§ 20. _Nature of the electric force or forces._ § 21. _Relation of the
electric and magnetic forces._ § 22. _Note on electrical excitation._
Received June 21, 1838.--Read June 21, 1838.
§ 20. _Nature of the electric force or forces._
1667. The theory of induction set forth and illustrated in the three
preceding series of experimental researches does not assume anything new as
to the nature of the electric force or forces, but only as to their
distribution. The effects may depend upon the association of one electric
fluid with the particles of matter, as in the theory of Franklin, Epinus,
Cavendish, and Mossotti; or they may depend upon the association of two
electric fluids, as in the theory of Dufay and Poisson; or they may not
depend upon anything which can properly be called the electric fluid, but
on vibrations or other affections of the matter in which they appear. The
theory is unaffected by such differences in the mode of viewing the nature
of the forces; and though it professes to perform the important office of
stating _how_ the powers are arranged (at least in inductive phenomena), it
does not, as far as I can yet perceive, supply a single experiment which
can be considered as a distinguishing test of the truth of any one of these
various views,
1668. But, to ascertain how the forces are arranged, to trace them in their
various relations to the particles of matter, to determine their general
laws, and also the specific differences which occur under these laws, is as
important as, if not more so than, to know whether the forces reside in a
fluid or not; and with the hope of assisting in this research, I shall
offer some further developments, theoretical and experimental, of the
conditions under which I suppose the particles of matter are placed when
exhibiting inductive phenomena.
1669. The theory assumes that all the _particles_, whether of insulating or
conducting matter, are as wholes conductors.
1670. That not being polar in their normal state, they can become so by the
influence of neighbouring charged particles, the polar state being
developed at the instant, exactly as in an insulated conducting _mass_
consisting of many particles.
1671. That the particles when polarized are in a forced state, and tend to
return to their normal or natural condition.
1672. That being as wholes conductors, they can readily be charged, either
_bodily_ or _polarly_.
1673. That particles which being contiguous[A] are also in the line of
inductive action can communicate or transfer their polar forces one to
another _more_ or _less_ readily.
[A] See note to 1164.--_Dec. 1838._
1674. That those doing so less readily require the polar forces to be
raised to a higher degree before this transference or communication takes
place.
1675. That the _ready_ communication of forces between contiguous particles
constitutes _conduction_, and the _difficult_ communication _insulation_;
conductors and insulators being bodies whose particles naturally possess
the property of communicating their respective forces easily or with
difficulty; having these differences just as they have differences of any
other natural property.
1676. That ordinary induction is the effect resulting from the action of
matter charged with excited or free electricity upon insulating matter,
tending to produce in it an equal amount of the contrary state.
1677. That it can do this only by polarizing the particles contiguous to
it, which perform the same office to the next, and these again to those
beyond; and that thus the action is propagated from the excited body to the
next conducting mass, and there renders the contrary force evident in
consequence of the effect of communication which supervenes in the
conducting mass upon the polarization of the particles of that body
(1675.).
1678. That therefore induction can only take place through or across
insulators; that induction is insulation, it being the necessary
consequence of the state of the particles and the mode in which the
influence of electrical forces is transferred or transmitted through or
across such insulating media.
1679. The particles of an insulating dielectric whilst under induction may
be compared to a series of small magnetic needles, or more correctly still
to a series of small insulated conductors. If the space round a charged
globe were filled with a mixture of an insulating dielectric, as oil of
turpentine or air, and small globular conductors, as shot, the latter being
at a little distance from each other so as to be insulated, then these
would in their condition and action exactly resemble what I consider to be
the condition and action of the particles of the insulating dielectric
itself (1337.). If the globe were charged, these little conductors would
all be polar; if the globe were discharged, they would all return to their
normal state, to be polarized again upon the recharging of the globe. The
state developed by induction through such particles on a mass of conducting
mutter at a distance would be of the contrary kind, and exactly equal in
amount to the force in the inductric globe. There would be a lateral
diffusion of force (1224. 1297.), because each polarized sphere would be in
an active or tense relation to all those contiguous to it, just as one
magnet can affect two or more magnetic needles near it, and these again a
still greater number beyond them. Hence would result the production of
curved lines of inductive force if the inducteous body in such a mixed
dielectric were an uninsulated metallic ball (1219. &c.) or other properly
shaped mass. Such curved lines are the consequences of the two electric
forces arranged as I have assumed them to be: and, that the inductive force
can be directed in such curved lines is the strongest proof of the presence
of the two powers and the polar condition of the dielectric particles.
1680. I think it is evident, that in the case stated, action at a distance
can only result through an action of the contiguous conducting particles.
There is no reason why the inductive body should polarize or affect
_distant_ conductors and leave those _near_ it, namely the particles of the
dielectric, unaffected: and everything in the form of fact and experiment
with conducting masses or particles of a sensible size contradicts such a
supposition.
1681. A striking character of the electric power is that it is limited and
exclusive, and that the two forces being always present are exactly equal
in amount. The forces are related in one of two ways, either as in the
natural normal condition of an uncharged insulated conductor; or as in the
charged state, the latter being a case of induction.
1682. Cases of induction are easily arranged so that the two forces being
limited in their direction shall present no phenomena or indications
external to the apparatus employed, Thus, if a Leyden jar, having its
external coating a little higher than the internal, be charged and then its
charging ball and rod removed, such jar will present no electrical
appearances so long as its outside is uninsulated. The two forces which may
be said to be in the coatings, or in the particles of the dielectric
contiguous to them, are entirely engaged to each other by induction through
the glass; and a carrier ball (1181.) applied either to the inside or
outside of the jar will show no signs of electricity. But if the jar be
insulated, and the charging ball and rod, in an uncharged state and
suspended by an insulating thread of white silk, be restored to their
place, then the part projecting above the jar will give electrical
indications and charge the carrier, and at the same time the _outside_
coating of the jar will be found in the opposite state and inductric
towards external surrounding objects.
1683. These are simple consequences of the theory. Whilst the charge of the
inner coating could induce only through the glass towards the outer
coating, and the latter contained no more of the contrary force than was
equivalent to it, no induction external to the jar could be perceived; but
when the inner coating was extended by the rod and ball so that it could
induce through the air towards external objects, then the tension of the
polarized glass molecules would, by their tendency to return to the normal
state, fall a little, and a portion of the charge passing to the surface of
this new part of the inner conductor, would produce inductive action
through the air towards distant objects, whilst at the same time a part of
the force in the outer coating previously directed inwards would now be at
liberty, and indeed be constrained to induct outwards through the air,
producing in that outer coating what is sometimes called, though I think
very improperly, free charge. If a small Leyden jar be converted into that
form of apparatus usually known by the name of the electric well, it will
illustrate this action very completely.
1684. The terms _free charge_ and _dissimulated electricity_ convey
therefore erroneous notions if they are meant to imply any difference as to
the mode or kind of action. The charge upon an insulated conductor in the
middle of a room is in the same relation to the walls of that room as the
charge upon the inner coating of a Leyden jar is to the outer coating of
the same jar. The one is not more _free_ or more _dissimulated_ than the
other; and when sometimes we make electricity appear where it was not
evident before, as upon the outside of a charged jar, when, after
insulating it, we touch the inner coating, it is only because we divert
more or less of the inductive force from one direction into another; for
not the slightest change is in such circumstances impressed upon the
character or action of the force.
* * * * *
1685. Having given this general theoretical view, I will now notice
particular points relating to the nature of the assumed electric polarity
of the insulating dielectric particles.
1686. The polar state may be considered in common induction as a forced
state, the particles tending to return to their normal condition. It may
probably be raised to a very high degree by approximation of the inductric
and inducteous bodies or by other circumstances; and the phenomena of
electrolyzation (861. 1652. 1796.) seem to imply that the quantity of power
which can thus be accumulated on a single particle is enormous. Hereafter
we may be able to compare corpuscular forces, as those of gravity,
cohesion, electricity, and chemical affinity, and in some way or other from
their effects deduce their relative equivalents; at present we are not able
to do so, but there seems no reason to doubt that their electrical, which
are at the same time their chemical forces (891. 918.), will be by far the
most energetic.
1687. I do not consider the powers when developed by the polarization as
limited to two distinct points or spots on the surface of each particle to
be considered as the poles of an axis, but as resident on large portions of
that surface, as they are upon the surface of a conductor of sensible size
when it is thrown into a polar state. But it is very probable,
notwithstanding, that the particles of different bodies may present
specific differences in this respect, the powers not being equally diffused
though equal in quantity; other circumstances also, as form and quality,
giving to each a peculiar polar relation. It is perhaps to the existence of
some such differences as these that we may attribute the specific actions
of the different dielectrics in relation to discharge(1394. 1508.). Thus
with respect to oxygen and nitrogen singular contrasts were presented when
spark and brush discharge were made to take place in these gases, as may be
seen by reference to the Table in paragraph 1518 of the Thirteenth Series;
for with nitrogen, when the small, negative or the large positive ball was
rendered inductric, the effects corresponded with those which in oxygen
were produced when the small positive or the large negative ball was
rendered inductric.
1688. In such solid bodies as glass, lac, sulphur, &c., the particles
appear to be able to become polarized in all directions, for a mass when
experimented upon so as to ascertain its inductive capacity in three or
more directions (1690.), gives no indication of a difference. Now as the
particles are fixed in the mass, and as the direction of the induction
through them must change with its change relative to the mass, the constant
effect indicates that they can be polarized electrically in any direction.
This accords with the view already taken of each particle as a whole being
a conductor (1669.), and, as an experimental fact, helps to confirm that
view.
1689. But though particles may thus be polarized in _any_ direction under
the influence of powers which are probably of extreme energy (1686.), it
does not follow that each particle may not tend to polarize to a greater
degree, or with more facility, in one direction than another; or that
different kinds may not have specific differences in this respect, as they
have differences of conducting and other powers (1296. 1326. 1395.). I
sought with great anxiety for a relation of this nature; and selecting
crystalline bodies as those in which all the particles are symmetrically
placed, and therefore best fitted to indicate any result which might depend
upon variation of the direction of the forces to the direction of the
particles in which they were developed, experimented very carefully with
them. I was the more strongly stimulated to this inquiry by the beautiful
electrical condition of the crystalline bodies tourmaline and boracite, and
hoped also to discover a relation between electric polarity and that of
crystallization, or even of cohesion itself (1316.). My experiments have
not established any connexion of the kind sought for. But as I think it of
equal importance to show either that there is or is not such a relation, I
shall briefly describe the results.
1690. The form of experiment was as follows. A brass ball 0.73 of an inch
in diameter, fixed at the end of a horizontal brass rod, and that at the
end of a brass cylinder, was by means of the latter connected with a large
Leyden battery (291.) by perfect metallic communications, the object being
to keep that ball, by its connexion with the charged battery in an
electrified state, very nearly uniform, for half an hour at a time. This
was the inductric ball. The inducteous ball was the carrier of the torsion
electrometer (1229. 1314.); and the dielectric between them was a cube cut
from a crystal, so that two of its faces should be perpendicular to the
optical axis, whilst the other four were parallel to it. A small projecting
piece of shell-lac was fixed on the inductric ball at that part opposite to
the attachment of the brass rod, for the purpose of preventing actual
contact between the ball and the crystal cube. A coat of shell-lac was also
attached to that side of the carrier ball which was to be towards the cube,
being also that side which was furthest from the repelled ball in the
electrometer when placed in its position in that instrument. The cube was
covered with a thin coat of shell-lac dissolved in alcohol, to prevent the
deposition of damp upon its surface from the air. It was supported upon a
small table of shell-lac fixed on the top of a stem of the same substance,
the latter being of sufficient strength to sustain the cube, and yet
flexible enough from its length to act as a spring, and allow the cube to
bear, when in its place, against the shell-lac on the inductric ball.
[Illustration:]
1691. Thus it was easy to bring the inducteous ball always to the same
distance from the inductric bull, and to uninsulate and insulate it again
in its place; and then, after measuring the force in the electrometer
(1181.), to return it to its place opposite to the inductric ball for a
second observation. Or it was easy by revolving the stand which supported
the cube to bring four of its faces in succession towards the inductric
ball, and so observe the force when the lines of inductive action (1304.)
coincided with, or were transverse to, the direction of the optical axis of
the crystal. Generally from twenty to twenty-eight observations were made
in succession upon the four vertical faces of a cube, and then an average
expression of the inductive force was obtained, and compared with similar
averages obtained at other times, every precaution being taken to secure
accurate results.
1692. The first cube used was of _rock crystal_; it was 0.7 of an inch in
the side. It presented a remarkable and constant difference, the average of
not less than 197 observations, giving 100 for the specific inductive
capacity in the direction coinciding with the optical axis of the cube,
whilst 93.59 and 93.31 were the expressions for the two transverse
directions.
1693. But with a second cube of rock crystal corresponding results were not
obtained. It was 0.77 of an inch in the side. The average of many
experiments gave 100 for the specific inductive capacity coinciding with
the direction of the optical axis, and 98.6 and 99.92 for the two other
directions.
1694. Lord Ashley, whom I have found ever ready to advance the cause of
science, obtained for me the loan of three globes of rock crystal belonging
to Her Grace the Duchess of Sutherland for the purposes of this
investigation. Two had such fissures as to render them unfit for the
experiments (1193. 1698.). The third, which was very superior, gave me no
indications of any difference in the inductive force for different
directions.
1695. I then used cubes of Iceland spar. One 0.5 of an inch in diameter
gave 100 for the axial direction, and 98.66 and 95.74 for the two cross
directions. The other, 0.8 of an inch in the side, gave 100 for the axial
direction, whilst 101.73 and 101.86 were the numbers for the cross
direction.
1696. Besides these differences there were others, which I do not think it
needful to state, since the main point is not confirmed. For though the
experiments with the first cube raised great expectation, they have not
been generalized by those which followed. I have no doubt of the results as
to that cube, but they cannot as yet be referred to crystallization. There
are in the cube some faintly coloured layers parallel to the optical axis,
and the matter which colours them may have an influence; but then the
layers are also nearly parallel to a cross direction, and if at all
influential should show some effect in that direction also, which they did
not.
1697. In some of the experiments one half or one part of a cube showed a
superiority to another part, and this I could not trace to any charge the
different parts had received. It was found that the varnishing of the cubes
prevented any communication of charge to them, except (in a few
experiments) a small degree of the negative state, or that which was
contrary to the state of the inductric ball (1564. 1566.).
1698. I think it right to say that, as far as I could perceive, the
insulating character of the cubes used was perfect, or at least so nearly
perfect, as to bear a comparison with shell-lac, glass, &c. (1255). As to
the cause of the differences, other than regular crystalline structure,
there may be several. Thus minute fissures in the crystal insensible to the
eye may be so disposed as to produce a sensible electrical difference
(1193.). Or the crystallization may be irregular; or the substance may not
be quite pure; and if we consider how minute a quantity of matter will
alter greatly the conducting power of water, it will seem not unlikely that
a little extraneous matter diffused through the whole or part of a cube,
may produce effects sufficient to account for all the irregularities of
action that have been observed.
1699. An important inquiry regarding the electrical polarity of the
particles of an insulating dielectric, is, whether it be the molecules of
the particular substance acted on, or the component or ultimate particles,
which thus act the part of insulated conducting polarizing portions
(1669.).
1700. The conclusion I have arrived at is, that it is the molecules of the
substance which polarize as wholes (1347.); and that however complicated
the composition of a body may be, all those particles or atoms which are
held together by chemical affinity to form one molecule of the resulting
body act as one conducting mass or particle when inductive phenomena and
polarization are produced in the substance of which it is a part.
1701. This conclusion is founded on several considerations. Thus if we
observe the insulating and conducting power of elements when they are used
as dielectrics, we find some, as sulphur, phosphorus, chlorine, iodine,
&c., whose particles insulate, and therefore polarize in a high degree;
whereas others, as the metals, give scarcely any indication of possessing a
sensible proportion of this power (1328.), their particles freely
conducting one to another. Yet when these enter into combination they form
substances having no direct relation apparently, in this respect, to their
elements; for water, sulphuric acid, and such compounds formed of
insulating elements, conduct by comparison freely; whilst oxide of lead,
flint glass, borate of lead, and other metallic compounds containing very
high proportions of conducting matter, insulate excellently well. Taking
oxide of lead therefore as the illustration, I conceive that it is not the
particles of oxygen and lead which polarize separately under the act of
induction, but the molecules of oxide of lead which exhibit this effect,
all the elements of one particle of the resulting body, being held together
as parts of one conducting individual by the bonds of chemical affinity;
which is but another term for electrical force (918.).
1702. In bodies which are electrolytes we have still further reason for
believing in such a state of things. Thus when water, chloride of tin,
iodide of lead, &c. in the solid state are between the electrodes of the
voltaic battery, their particles polarize as those of any other insulating
dielectric do (1164.); but when the liquid state is conferred on these
substances, the polarized particles divide, the two halves, each in a
highly charged state, travelling onwards until they meet other particles in
an opposite and equally charged state, with which they combine, to the
neutralization of their chemical, i.e. their electrical forces, and the
reproduction of compound particles, which can again polarize as wholes, and
again divide to repeat the same series of actions (1347.).
1703. But though electrolytic particles polarize as wholes, it would appear
very evident that in them it is not a matter of entire indifference _how_
the particle polarizes (1689.), since, when free to move (380, &c.) the
polarities are ultimately distributed in reference to the elements; and
sums of force equivalent to the polarities, and very definite in kind and
amount, separate, as it were, from each other, and travel onwards with the
elementary particles. And though I do not pretend to know what an atom is,
or how it is associated or endowed with electrical force, or how this force
is arranged in the cases of combination and decomposition, yet the strong
belief I have in the electrical polarity of particles when under inductive
action, and the hearing of such an opinion on the general effects of
induction, whether ordinary or electrolytic, will be my excuse, I trust,
for a few hypothetical considerations.
1704 In electrolyzation it appears that the polarized particles would
(because of the gradual change which has been induced upon the chemical,
i.e. the electrical forces of their elements (918.)) rather divide than
discharge to each other without division (1348.); for if their division,
i.e. their decomposition and recombination, be prevented by giving them the
solid state, then they will insulate electricity perhaps a hundredfold more
intense than that necessary for their electrolyzation (419, &c.). Hence the
tension necessary for direct conduction in such bodies appears to be much
higher than that for decomposition (419. 1164. 1344.).
1705. The remarkable stoppage of electrolytic conduction by solidification
(380. 1358.), is quite consistent with these views of the dependence of
that process on the polarity which is common to all insulating matter when
under induction, though attended by such peculiar electro-chemical results
in the case of electrolytes. Thus it may be expected that the first effect
of induction is so to polarize and arrange the particles of water that the
positive or hydrogen pole of each shall be from the positive electrode and
towards the negative electrode, whilst the negative or oxygen pole of each
shall be in the contrary direction; and thus when the oxygen and hydrogen
of a particle of water have separated, passing to and combining with other
hydrogen and oxygen particles, unless these new particles of water could
turn round they could not take up that position necessary for their
successful electrolytic polarization. Now solidification, by fixing the
water particles and preventing them from assuming that essential
preliminary position, prevents also their electrolysis (413.); and so the
transfer of forces in that manner being prevented (1347. 1703.), the
substance acts as an ordinary insulating dielectric (for it is evident by
former experiments (419. 1704.) that the insulating tension is higher than
the electrolytic tension), induction through it rises to a higher degree,
and the polar condition of the molecules as wholes, though greatly exalted,
is still securely maintained.
1706. When decomposition happens in a fluid electrolyte, I do not suppose
that all the molecules in the same sectional plane (1634.) part with and
transfer their electrified particles or elements at once. Probably the
_discharge force_ for that plane is summed up on one or a few particles,
which decomposing, travelling and recombining, restore the balance of
forces, much as in the case of spark disruptive discharge (1406.); for as
those molecules resulting from particles which have just transferred power
must by their position (1705.) be less favourably circumstanced than
others, so there must be some which are most favourably disposed, and
these, by giving way first, will for the time lower the tension and produce
discharge.
1707. In former investigations of the action of electricity (821, &c.) it
was shown, from many satisfactory cases, that the quantity of electric
power transferred onwards was in proportion to and was definite for a given
quantity of matter moving as anion or cathion onwards in the electrolytic
line of action; and there was strong reason to believe that each of the
particles of matter then dealt with, had associated with it a definite
amount of electrical force, constituting its force of chemical affinity,
the chemical equivalents and the electro-chemical equivalents being the
same (836.). It was also found with few, and I may now perhaps say with no
exceptions (1341.), that only those compounds containing elements in single
proportions could exhibit the characters and phenomena of electrolytes
(697.); oxides, chlorides, and other bodies containing more than one
proportion of the electro-negative element refusing to decompose under the
influence of the electric current.
1708. Probable reasons for these conditions and limitations arise out of
the molecular theory of induction. Thus when a liquid dielectric, as
chloride of tin, consists of molecules, each composed of a single particle
of each of the elements, then as these can convey equivalent opposite
forces by their separation in opposite directions, both decomposition and
transfer can result. But when the molecules, as in the bichloride of tin,
consist of one particle or atom of one element, and two of the other, then
the simplicity with which the particles may be supposed to be arranged and
to act, is destroyed. And, though it may be conceived that when the
molecules of bichloride of tin are polarized as wholes by the induction
across them, the positive polar force might accumulate on the one particle
of tin whilst the negative polar force accumulated on the two particles of
chlorine associated with it, and that these might respectively travel right
and left to unite with other two of chlorine and one of tin, in analogy
with what happens in cases of compounds consisting of single proportions,
yet this is not altogether so evident or probable. For when a particle of
tin combines with two of chlorine, it is difficult to conceive that there
should not be some relation of the three in the resulting molecule
analogous to fixed position, the one particle of metal being perhaps
symmetrically placed in relation to the two of chlorine: and, it is not
difficult to conceive of such particles that they could not assume that
position dependent both on their polarity and the relation of their
elements, which appears to be the first step in the process of
electrolyzation (1345. 1705.).
§ 21. _Relation of the electric and magnetic forces._
1709. I have already ventured a few speculations respecting the probable
relation of magnetism, as the transverse force of the current, to the
divergent or transverse force of the lines of inductive action belonging to
static electricity (1658, &c.).
1710. In the further consideration of this subject it appeared to me to be
of the utmost importance to ascertain, if possible, whether this lateral
action which we call magnetism, or sometimes the induction of electrical
currents (26. 1048, &c.), is extended to a distance _by the action of the
intermediate particles_ in analogy with the induction of static
electricity, or the various effects, such as conduction, discharge, &c.,
which are dependent on that induction; or, whether its influence at a
distance is altogether independent of such intermediate particles (1662.).
1711. I arranged two magneto-electric helices with iron cores end to end,
but with an interval of an inch and three quarters between them, in which
interval was placed the end or pole of a bar magnet. It is evident, that on
moving the magnetic pole from one core towards the other, a current would
tend to form in both helices, in the one because of the lowering, and in
the other because of the strengthening of the magnetism induced in the
respective soft iron cores. The helices were connected together, and also
with a galvanometer, so that these two currents should coincide in
direction, and tend by their joint force to deflect the needle of the
instrument. The whole arrangement was so effective and delicate, that
moving the magnetic pole about the eighth of an inch to and fro two or
three times, in periods equal to those required for the vibrations of the
galvanometer needle, was sufficient to cause considerable vibration in the
latter; thus showing readily the consequence of strengthening the influence
of the magnet on the one core and helix, and diminishing it on the other.
1712. Then without disturbing the distances of the magnet and cores, plates
of substances were interposed. Thus calling the two cores A and B, a plate
of shell-lac was introduced between the magnetic pole and A for the time
occupied by the needle in swinging one way; then it was withdrawn for the
time occupied in the return swing; introduced again for another equal
portion of time; withdrawn for another portion, and so on eight or nine
times; but not the least effect was observed on the needle. In other cases
the plate was alternated, i.e. it was introduced between the magnet and A
for one period of time, withdrawn and introduced between the magnet and B
for the second period, withdrawn and restored to its first place for the
third period, and so on, but with no effect on the needle.
1713. In these experiments _shell-lac_ in plates 0.9 of an inch in
thickness, _sulphur_ in a plate 0.9 of an inch in thickness, and _copper_
in a plate 0.7 of an inch in thickness were used without any effect. And I
conclude that bodies, contrasted by the extremes of conducting and
insulating power, and opposed to each other as strongly as metals, air, and
sulphur, show no difference with respect to magnetic forces when placed in
their lines of action, at least under the circumstances described.
1714. With a plate of iron, or even a small piece of that metal, as the
head of a nail, a very different effect was produced, for then the
galvanometer immediately showed its sensibility, and the perfection of the
general arrangement.
1715. I arranged matters so that a plate of _copper_ 0.2 of an inch in
thickness, and ten inches in diameter, should have the part near the edge
interposed between the magnet and the core, in which situation it was first
rotated rapidly, and then held quiescent alternately, for periods according
with that required for the swinging of the needle; but not the least effect
upon the galvanometer was produced.
1716. A plate of shell-lac 0.6 of an inch in thickness was applied in the
same manner, but whether rotating or not it produced no effect.
1717. Occasionally the plane of rotation was directly across the magnetic
curve: at other times it was made as oblique as possible; the direction of
the rotation being also changed in different experiments, but not the least
effect was produced.
1718. I now removed the helices with their soft iron cores, and replaced
them by two _flat helices_ wound upon card board, each containing forty-two
feet of silked copper wire, and having no associated iron. Otherwise the
arrangement was as before, and exceedingly sensible; for a very slight
motion of the magnet between the helices produced an abundant vibration of
the galvanometer needle.
1719. The introduction of plates of shell-lac, sulphur, or copper into the
intervals between the magnet and these helices (1713.), produced not the
least effect, whether the former were quiescent or in rapid revolution
(1715.). So here no evidence of the influence of the intermediate particles
could be obtained (1710.).
1720. The magnet was then removed and replaced by a flat helix,
corresponding to the two former, the three being parallel to each other.
The middle helix was so arranged that a voltaic current could be sent
through it at pleasure. The former galvanometer was removed, and one with a
double coil employed, one of the lateral helices being connected with one
coil, and the other helix with the other coil, in such manner that when a
voltaic current was sent through the middle helix its inductive action
(26.) on the lateral helices should cause currents in them, having contrary
directions in the coils of the galvanometer. By a little adjustment of the
distances these induced currents were rendered exactly equal, and the
galvanometer needle remained stationary notwithstanding their frequent
production in the instrument. I will call the middle coil C, and the
external coils A and B.
1721. A plate of copper 0.7 of an inch thick and six inches square, was
placed between coils C and B, their respective distances remaining
unchanged; and then a voltaic current from twenty pairs of 4 inch plates
was sent through the coil C, and intermitted, in periods fitted to produce
an effect on the galvanometer (1712.). if any difference had been produced
in the effect of C on A and B. But notwithstanding the presence of air in
one interval and copper in the other, the inductive effect was exactly
alike on the two coils, and as if air had occupied both intervals. So that
notwithstanding the facility with which any induced currents might form in
the thick copper plate, the coil outside of it was just as much affected by
the central helix C as if no such conductor as the copper had been there
(65.).
1722. Then, for the copper plate was substituted one of sulphur 0.9 of an
inch thick; still the results were exactly the same, i.e. there was no
action at the galvanometer.
1723. Thus it appears that when a voltaic current in one wire is exerting
its inductive action to produce a contrary or a similar current in a
neighbouring wire, according as the primary current is commencing or
ceasing, it makes not the least difference whether the intervening space is
occupied by such insulating bodies as air, sulphur and shell-lac, or such
conducting bodies as copper, and the other non-magnetic metals.
1724. A correspondent effect was obtained with the like forces when
resident in a magnet thus. A single flat helix (1718.) was connected with a
galvanometer, and a magnetic pole placed near to it; then by moving the
magnet to and from the helix, or the helix to and from the magnet, currents
were produced indicated by the galvanometer.
1725. The thick copper plate (1721.) was afterwards interposed between the
magnetic pole and the helix; nevertheless on moving these to and fro,
effects, exactly the same in direction and amount, were obtained as if the
copper had not been there. So also on introducing a plate of sulphur into
the interval, not the least influence on the currents produced by motion of
the magnet or coils could be obtained.
1726. These results, with many others which I have not thought it needful
to describe, would lead to the conclusion that (judging by the _amount_ of
effect produced at a distance by forces transverse to the electric current,
i.e. magnetic forces,) the intervening matter, and therefore the
intervening particles, have nothing to do with the phenomena; or in other
words, that though the inductive force of static electricity is transmitted
to a distance by the action of the intermediate particles (1164. 1666.),
the transverse inductive force of currents, which can also act at a
distance, is not transmitted by the intermediate particles in a similar
way.
1727. It is however very evident that such a conclusion cannot be
considered as proved. Thus when the metal copper is between the pole and
the helix (1715. 1719. 1725.) or between the two helices (1721.) we know
that its particles are affected, and can by proper arrangements make their
peculiar state for the time very evident by the production of either
electrical or magnetical effects. It seems impossible to consider this
effect on the particles of the intervening matter as independent of that
produced by the inductric coil or magnet C, on the inducteous coil or core
A (1715. 1721.); for since the inducteous body is equally affected by the
inductric body whether these intervening and affected particles of copper
are present or not (1723. 1725.), such a supposition would imply that the
particles so affected had no reaction back on the original inductric
forces. The more reasonable conclusion, as it appears to me, is, to
consider these affected particles as efficient in continuing the action
onwards from the inductric to the inducteous body, and by this very
communication producing the effect of _no loss_ of induced power at the
latter.
1728. But then it may be asked what is the relation of the particles of
insulating bodies, such as air, sulphur, or lac, when _they_ intervene in
the line of magnetic action? The answer to this is at present merely
conjectural. I have long thought there must be a particular condition of
such bodies corresponding to the state which causes currents in metals and
other conductors (26. 53. 191. 201. 213.); and considering that the bodies
are insulators one would expect that state to be one of tension. I have 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. Nevertheless, as any
such state must be of exceedingly low intensity, because of the feeble
intensity of the currents which are used to induce it, it may well be that
the state may exist, and may be discoverable by some more expert
experimentalist, though I have not been able to make it sensible.
1729. It appears to me possible, therefore, and even probable, that
magnetic action may be communicated to a distance by the action of the
intervening particles, in a manner having a relation to the way in which
the inductive forces of static electricity are transferred to a distance
(1677.); the intervening particles assuming for the time more or less of a
peculiar condition, which (though with a very imperfect idea) I have
several times expressed by the term _electro-tonic state_ (60. 242. 1114.
1661.). I hope it will not be understood that I hold the settled opinion
that such is the case. I would rather in fact have proved the contrary,
namely, that magnetic forces are quite independent of the matter
intervening between the inductric and the inductions bodies; but I cannot
get over the difficulty presented by such substances as copper, silver,
lead, gold, carbon, and even aqueous solutions (201. 213.), which though
they are known to assume a peculiar state whilst intervening between the
bodies acting and acted upon (1727.), no more interfere with the final
result than those which have as yet had no peculiarity of condition
discovered in them.
1730. A remark important to the whole of this investigation ought to be
made here. Although I think the galvanometer used as I have described it
(1711. 1720.) is quite sufficient to prove that the final amount of action
on each of the two coils or the two cores A and B (1713. 1719.) is equal,
yet there is an effect which _may_ be consequent on the difference of
action of two interposed bodies which it would not show. As time enters as
an element into these actions[A] (125.), it is very possible that the
induced actions on the helices or cores A, B, though they rise to the same
degree when air and copper, or air and lac are contrasted as intervening
substances, do not do so in the same time; and yet, because of the length
of time occupied by a vibration of the needle, this difference may not be
visible, both effects rising to their maximum in periods so short as to
make no sensible portion of that required for a vibration of the needle,
and so exert no visible influence upon it.
[A] See Annnles de Chimie, 1833, tom. li. pp. 422, 428.
* * * * *
1731. If the lateral or transverse force of electrical currents, or what
appears to be the same thing, magnetic power, could be proved to be
influential at a distance independently of the intervening contiguous
particles, then, as it appears to me, a real distinction of a high and
important kind, would be established between the natures of these two
forces (1654. 1664.). I do not mean that the powers are independent of each
other and might be rendered separately active, on the contrary they are
probably essentially associated (1654.), but it by no means follows that
they are of the same nature. In common statical induction, in conduction,
and in electrolyzation, the forces at the opposite extremities of the
particles which coincide with the lines of action and have commonly been
distinguished by the term electric, are polar, and in the cases of
contiguous particles act only to insensible distances; whilst those which
are transverse to the direction of these lines, and are called magnetic,
are circumferential, act at a distance, and if not through the mediation of
the intervening particles, have their relations to ordinary matter entirely
unlike those of the electrical forces with which they are associated.
1732. To decide this question of the identity or distinction of the two
kinds of power, and establish their true relation, would be exceedingly
important. The question seems fully within the reach of experiment, and
offers a high reward to him who will attempt its settlement.
1733. I have already expressed a hope of finding an effect or condition
which shall be to statical electricity what magnetic force is to current
electricity (1658.). If I could have proved to my own satisfaction that
magnetic forces extended their influence to a distance by the conjoined
action of the intervening particles in a manner analogous to that of
electrical forces, then I should have thought that the natural tension of
the lines of inductive action (1659.), or that state so often hinted at as
the electro-tonic state (1661. 1662.), was this related condition of
statical electricity.
1734. It may be said that the state of _no lateral action_ is to static or
inductive force the equivalent of _magnetism_ to current force; but that
can only be upon the view that electric and magnetic action are in their
nature essentially different (1664.). If they are the same power, the whole
difference in the results being the consequence of the difference of
_direction_, then the normal or _undeveloped_ state of electric force will
correspond with the state of _no lateral action_ of the magnetic state of
the force; the electric current will correspond with the lateral effects
commonly called magnetism; but the state of static induction which is
between the normal condition and the current will still require a
corresponding lateral condition in the magnetic series, presenting its own
peculiar phenomena; for it can hardly be supposed that the normal electric,
and the inductive or polarized electric, condition, can both have the same
lateral relation. If magnetism be a separate and a higher relation of the
powers developed, then perhaps the argument which presses for this third
condition of that force would not be so strong.
1735. I cannot conclude these general remarks upon the relation of the
electric and magnetic forces without expressing my surprise at the results
obtained with the copper plate (1724. 1725.). The experiments with the flat
helices represent one of the simplest cases of the induction of electrical
currents (1720.); the effect, as is well known, consisting in the
production of a momentary current in a wire at the instant when a current
in the contrary direction begins to pass through a neighbouring parallel
wire, and the production of an equally brief current in the reverse
direction when the determining current is stopped (26.). Such being the
case, it seems very extraordinary that this induced current which takes
place in the helix A when there is only air between A and C (1720.). should
be equally strong when that air is replaced by an enormous mass of that
excellently conducting metal copper (1721.). It might have been supposed
that this mass would have allowed of the formation and discharge of almost
any quantity of currents in it, which the helix C was competent to induce,
and so in some degree have diminished if not altogether prevented the
effect in A: instead of which, though we can hardly doubt that an infinity
of currents are formed at the moment in the copper plate, still not the
smallest diminution or alteration of the effect in A appears (65.). Almost
the only way of reconciling this effect with generally received notions is,
as it appears to me, to admit that magnetic action is communicated by the
action of the intervening particles (1729. 1733.).
1736. This condition of things, which is very remarkable, accords perfectly
with the effects observed in solid helices where wires are coiled over
wires to the amount of five or six or more layers in succession, no
diminution of effect on the outer ones being occasioned by those within.
§ _22. Note on electrical excitation._
1737. That the different modes in which electrical excitement takes place
will some day or other be reduced under one common law can hardly be
doubted, though for the present we are bound to admit distinctions. It will
be a great point gained when these distinctions are, not removed, but
understood.
1738. The strict relation of the electrical and chemical powers renders the
chemical mode of excitement the most instructive of all, and the case of
two isolated combining particles is probably the simplest that we possess.
Here however the action is local, and we still want such a test of
electricity as shall apply to it, to cases of current electricity, and also
to those of static induction. Whenever by virtue of the previously combined
condition of some of the acting particles (923.) we are enabled, as in the
voltaic pile, to expand or convert the local action into a current, then
chemical action can be traced through its variations to the production of
_all_ the phenomena of tension and the static state, these being in every
respect the same as if the electric forces producing them had been
developed by friction.
1739. It was Berzelius, I believe, who first spoke of the aptness of
certain particles to assume opposite states when in presence of each other
(959.). Hypothetically we may suppose these states to increase in intensity
by increased approximation, or by heat, &c. until at a certain point
combination occurs, accompanied by such an arrangement of the forces of the
two particles between themselves as is equivalent to a discharge, producing
at the same time a particle which is throughout a conductor (1700.).
1740. This aptness to assume an excited electrical state (which is probably
polar in those forming non-conducting matter) appears to be a primary fact,
and to partake of the nature of induction (1162.), for the particles do not
seem capable of retaining their particular state independently of each
other (1177.) or of matter in the opposite state. What appears to be
definite about the particles of matter is their assumption of a
_particular_ state, as the positive or negative, in relation to each other,
and not of either one or other indifferently; and also the acquirement of
force up to a certain amount.
1741. It is easily conceivable that the same force which causes local
action between two free particles shall produce current force if one of the
particles is previously in combination, forming part of an electrolyte
(923. 1738.). Thus a particle of zinc, and one of oxygen, when in presence
of each other, exert their inductive forces (1740.), and these at last rise
up to the point of combination. If the oxygen be previously in union with
hydrogen, it is held so combined by an analogous exertion and arrangement
of the forces; and as the forces of the oxygen and hydrogen are for the
time of combination mutually engaged and related, so when the superior
relation of the forces between the oxygen and zinc come into play, the
induction of the former or oxygen towards the metal cannot be brought on
and increased without a corresponding deficiency in its induction towards
the hydrogen with which it is in combination (for the amount of force in a
particle is considered as definite), and the latter therefore has its force
turned towards the oxygen of the next particle of water; thus the effect
may be considered as extended to sensible distances, and thrown into the
condition of static induction, which being discharged and then removed by
the action of other particles produces currents.
1742. In the common voltaic battery, the current is occasioned by the
tendency of the zinc to take the oxygen of the water from the hydrogen, the
effective action being at the place where the oxygen leaves the previously
existing electrolyte. But Schoenbein has arranged a battery in which the
effective action is at the other extremity of this essential part of the
arrangement, namely, where oxygen goes to the electrolyte[A]. The first may
be considered as a case where the current is put into motion by the
abstraction of oxygen from hydrogen, the latter by that of hydrogen from
oxygen. The direction of the electric current is in both cases the same,
when referred to the direction in which the elementary particles of the
electrolyte are moving (923. 962.), and both are equally in accordance with
the hypothetical view of the inductive action of the particles just
described (1740.).
[A] Philosophical Magazine, 1838, xii. 225, 315. also De la Rive's
results with peroxide of manganese. Annales de Chimie, 1836, lxi. p.
40.--_Dec. 1838._
1743. In such a view of voltaic excitement, the action of the particles may
be divided into two parts, that which occurs whilst the force in a particle
of oxygen is rising towards a particle of zinc acting on it, and falling
towards the particle of hydrogen with which it is associated (this being
the progressive period of the inductive action), and that which occurs when
the change of association takes place, and the particle of oxygen leaves
the hydrogen and combines with the zinc. The former appears to be that
which produces the current, or if there be no current, produces the state
of tension at the termination of the battery; whilst the latter, by
terminating for the time the influence of the particles which have been
active, allows of others coming into play, and so the effect of current is
continued.
1744. It seems highly probable, that excitement by friction may very
frequently be of the same character. Wollaston endeavoured to refer such
excitement to chemical action[A]; but if by chemical action ultimate union
of the acting particles is intended, then there are plenty of cases which
are opposed to such a view. Davy mentions some such, and for my own part I
feel no difficulty in admitting other means of electrical excitement than
chemical action, especially if by chemical action is meant a final
combination of the particles.
[A] Philosophical Transactions, 1801, p. 427.
1745. Davy refers experimentally to the opposite states which two particles
having opposite chemical relations can assume when they are brought into
the close vicinity of each other, but _not_ allowed to combine[A]. This, I
think, is the first part of the action already described (1743.); but in my
opinion it cannot give rise to a continuous current unless combination take
place, so as to allow other particles to act successively in the same
manner, and not even then unless one set of the particles be present as an
element of an electrolyte (923. 963.); i.e. mere quiescent contact alone
without chemical action does not in such cases produce a _current_.
[A] Philosophical Transactions, 1807, p. 31.
1746. Still it seems very possible that such a relation may produce a high
charge, and thus give rise to excitement by friction. When two bodies are
rubbed together to produce electricity in the usual way, one at least must
be an insulator. During the act of rubbing, the particles of opposite kinds
must be brought more or less closely together, the few which are most
favourably circumstanced being in such close contact as to be short only of
that which is consequent upon chemical combination. At such moments they
may acquire by their mutual induction (1740.) and partial discharge to each
other, very exalted opposite states, and when, the moment after, they are
by the progress of the rub removed from each other's vicinity, they will
retain this state if both bodies be insulators, and exhibit them upon their
complete separation.
1747. All the circumstances attending friction seem to me to favour such a
view. The irregularities of form and pressure will cause that the particles
of the two rubbing surfaces will be at very variable distances, only a few
at once being in that very close relation which is probably necessary for
the development of the forces; further, those which are nearest at one time
will be further removed at another, and others will become the nearest, and
so by continuing the friction many will in succession be excited. Finally,
the lateral direction of the separation in rubbing seems to me the best
fitted to bring many pairs of particles, first of all into that close
vicinity necessary for their assuming the opposite states by relation to
each other, and then to remove them from each other's influence whilst they
retain that state.
1748. It would be easy, on the same view, to explain hypothetically, how,
if one of the rubbing bodies be a conductor, as the amalgam of an
electrical machine, the state of the other when it comes from under the
friction is (as a mass) exalted; but it would be folly to go far into such
speculation before that already advanced has been confirmed or corrected by
fit experimental evidence. I do not wish it to be supposed that I think all
excitement by friction is of this kind; on the contrary, certain
experiments lead me to believe, that in many cases, and perhaps in all,
effects of a thermo-electric nature conduce to the ultimate effect; and
there are very probably other causes of electric disturbance influential at
the same time, which we have not as yet distinguished.
_Royal Institution.
June, 1838._