or three
times the number of molecules of the dielectric are brought into the
polarized condition, and employed in sustaining the inductive action, and
consequently the tension belonging to the smaller number on which the
limited force was originally accumulated, must fall in a proportionate
degree.
1373. For the same reason diminishing these opposing surfaces must increase
the intensity, and the effect will increase until the surfaces become
points. But in this case, the tension of the particles of the dielectric
next the points is higher than that of particles midway, because of the
lateral action and consequent bulging, as it were, of the lines of
inductive force at the middle distance (1369.).
1374. The more exalted effects of induction on a point _p_, or any small
surface, as the rounded end of a rod, when it is opposed to a large
surface, as that of a ball or plate, rather than to another point or end,
the distance being in both cases the same, fall into harmonious relation
with my theory (1302.). For in the latter case, the small surface _p_ is
affected only by those particles which are brought into the inductive
condition by the equally small surface of the opposed conductor, whereas
when that is a ball or plate the lines of inductive force from the latter
are concentrated, as it were, upon the end _p_. Now though the molecules of
the dielectric against the large surface may have a much lower state of
tension than those against the corresponding smaller surface, yet they are
also far more numerous, and, as the lines of inductive force converge
towards a point, are able to communicate to the particles contained in any
cross section (1369.) nearer the small surface an amount of tension equal
to their own, and consequently much higher for each individual particle; so
that, at the surface of the smaller conductor, the tension of a particle
rises much, and if that conductor were to terminate in a point, the tension
would rise to an infinite degree, except that it is limited, as before
(1368.), by discharge. The nature of the discharge from small surfaces and
points under induction will be resumed hereafter (1425. &c.)
1375. _Rarefaction_ of the air does not alter the _intensity_ of inductive
action (1284. 1287.); nor is there any reason, as far as I can perceive,
why it should. If the quantity of electricity and the distance remain the
same, and the air be rarefied one-half, then, though one-half of the
particles of the dielectric are removed, the other half assume a double
degree of tension in their polarity, and therefore the inductive forces are
balanced, and the result remains unaltered as long as the induction and
insulation are sustained. But the case of _discharge_ is very different;
for as there are only half the number of dielectric particles in the
rarefied atmosphere, so these are brought up to the discharging intensity
by half the former quantity of electricity; discharge, therefore, ensues,
and such a consequence of the theory is in perfect accordance with Mr.
Harris's results (1365.).
1376. The _increase_ of electricity required to cause discharge over the
same distance, when the pressure of the air or its density is increased,
flows in a similar manner, and on the same principle (1375.), from the
molecular theory.
1377. Here I think my view of induction has a decided advantage over
others, especially over that which refers the retention of electricity on
the surface of conductors in air to the _pressure of the atmosphere_
(1305.). The latter is the view which, being adopted by Poisson and
Biot[A], is also, I believe, that generally received; and it associates two
such dissimilar things, as the ponderous air and the subtile and even
hypothetical fluid or fluids of electricity, by gross mechanical relations;
by the bonds of mere static pressure. My theory, on the contrary, sets out
at once by connecting the electric forces with the particles of matter; it
derives all its proofs, and even its origin in the first instance, from
experiment; and then, without any further assumption, seems to offer at
once a full explanation of these and many other singular, peculiar, and, I
think, heretofore unconnected effects.
[A] Encyclopædia Britannica, Supplement, vol. iv. Article Electricity,
pp. 76, 81. &c.
1378. An important assisting experimental argument may here be adduced,
derived from the difference of specific inductive capacity of different
dielectrics (1269. 1274. 1278.). Consider an insulated sphere electrified
positively and placed in the centre of another and larger sphere
uninsulated, a uniform dielectric, as air, intervening. The case is really
that of my apparatus (1187.), and also, in effect, that of any ball
electrified in a room and removed to some distance from irregularly-formed
conductors. Whilst things remain in this state the electricity is
distributed (so to speak) uniformly over the surface of the electrified
sphere. But introduce such a dielectric as sulphur or lac, into the space
between the two conductors on one side only, or opposite one part of the
inner sphere, and immediately the electricity on the latter is diffused
unequally (1229. 1270. 1309.), although the form of the conducting
surfaces, their distances, and the _pressure_ of the atmosphere remain
perfectly unchanged.
1379. Fusinieri took a different view from that of Poisson, Biot, and
others, of the reason why rarefaction of air caused easy diffusion of
electricity. He considered the effect as due to the removal of the
_obstacle_ which the air presented to the expansion of the substances from
which the electricity passed[A]. But platina balls show the phenomena _in
vacuo_ as well as volatile metals and other substances; besides which, when
the rarefaction is very considerable, the electricity passes with scarcely
any resistance, and the production of no sensible heat; so that I think
Fusinieri's view of the matter is likely to gain but few assents.
[A] Bib. Univ. 1831, xlviii. 375.
1380. I have no need to remark upon the discharging or collecting power of
flame or hot air. I believe, with Harris, that the mere heat does nothing
(1367.), the rarefaction only being influential. The effect of rarefaction
has been already considered generally (1375.); and that caused by the heat
of a burning light, with the pointed form of the wick, and the carrying
power of the carbonaceous particles which for the time are associated with
it, are fully sufficient to account for all the effects.
1381. We have now arrived at the important question, how will the inductive
tension requisite for insulation and disruptive discharge be sustained in
gases, which, having the same physical state and also the _same
pressure_ and the _same temperature_ as _air_, differ from it in specific
gravity, in chemical qualities, and it may be in peculiar relations, which
not being as yet recognized, are purely electrical (1361.)?
1382. Into this question I can enter now only as far as is essential for
the present argument, namely, that insulation and inductive tension do not
depend merely upon the charged conductors employed, but also, and
essentially, upon the interposed dielectric, in consequence of the
molecular action of its particles (1292.).
1383. A glass vessel _a_ (fig. 127.)[A] was ground at the top and bottom so
as to be closed by two ground brass plates, _b_ and _c_; _b_ carried a
stuffing-box, with a sliding rod _d_ terminated by a brass ball _s_ below,
and a ring above. The lower plate was connected with a foot, stop-cock, and
socket, _e_, _f_ and _g_; and also with a brass ball _l_, which by means of
a stem attached to it and entering the socket _g_, could be fixed at
various heights. The metallic parts of this apparatus were not varnished,
but the glass was well-covered with a coat of shell-lac previously
dissolved in alcohol. On exhausting the vessel at the air-pump it could be
filled with any other gas than air, and, in such cases, the gas so passed
in was dried whilst entering by fused chloride of calcium.
[A] The drawing is to a scale of 1/6.
1384. The other part of the apparatus consisted of two insulating pillars,
_h_ and _i_, to which were fixed two brass balls, and through these passed
two sliding rods, _k_ and _m_, terminated at each end by brass balls; _n_
is the end of an insulated conductor, which could be rendered either
positive or negative from an electrical machine; _o_ and _p_ are wires
connecting it with the two parts previously described, and _q_ is a wire
which, connecting the two opposite sides of the collateral arrangements,
also communicates with a good discharging train _r_ (292.).
1385. It is evident that the discharge from the machine electricity may
pass either between _s_ and _l_, or S and L. The regulation adopted in the
first experiments was to keep _s_ and _l_ with their distance _unchanged_,
but to introduce first one gas and then another into the vessel _a_, and
then balance the discharge at the one place against that at the other; for
by making the interval at _a_ sufficiently small, all the discharge would
pass there, or making it sufficiently large it would all occur at the
interval _v_ in the receiver. On principle it seemed evident, that in this
way the varying interval _u_ might be taken as a measure, or rather
indication of the resistance to discharge through the gas at the constant
interval _v_. The following are the constant dimensions.
Ball _s_ 0.93 of an inch.
Ball S 0.96 of an inch.
Ball _l_ 2.02 of an inch.
Ball L 0.62 of an inch.
Interval _v_ 0.62 of an inch.
1386. On proceeding to experiment it was found that when air or any gas was
in the receiver _a_, the interval _u_ was not a fixed one; it might be
altered through a certain range of distance, and yet sparks pass either
there or at _v_ in the receiver. The extremes were therefore noted, i.e.
the greatest distance short of that at which the discharge _always_ took
place at _v_ in the gas, and the least distance short of that at which it
_always_ took place at _u_ in the air. Thus, with air in the receiver, the
extremes at _u_ were 0.56 and 0.79 of an inch, the range of 0.23 between
these distances including intervals at which sparks passed occasionally
either at one place or the other.
1387. The small balls _s_ and S could be rendered either positive or
negative from the machine, and as gases were expected and were found to
differ from each other in relation to this change (1399.), the results
obtained under these differences of charge were also noted.
1388. The following is a Table of results; the gas named is that in the
vessel _a_. The smallest, greatest, and mean interval at _u_ in air is
expressed in parts of an inch, the interval _v_ being constantly 0.62 of an
inch.
Smallest. Greatest. Mean.
_
| Air, _s_ and S, pos. 0.60 0.79 0.695
|_Air, _s_ and S, neg. 0.59 0.68 0.635
_
| Oxygen, _s_ and S, pos. 0.41 0.60 0.505
|_Oxygen, _s_ and S, neg. 0.50 0.52 0.510
_
| Nitrogen, _s_ and S, pos. 0.55 0.68 0.615
|_Nitrogen, _s_ and S, neg. 0.59 0.70 0.645
_
| Hydrogen, _s_ and S, pos. 0.30 0.44 0.370
|_Hydrogen, _s_ and S, neg. 0.25 0.30 0.275
_
| Carbonic acid, _s_ and S, pos. 0.56 0.72 0.640
|_Carbonic acid, _s_ and S, neg. 0.58 0.60 0.590
_
| Olefiant gas, _s_ and S, pos. 0.64 0.86 0.750
|_Olefiant gas, _s_ and S, neg. 0.69 0.77 0.730
_
| Coal gas, _s_ and S, pos. 0.37 0.61 0.490
|_Coal gas, _s_ and S, neg. 0.47 0.58 0.525
_
| Muriatic acid gas, _s_ and S, pos. 0.89 1.32 1.105
|_Muriatic acid gas, _s_ and S, neg. 0.67 0.75 0.710
1389. The above results were all obtained at one time. On other occasions
other experiments were made, which gave generally the same results as to
order, though not as to numbers. Thus:
Hydrogen, _s_ and S, pos. 0.23 0.57 0.400
Carbonic acid, _s_ and S, pos. 0.51 1.05 0.780
Olefiant gas, _s_ and S, pos. 0.66 1.27 0.965
I did not notice the difference of the barometer on the days of
experiment[A].
[A] Similar experiments in different gases are described at 1507.
1508.--_Dec. 1838._
1390. One would have expected only two distances, one for each interval,
for which the discharge might happen either at one or the other; and that
the least alteration of either would immediately cause one to predominate
constantly over the other. But that under common circumstances is not the
case. With air in the receiver, the variation amounted to 0.2 of an inch
nearly on the smaller interval of 0.6, and with muriatic acid gas, the
variation was above 0.4 on the smaller interval of 0.9. Why is it that when
a fixed interval (the one in the receiver) will pass a spark that cannot go
across 0.6 of air at one time, it will immediately after, and apparently
under exactly similar circumstances, not pass a spark that can go across
0.8 of air?
1391. It is probable that part of this variation will be traced to
particles of dust in the air drawn into and about the circuit (1568.). I
believe also that part depends upon a variable charged condition of the
surface of the glass vessel _a_. That the whole of the effect is not
traceable to the influence of circumstances in the vessel _a_, may be
deduced from the fact, that when sparks occur between balls in free air
they frequently are not straight, and often pass otherwise than by the
shortest distance. These variations in air itself, and at different parts
of the very same balls, show the presence and influence of circumstances
which are calculated to produce effects of the kind now under
consideration.
1392. When a spark had passed at either interval, then, generally, more
tended to appear at the _same_ interval, as if a preparation had been made
for the passing of the latter sparks. So also on continuing to work the
machine quickly the sparks generally followed at the same place. This
effect is probably due in part to the warmth of the air heated by the
preceding spark, in part to dust, and I suspect in part, to something
unperceived as yet in the circumstances of discharge.
1393. A very remarkable difference, which is _constant_ in its direction,
occurs when the electricity communicated to the balls _s_ and S is changed
from positive to negative, or in the contrary direction. It is that the
range of variation is always greater when the small bulls are positive than
when they are negative. This is exhibited in the following Table, drawn
from the former experiments.
Pos. Neg.
In Air the range was 0.19 0.09
Oxygen 0.19 0.02
Nitrogen 0.18 0.11
Hydrogen 0.14 0.05
Carbonic acid 0.16 0.02
Olefiant gas 0.22 0.08
Coal gas 0.24 0.12
Muriatic acid 0.43 0.08
I have no doubt these numbers require considerable correction, but the
general result is striking, and the differences in several cases very
great.
* * * * *