discharge, I again put the apparatus together, and charged it for fifteen
minutes as before, the inner ball positively. I then discharged it,
instantly removing the upper hemisphere with the interior ball, and,
leaving the shell-lac cup in the lower uninsulated hemisphere, examined its
inner surface by the carrier ball as before (1237.). In this way I found
the surface of the shell-lac actually _negative_, or in the reverse state
to the ball which had been in it; this state quickly disappeared, and was
succeeded by a positive condition, gradually increasing in intensity for
some time, in the same manner as before. The first negative condition of
the surface opposite the positive charging ball is a natural consequence of
the state of things, the charging ball being in contact with the shell-lac
only in a few points. It does not interfere with the general result and
peculiar state now under consideration, except that it assists in
illustrating in a very marked manner the ultimate assumption by the
surfaces of the shell-lac of an electrified condition, similar to that of
the metallic surfaces opposed to or against them.
1239. _Glass_ was then examined with respect to its power of assuming this
peculiar state. I had a thick flint-glass hemispherical cup formed, which
would fit easily into the space _o_ of the lower hemisphere (1188. 1189.);
it had been heated and varnished with a solution of shell-lac in alcohol,
for the purpose of destroying the conducting power of the vitreous surface
(1254.). Being then well-warmed and experimented with, I found it could
also assume the _same state_, but not apparently to the same degree, the
return action amounting in different cases to quantities from 6° to 18°.
1240. _Spermaceti_ experimented with in the same manner gave striking
results. When the original charge had been sustained for fifteen or twenty
minutes at about 500°, the return charge was equal to 95° or 100°, and was
about fourteen minutes arriving at the maximum effect. A charge continued
for not more than two or three seconds was here succeeded by a return
charge of 50° or 60°. The observations formerly made (1234.) held good with
this substance. Spermaceti, though it will insulate a low charge for some
time, is a better conductor than shell-lac, glass, and sulphur; and this
conducting power is connected with the readiness with which it exhibits the
particular effect under consideration.
1241. _Sulphur._--I was anxious to obtain the amount of effect with this
substance, first, because it is an excellent insulator, and in that respect
would illustrate the relation of the effect to the degree of conducting
power possessed by the dielectric (1247.); and in the next place, that I
might obtain that body giving the smallest degree of the effect now under
consideration for the investigation of the question of specific inductive
capacity (1277.).
1242. With a good hemispherical cup of sulphur cast solid and sound, I
obtained the return charge, but only to an amount of 17° or 18°. Thus glass
and sulphur, which are bodily very bad conductors of electricity, and
indeed almost perfect insulators, gave very little of this return charge.
1243. I tried the same experiment having _air_ only in the inductive
apparatus. After a continued high charge for some time I could obtain a
little effect of return action, but it was ultimately traced to the
shell-lac of the stem.
1244. I sought to produce something like this state with one electric power
and without induction; for upon the theory of an electric fluid or fluids,
that did not seem impossible, and then I should have obtained an absolute
charge (1169. 1177.), or something equivalent to it. In this I could not
succeed. I excited the outside of a cylinder of shell-lac very highly for
some time, and then quickly discharging it (1203.), waited and watched
whether any return charge would appear, but such was not the case. This is
another fact in favour of the inseparability of the two electric forces
(1177.), and another argument for the view that induction and its
concomitant phenomena depend upon a polarity of the particles of matter.
1245. Although inclined at first to refer these effects to a peculiar
masked condition of a certain portion of the forces, I think I have since
correctly traced them to known principles of electrical action. The effects
appear to be due to an actual penetration of the charge to some distance
within the electric, at each of its two surfaces, by what we call
_conduction_; so that, to use the ordinary phrase, the electric forces
sustaining the induction are not upon the metallic surfaces only, but upon
and within the dielectric also, extending to a greater or smaller depth
from the metal linings. Let _c_ (fig. 113.) be the section of a plate of
any dielectric, _a_ and _b_ being the metallic coatings; let _b_ be
uninsulated, and _a_ be charged positively; after ten or fifteen minutes,
if _a_ and _b_ be discharged, insulated, and immediately examined, no
electricity will appear in them; but in a short time, upon a second
examination, they will appear charged in the same way, though not to the
same degree, as they were at first. Now suppose that a portion of the
positive force has, under the coercing influence of all the forces
concerned, penetrated the dielectric and taken up its place at the line
_p_, a corresponding portion of the negative force having also assumed its
position at the line _n_; that in fact the electric at these two parts has
become charged positive and negative; then it is clear that the induction
of these two forces will be much greater one towards the other, and less in
an external direction, now that they are at the small distance _np_ from
each other, than when they were at the larger interval _ab_. Then let _a_
and _b_ be discharged; the discharge destroys or neutralizes all external
induction, and the coatings are therefore found by the carrier ball
unelectrified; but it also removes almost the whole of the forces by which
the electric charge was driven into the dielectric, and though probably a
part of that charge goes forward in its passage and terminates in what we
call discharge, the greater portion returns on its course to the surfaces
of _c_, and consequently to the conductors _a_ and _b_, and constitutes the
recharge observed.
1246. The following is the experiment on which I rest for the truth of this
view. Two plates of spermaceti, _d_ and, _f_ (fig. 114.), were put together
to form the dielectric, _a_ and _b_ being the metallic coatings of this
compound plate, as before. The system was charged, then discharged,
insulated, examined, and found to give no indications of electricity to the
carrier ball. The plates _d_ and _f_were then separated from each other,
and instantly _a_ with _d_ was found in a positive state, and _b_ with _f_
in a negative state, nearly all the electricity being in the linings _a_
and _b_. Hence it is clear that, of the forces sought for, the positive was
in one-half of the compound plate and the negative in the other half; for
when removed bodily with the plates from each other's inductive influence,
they appeared in separate places, and resumed of necessity their power of
acting by induction on the electricity of surrounding bodies. Had the
effect depended upon a peculiar relation of the contiguous particles of
matter only, then each half-plate, _d_ and _f_, should have shown positive
force on one surface and negative on the other.
1247. Thus it would appear that the best solid insulators, such as
shell-lac, glass, and sulphur, have conductive properties to such an
extent, that electricity can penetrate them bodily, though always subject
to the overruling condition of induction (1178.). As to the depth to which
the forces penetrate in this form of charge of the particles,
theoretically, it should be throughout the mass, for what the charge of the
metal does for the portion of dielectric next to it, should be close by the
charged dielectric for the portion next beyond it again; but probably in
the best insulators the sensible charge is to a very small depth only in
the dielectric, for otherwise more would disappear in the first instance
whilst the original charge is sustained, less time would be required for
the assumption of the particular state, and more electricity would
re-appear as return charge.
1248. The condition of _time_ required for this penetration of the charge
is important, both as respects the general relation of the cases to
conduction, and also the removal of an objection that might otherwise
properly be raised to certain results respecting specific inductive
capacities, hereafter to be given (1269. 1277.)
1249. It is the assumption for a time of this charged state of the glass
between the coatings in the Leyden jar, which gives origin to a well-known
phenomenon, usually referred to the diffusion of electricity over the
uncoated portion of the glass, namely, the _residual charge_. The extent of
charge which can spontaneously be recovered by a large battery, after
perfect uninsulation of both surfaces, is very considerable, and by far the
largest portion of this is due to the return of electricity in the manner
described. A plate of shell-lac six inches square, and half an inch thick,
or a similar plate of spermaceti an inch thick, being coated on the sides
with tinfoil as a Leyden arrangement, will show this effect exceedingly
well.
* * * * *
1250. The peculiar condition of dielectrics which has now been described,
is evidently capable of producing an effect interfering with the results
and conclusions drawn from the use of the two inductive apparatus, when
shell-lac, glass, &c. is used in one or both of them (1192. 1207.), for
upon dividing the charge in such cases according to the method described
(1198. 1207.), it is evident that the apparatus just receiving its half
charge must fall faster in its tension than the other. For suppose app. i.
first charged, and app. ii. used to divide with it; though both may
actually lose alike, yet app. i., which has been diminished one-half, will
be sustained by a certain degree of return action or charge (1234.), whilst
app. ii. will sink the more rapidly from the coming on of the particular
state. I have endeavoured to avoid this interference by performing the
whole process of comparison as quickly as possible, and taking the force of
app. ii. immediately after the division, before any sensible diminution of
the tension arising from the assumption of the peculiar state could be
produced; and I have assumed that as about three minutes pass between the
first charge of app. i. and the division, and three minutes between the
division and discharge, when the force of the non-transferable electricity
is measured, the contrary tendencies for those periods would keep that
apparatus in a moderately steady and uniform condition for the latter
portion of time.
1251. The particular action described occurs in the shell-lac of the stems,
as well as in the _dielectric_ used within the apparatus. It therefore
constitutes a cause by which the outside of the stems may in some
operations become charged with electricity, independent of the action of
dust or carrying particles (1203.).
¶ v. _On specific induction, or specific inductive capacity._
1252. I now proceed to examine the great question of specific inductive
capacity, i.e. whether different dielectric bodies actually do possess any
influence over the degree of induction which takes place through them. If
any such difference should exist, it appeared to me not only of high
importance in the further comprehension of the laws and results of
induction, but an additional and very powerful argument for the theory I
have ventured to put forth, that the whole depends upon a molecular action,
in contradistinction to one at sensible distances.
The question may be stated thus: suppose A an electrified plate of metal
suspended in the air, and B and C two exactly similar plates, placed
parallel to and on each side of A at equal distances and uninsulated; A
will then induce equally towards B and C. If in this position of the plates
some other dielectric than air, as shell-lac, be introduced between A and
C, will the induction between them remain the same? Will the relation of C
and B to A be unaltered, notwithstanding the difference of the dielectrics
interposed between them?[A]
[A] Refer for the practical illustration of this statement to the
supplementary note commencing 1307, &c.--_Dec. 1838._
1253. As far as I recollect, it is assumed that no change will occur under
such variation of circumstances, and that the relations of B find C to A
depend entirely upon their distance. I only remember one experimental
illustration of the question, and that is by Coulomb[A], in which he shows
that a wire surrounded by shell-lac took exactly the same quantity of
electricity from a charged body as the same wire in air. The experiment
offered to me no proof of the truth of the supposition: for it is not the
mere films of dielectric substances surrounding the charged body which have
to be examined and compared, but the _whole mass_ between that body and the
surrounding conductors at which the induction terminates. Charge depends
upon induction (1171. 1178.); and if induction is related to the particles
of the surrounding dielectric, then it is related to _all_ the particles of
that dielectric inclosed by the surrounding conductors, and not merely to
the few situated next to the charged body. Whether the difference I sought
for existed or not, I soon found reason to doubt the conclusion that might
be drawn from Coulomb's result; and therefore had the apparatus made,
which, with its use, has been already described (1187, &c.), and which
appears to me well-suited for the investigation of the question.
[A] Mémoires de l'Académie, 1787, pp. 452, 453.
1254. Glass, and many bodies which might at first be considered as very fit
to test the principle, proved exceedingly unfit for that purpose. Glass,
principally in consequence of the alkali it contains, however well-warmed
and dried it may be, has a certain degree of conducting power upon its
surface, dependent upon the moisture of the atmosphere, which renders it
unfit for a test experiment. Resin, wax, naphtha, oil of turpentine, and
many other substances were in turn rejected, because of a slight degree of
conducting power possessed by them; and ultimately shell-lac and sulphur
were chosen, after many experiments, as the dielectrics best fitted for the
investigation. No difficulty can arise in perceiving how the possession of
a feeble degree of conducting power tends to make a body produce effects,
which would seem to indicate that it had a greater capability of allowing
induction through it than another body perfect in its insulation. This
source of error has been that which I have found most difficult to obviate
in the proving experiments.
* * * * *
1255. _Induction through shell-lac._--As a preparatory experiment, I first
ascertained generally that when a part of the surface of a thick plate of
shell-lac was excited or charged, there was no sensible difference in the
character of the induction sustained by that charged part, whether exerted
through the air in the one direction, or through the shell-lac of the plate
in the other; provided the second surface of the plate had not, by contact
with conductors, the action of dust, or any other means, become charged
(1203.). Its solid condition enabled it to retain the excited particles in
a permanent position, but that appeared to be all; for these particles
acted just as freely through the shell-lac on one side as through the air
on the other. The same general experiment was made by attaching a disc of
tinfoil to one side of the shell-lac plate, and electrifying it, and the
results were the same. Scarcely any other solid substance than shell-lac
and sulphur, and no liquid substance that I have tried, will bear this
examination. Glass in its ordinary state utterly fails; yet it was
essentially necessary to obtain this prior degree of perfection in the
dielectric used, before any further progress could be made in the principal
investigation.
1256. _Shell-lac and air_ were compared in the first place. For this
purpose a thick hemispherical cup of shell-lac was introduced into the
lower hemisphere of one of the inductive apparatus (1187, &c.), so as
nearly to fill the lower half of the space _o, o_ (fig. 104.) between it
and the inner ball; and then charges were divided in the manner already
described (1198. 1207.), each apparatus being used in turn to receive the
first charge before its division by the other. As the apparatus were known
to have equal inductive power when air was in both (1209. 1211.), any
differences resulting from the introduction of the shell-lac would show a
peculiar action in it, and if unequivocally referable to a specific
inductive influence, would establish the point sought to be sustained. I
have already referred to the precautions necessary in making the
experiments (1199, &c.); and with respect to the error which might be
introduced by the assumption of the peculiar state, it was guarded against,
as far as possible, in the first place, by operating quickly (1248); and,
afterwards, by using that dielectric as glass or sulphur, which assumed the
peculiar state most slowly, and in the least degree (1239. 1241.).
1257. The shell-lac hemisphere was put into app. i., and app. ii. left
filled with air. The results of an experiment in which the charge through
air was divided and reduced by the shell-lac app. were as follows:
App. i. Lac. App. ii. Air.
Balls 255°.
0° . . . .
. . . . 304°
. . . . 297
Charge divided.
113 . . . .
. . . . 121
0 . . . . after being discharged.
. . . . 7 after being discharged.
1258. Here 297°, minus 7°, or 290°, may be taken as the divisible charge of
app. ii. (the 7° being fixed stem action (1203. 1232.)), of which 145° is
the half. The lac app. i. gave 113° as the power or tension it had acquired
after division; and the air app. ii. gave 121°, minus 7°, or 114°, as the
force it possessed from what it retained of the divisible charge of 290°.
These two numbers should evidently be alike, and they are very nearly so,
indeed far within the errors of experiment and observation, but these
numbers differ very much from 145°, or the force which the half charge
would have had if app. i. had contained air instead of shell-lac; and it
appears that whilst in the division the induction through the air has lost
176° of force, that through the lac has only gained 113°.
1259. If this difference be assumed as depending entirely on the greater
facility possessed by shell-lac of allowing or causing inductive action
through its substance than that possessed by air, then this capacity for
electric induction would be inversely as the respective loss and gain
indicated above; and assuming the capacity of the air apparatus as 1, that
of the shell-lac apparatus would be 176/113 or 1.55.
1260. This extraordinary difference was so unexpected in its amount, as to
excite the greatest suspicion of the general accuracy of the experiment,
though the perfect discharge of app. i. after the division, showed that the
113° had been taken and given up readily. It was evident that, if it really
existed, it ought to produce corresponding effects in the reverse order;
and that when induction through shell-lac was converted into induction
through air, the force or tension of the whole ought to be _increased_. The
app. i. was therefore charged in the first place, and its force divided
with app. ii. The following were the results:
App. i. Lac. App. ii. Air.
. . . . 0°
215° . . . .
204 . . . .
Charge divided.
. . . . 118
118 . . . .
. . . . 0 after being discharged.
0 . . . . after being discharged.
1261. Here 204° must be the utmost of the divisible charge. The app. i. and
app. ii. present 118° as their respective forces; both now much _above_ the
half of the first force, or 102°, whereas in the former case they were
below it. The lac app. i. has lost only 86°, yet it has given to the air
app. ii. 118°, so that the lac still appears much to surpass the air, the
capacity of the lac app. i. to the air app. ii. being as 1.37 to 1.
1262. The difference of 1.55 and 1.37 as the expression of the capacity for
the induction of shell-lac seems considerable, but is in reality very
admissible under the circumstances, for both are in error in _contrary
directions_. Thus in the last experiment the charge fell from 215° to 204°
by the joint effects of dissipation and absorption (1192. 1250.), during
the time which elapsed in the electrometer operations, between the
applications of the carrier ball required to give those two results. Nearly
an equal time must have elapsed between the application of the carrier
which gave the 204° result, and the division of the charge between the two
apparatus; and as the fall in force progressively decreases in amount
(1192.), if in this case it be taken at 6° only, it will reduce the whole
transferable charge at the time of division to 198° instead of 204°; this
diminishes the loss of the shell-lac charge to 80° instead of 86°; and then
the expression of specific capacity for it is increased, and, instead of
1.37, is 1.47 times that of air.
1263. Applying the same correction to the former experiment in which air
was _first_ charged, the result is of the _contrary_ kind. No shell-lac
hemisphere was then in the apparatus, and therefore the loss would be
principally from dissipation, and not from absorption: hence it would be
nearer to the degree of loss shown by the numbers 304° and 297°, and being
assumed as 6° would reduce the divisible charge to 284°. In that case the
air would have lost 170°, and communicated only 113° to the shell-lac; and
the relative specific capacity of the latter would appear to be 1.50, which
is very little indeed removed from 1.47, the expression given by the second
experiment when corrected in the same way.
1264. The shell-lac was then removed from app. i. and put into app. ii. and
the experiments of division again made. I give the results, because I think
the importance of the point justifies and even requires them.
App. i. Air. App. ii. Lac.
Balls 200°.
. . . . 0°.
286° . . . .
283 . . . .
Charge divided.
. . . . 110
109 . . . .
. . . . 0.25 after discharge.
Trace . . . . after discharge.
Here app. i. retained 109°, having lost 174° in communicating 110° to app.
ii.; and the capacity of the air app. is to the lac app., therefore, as 1
to 1.58. If the divided charge be corrected for an assumed loss of only 3°,
being the amount of previous loss in the same time, it will make the
capacity of the shell-lac app. 1.55 only.
1265. Then app. ii. was charged, and the charge divided thus:
App. i. Air. App. ii. Lac,
0° . . . .
. . . . 250°
. . . . 251
Charge divided.
146 . . . .
. . . . 149
a little . . . . after discharge.
. . . . a little after discharge.
Here app. i. acquired a charge of 146°, while app. ii. lost only 102° in
communicating that amount of force; the capacities being, therefore, to
each other as 1 to 1.43. If the whole transferable charge be corrected for
a loss of 4° previous to division, it gives the expression of l.49 for the
capacity of the shell-lac apparatus.
1266. These four expressions of 1.47, 1.50, 1.55, and 1.49 for the power of
the shell-lac apparatus, through the different variations of the
experiment, are very near to each other; the average is close upon 1.5,
which may hereafter be used as the expression of the result. It is a very
important result; and, showing for this particular piece of shell-lac a
decided superiority over air in allowing or causing the act of induction,
it proved the growing necessity of a more close and rigid examination of
the whole question.
1267. The shell-lac was of the best quality, and had been carefully
selected and cleaned; but as the action of any conducting particles in it
would tend, virtually, to diminish the quantity or thickness of the
dielectric used, and produce effects as if the two inducing surfaces of the
conductors in that apparatus were nearer together than in the one with air
only, I prepared another shell-lac hemisphere, of which the material had
been dissolved in strong spirit of wine, the solution filtered, and then
carefully evaporated. This is not an easy operation, for it is difficult to
drive off the last portions of alcohol without injuring the lac by the heat
applied; and unless they be dissipated, the substance left conducts too
well to be used in these experiments. I prepared two hemispheres this way,
one of them unexceptionable; and with it I repeated the former experiments
with all precautions. The results were exactly of the same kind; the
following expressions for the capacity of the shell-lac apparatus, whether
it were app. i. or ii., being given directly by the experiments, 1.46,
1.50, 1.52, 1.51; the average of these and several others being very nearly
1.5.
1268. As a final check upon the general conclusion, I then actually brought
the surfaces of the air apparatus, corresponding to the place of the
shell-lac in its apparatus, nearer together, by putting a metallic lining
into the lower hemisphere of the one not containing the lac (1213.). The
distance of the metal surface from the carrier ball was in this way
diminished from 0.62 of an inch to 0.435 of an inch, whilst the interval
occupied by the lac in the other apparatus remained O.62 of an inch as
before. Notwithstanding this change, the lac apparatus showed its former
superiority; and whether it or the air apparatus was charged first, the
capacity of the lac apparatus to the air apparatus was by the experimental
results as 1.45 to 1.
1269. From all the experiments I have made, and their constant results, I
cannot resist the conclusion that shell-lac does exhibit a case of
_specific inductive capacity_. I have tried to check the trials in every
way, and if not remove, at least estimate, every source of error. That the
final result is not due to common conduction is shown by the capability of
the apparatus to retain the communicated charge; that it is not due to the
conductive power of inclosed small particles, by which they could acquire a
polarized condition as conductors, is shown by the effects of the shell-lac
purified by alcohol; and, that it is not due to any influence of the
charged state, formerly described (1250.), first absorbing and then
evolving electricity, is indicated by the _instantaneous_ assumption and
discharge of those portions of the power which are concerned in the
phenomena, that instantaneous effect occurring in these cases, as in all
others of ordinary induction, by charged conductors. The latter argument is
the more striking in the case where the air apparatus is employed to divide
the charge with the lac apparatus, for it obtains its portion of
electricity in an _instant_, and yet is charged far above the _mean_.
1270. Admitting for the present the general fact sought to be proved; then
1.5, though it expresses the capacity of the apparatus containing the
hemisphere of shell-lac, by no means expresses the relation of lac to air.
The lac only occupies one-half of the space _o, o_, of the apparatus
containing it, through which the induction is sustained; the rest is filled
with air, as in the other apparatus; and if the effect of the two upper
halves of the globes be abstracted, then the comparison of the shell-lac
powers in the lower half of the one, with the power of the air in the lower
half of the other, will be as 2:1; and even this must be less than the
truth, for the induction of the upper part of the apparatus, i.e. of the
wire and ball B. (fig. 104.) to external objects, must be the same in both,
and considerably diminish the difference dependent upon, and really
producible by, the influence of the shell-lac within.
* * * * *
1271. _Glass._--I next worked with glass as the dielectric. It involved the
possibility of conduction on its surface, but it excluded the idea of
conducting particles within its substance (1267.) other than those of its
own mass. Besides this it does not assume the charged state (1239.) so
readily, or to such an extent, as shell-lac.
1272. A thin hemispherical cup of glass being made hot was covered with a
coat of shell-lac dissolved in alcohol, and after being dried for many
hours in a hot place, was put into the apparatus and experimented with. It
exhibited effects so slight, that, though they were in the direction
indicating a superiority of glass over air, they were allowed to pass as
possible errors of experiment; and the glass was considered as producing no
sensible effect.
1273. I then procured a thick hemispherical flint glass cup resembling that
of shell-lac (1239.), but not filling up the space _o, o_, so well. Its
average thickness was 0.4 of an inch, there being an additional thickness
of air, averaging 0.22 of an inch, to make up the whole space of 0.62 of an
inch between the inductive metallic surfaces. It was covered with a film of
shell-lac as the former was, (1272.) and being made very warm, was
introduced into the apparatus, also warmed, and experiments made with it as
in the former instances (1257. &c.). The general results were the same as
with shell-lac, i.e. glass surpassed air in its power of favouring
induction through it. The two best results as respected the state of the
apparatus for retention of charge, &c., gave, when the air apparatus was
charged first 1.336, and when the glass apparatus was charged first 1.45,
as the specific inductive capacity for glass, both being without
correction. The average of nine results, four with the glass apparatus
first charged, and five with the air apparatus first charged, gave 1.38 as
the power of the glass apparatus; 1.22 and 1.46 being the minimum and
maximum numbers with all the errors of experiment upon them. In all the
experiments the glass apparatus took up its inductive charge instantly, and
lost it as readily (1269.); and during the short time of each experiment,
acquired the peculiar state in a small degree only, so that the influence
of this state, and also of conduction upon the results, must have been
small.
1274. Allowing specific inductive capacity to be proved and active in this
case, and 1.38 as the expression for the glass apparatus, then the specific
inductive capacity of flint glass will be above 1.76, not forgetting that
this expression is for a piece of glass of such thickness as to occupy not
quite two-thirds of the space through which the induction is sustained
(1253. 1273.).
* * * * *
1275. _Sulphur._--The same hemisphere of this substance was used in app.
ii. as was formerly referred to (1242.). The experiments were well made,
i.e. the sulphur itself was free from charge both before and after each
experiment, and no action from the stem appeared (1203. 1232.), so that no
correction was required on that account. The following are the results when
the air apparatus was first charged and divided:
App. i. Air, App. ii. Sulphur.
Balls 280°.
0° . . . .
. . . . 0°
438 . . . .
434 . . . .
Charge divided.
. . . . 162
164 . . . .
. . . . 160
162 . . . .
. . . . 0 after discharge.
0 . . . . after discharge.
Here app. i. retained 164°, having lost 276° in communicating 162° to app.
ii., and the capacity of the air apparatus is to that of the sulphur
apparatus as 1 to 1.66.
1276. Then the sulphur apparatus was charged first, thus:
. . . . 0°
0° . . . .
. . . . 395
. . . . 388
Charge divided.
237 . . . .
. . . . 238
0 . . . . after discharge.
. . . . 0 after discharge.
Here app. ii. retained 238°, and gave up 150° in communicating a charge of
237° to app. i., and the capacity of the air apparatus is to that of the
sulphur apparatus as 1 to 1.58. These results are very near to each other,
and we may take the mean 1.62 as representing the specific inductive
capacity of the sulphur apparatus; in which case the specific inductive
capacity of sulphur itself as compared to air = 1 (1270.) will be about or
above 2.24.
1277. This result with sulphur I consider as one of the most
unexceptionable. The substance when fused was perfectly clear, pellucid,
and free from particles of dirt (1267.), so that no interference of small
conducting bodies confused the result. The substance when solid is an
excellent insulator, and by experiment was found to take up, with great
slowness, that state (1244. 1242.) which alone seemed likely to disturb the
conclusion. The experiments themselves, also, were free from any need of
correction. Yet notwithstanding these circumstances, so favourable to the
exclusion of error, the result is a higher specific inductive capacity for
sulphur than for any other body as yet tried; and though this may in part
be clue to the sulphur being in a better shape, i.e. filling up more
completely the space _o, o_, (fig. 104.) than the cups of shell-lac and
glass, still I feel satisfied that the experiments altogether fully prove
the existence of a difference between dielectrics as to their power of
favouring an inductive action through them; which difference may, for the
present, be expressed by the term _specific inductive capacity_.
1278. Having thus established the point in the most favourable cases that I
could anticipate, I proceeded to examine other bodies amongst solids,
liquids, and gases. These results I shall give with all convenient brevity.
* * * * *
1279. _Spermaceti._--A good hemisphere of spermaceti being tried as to
conducting power whilst its two surfaces were still in contact with the
tinfoil moulds used in forming it, was found to conduct sensibly even
whilst warm. On removing it from the moulds and using it in one of the
apparatus, it gave results indicating a specific inductive capacity between
1.3 and 1.6 for the apparatus containing it. But as the only mode of
operation was to charge the air apparatus, and then after a quick contact
with the spermaceti apparatus, ascertain what was left in the former
(1281.), no great confidence can be placed in the results. They are not in
opposition to the general conclusion, but cannot be brought forward as
argument in favour of it.
* * * * *
1280. I endeavoured to find some liquids which would insulate well, and
could be obtained in sufficient quantity for these experiments. Oil of
turpentine, native naphtha rectified, and the condensed oil gas fluid,
appeared by common experiments to promise best as to insulation. Being left
in contact with fused carbonate of potassa, chloride of lime, and quick
lime for some days and then filtered, they were found much injured in
insulating power; but after distillation acquired their best state, though
even then they proved to be conductors when extensive metallic contact was
made with them.
1281. _Oil of turpentine rectified._--I filled the lower half of app. i.
with the fluid: and as it would not hold a charge sufficiently to enable me
first to measure and then divide it, I charged app. ii. containing air, and
dividing its charge with app. i. by a quick contact, measured that
remaining in app. ii.: for, theoretically, if a quick contact would divide
up to equal tension between the two apparatus, yet without sensible loss
from the conducting power of app. i.; and app. ii. were left charged to a
degree of tension above half the original charge, it would indicate that
oil of turpentine had less specific inductive capacity than air; or, if
left charged below that mean state of tension, it would imply that the
fluid had the greater inductive capacity. In an experiment of this kind,
app. ii. gave as its charge 390° before division with app. i., and 175°
afterwards, which is less than the half of 390°. Again, being at 176°
before division, it was 79° after, which is also less than half the divided
charge. Being at 79°, it was a third time divided, and then fell to 36°,
less than the half of 79°. Such are the best results I could obtain; they
are not inconsistent with the belief that oil of turpentine has a greater
specific capacity than air, but they do not prove the fact, since the
disappearance of more than half the charge may be due to the conducting
power merely of the fluid.
1282. _Naphtha._--This liquid gave results similar in their nature and
direction to those with oil of turpentine.
* * * * *
1283. A most interesting class of substances, in relation to specific
inductive capacity, now came under review, namely, the gases or aëriform
bodies. These are so peculiarly constituted, and are bound together by so
many striking physical and chemical relations, that I expected some
remarkable results from them: air in various states was selected for the
first experiments.
1284. _Air, rare and dense._--Some experiments of division (1208.) seemed
to show that dense and rare air were alike in the property under
examination. A simple and better process was to attach one of the apparatus
to an air-pump, to charge it, and then examine the tension of the charge
when the air within was more or less rarefied. Under these circumstances it
was found, that commencing with a certain charge, that charge did not
change in its tension or force as the air was rarefied, until the
rarefaction was such that _discharge_ across the space _o_, _o_ (fig. 104.)
occurred. This discharge was proportionate to the rarefaction; but having
taken place, and lowered the tension to a certain degree, that degree was
not at all affected by restoring the pressure and density of the air to
their first quantities.
inches of mercury.
Thus at a pressure of 30 the charge was 88°
Again 30 the charge was 88
Again 30 the charge was 87
Reduced to 11 the charge was 87
Raised again to 30 the charge was 86
Being now reduced to 3.4 the charge fell to 81
Raised again to 30 the charge was still 81
1285. The charges were low in these experiments, first that they might not
pass off at low pressure, and next that little loss by dissipation might
occur. I now reduced them still lower, that I might rarefy further, and for
this purpose in the following experiment used a measuring interval in the
electrometer of only 15° (1185.). The pressure of air within the apparatus
being reduced to 1.9 inches of mercury, the charge was found to be 29°;
then letting in air till the pressure was 30 inches, the charge was still
29°.
1286. These experiments were repeated with pure oxygen with the same
consequences.
1287. This result of _no variation_ in the electric tension being produced
by variation in the density or pressure of the air, agrees perfectly with
those obtained by Mr. Harris[A], and described in his beautiful and
important investigations contained in the Philosophical Transactions;
namely that induction is the same in rare and dense air, and that the
divergence of an electrometer under such variations of the air continues
the same, provided no electricity pass away from it. The effect is one
entirely independent of that power which dense air has of causing a higher
charge to be _retained_ upon the surface of conductors in it than can be
retained by the same conductors in rare air; a point I propose considering
hereafter.
[A] Philosophical Transactions, 1834, pp. 223, 224, 237, 244.
1288. I then compared _hot and cold air_ together, by raising the
temperature of one of the inductive apparatus as high as it could be
without injury, and then dividing charges between it and the other
apparatus containing cold air. The temperatures were about 50° and 200°,
Still the power or capacity appeared to be unchanged; and when I
endeavoured to vary the experiment, by charging a cold apparatus and then
warming it by a spirit lamp, I could obtain no proof that the inductive
capacity underwent any alteration.
1289. I compared _damp and dry air_ together, but could find no difference
in the results.
* * * * *
1290. _Gases._--A very long series of experiments was then undertaken for
the purpose of comparing _different gases_ one with another. They were all
found to insulate well, except such as acted on the shell-lac of the
supporting stem; these were chlorine, ammonia, and muriatic acid. They were
all dried by appropriate means before being introduced into the apparatus.
It would have been sufficient to have compared each with air; but, in
consequence of the striking result which came out, namely, that _all had
the same power of_ or _capacity for_, sustaining induction through them,
(which perhaps might have been expected after it was found that no
variation of density or pressure produced any effect,) I was induced to
compare them, experimentally, two and two in various ways, that no
difference might escape me, and that the sameness of result might stand in
full opposition to the contrast of property, composition, and condition
which the gases themselves presented.
1291. The experiments were made upon the following pairs of gases.
1. Nitrogen and Oxygen.
2. Oxygen Air.
3. Hydrogen Air.
4. Muriatic acid gas Air.
5. Oxygen Hydrogen.
5. Oxygen Carbonic acid.
7. Oxygen Olefiant gas.
8. Oxygen Nitrous gas.
9. Oxygen Sulphurous acid.
10. Oxygen Ammonia.
11. Hydrogen Carbonic acid.
12 Hydrogen Olefiant gas.
13. Hydrogen Sulphurous acid.
14. Hydrogen Fluo-silicic acid.
15. Hydrogen Ammonia.
16, Hydrogen Arseniuretted hydrogen.
17. Hydrogen Sulphuretted hydrogen.
18, Nitrogen Olefiant gas.
19. Nitrogen Nitrous gas.
20. Nitrogen Nitrous oxide.
21. Nitrogen Ammonia.
22. Carbonic oxide Carbonic acid.
23. Carbonic oxide Olefiant gas.
24. Nitrous oxide Nitrous gas.
25. Ammonia Sulphurous acid.
1292. Notwithstanding the striking contrasts of all kinds which these gases
present of property, of density, whether simple or compound, anions or
cations (665.), of high or low pressure (1284. 1286.), hot or cold (1288.),
not the least difference in their capacity to favour or admit electrical
induction through them could be perceived. Considering the point
established, that in all these gases induction takes place by an action of
contiguous particles, this is the more important, and adds one to the many
striking relations which hold between bodies having the gaseous condition
and form. Another equally important electrical relation, which will be
examined in the next paper[A], is that which the different gases have to
each other at the _same pressure_ of causing the retention of the _same or
different degrees of charge_ upon conductors in them. These two results
appear to bear importantly upon the subject of electrochemical excitation
and decomposition; for as _all_ these phenomena, different as they seem to
be, must depend upon the electrical forces of the particles of matter, the
very distance at which they seem to stand from each other will do much, if
properly considered, to illustrate the principle by which they are held in
one common bond, and subject, as they must be, to one common law.
[A] See in relation to this point 1382. &c.--_Dec. 1838._
1293. It is just possible that the gases may differ from each other in
their specific inductive capacity, and yet by quantities so small as not to
be distinguished in the apparatus I have used. It must be remembered,
however, that in the gaseous experiments the gases occupy all the space _o,
o_, (fig. 104.) between the inner and the outer ball, except the small
portion filled by the stem; and the results, therefore, are twice as
delicate as those with solid dielectrics.
1294. The insulation was good in all the experiments recorded, except Nos.
10, 15, 21, and 25, being those in which ammonia was compared with other
gases. When shell-lac is put into ammoniacal gas its surface gradually
acquires conducting power, and in this way the lac part of the stem within
was so altered, that the ammonia apparatus could not retain a charge with
sufficient steadiness to allow of division. In these experiments,
therefore, the other apparatus was charged; its charge measured and divided
with the ammonia apparatus by a quick contact, and what remained untaken
away by the division again measured (1281.). It was so nearly one-half of
the original charge, as to authorize, with this reservation, the insertion
of ammoniacal gas amongst the other gases, as having equal power with them.
¶ vi. _General results as to induction._
1295. Thus _induction_ appears to be essentially an action of contiguous
particles, through the intermediation of which the electric force,
originating or appearing at a certain place, is propagated to or sustained
at a distance, appearing there as a force of the same kind exactly equal in
amount, but opposite in its direction and tendencies (1164.). Induction
requires no sensible thickness in the conductors which may be used to limit
its extent; an uninsulated leaf of gold may be made very highly positive on
one surface, and as highly negative on the other, without the least
interference of the two states whilst the inductions continue. Nor is it
affected by the nature of the limiting conductors, provided time be
allowed, in the case of those which conduct slowly, for them to assume
their final state (1170.).
1296. But with regard to the _dielectrics_ or insulating media, matters are
very different (1167.). Their thickness has an immediate and important
influence on the degree of induction. As to their quality, though all gases
and vapours are alike, whatever their state; yet amongst solid bodies, and
between them and gases, there are differences which prove the existence of
_specific inductive capacities_, these differences being in some cases very
great.
1297. The direct inductive force, which may be conceived to be exerted in
lines between the two limiting and charged conducting surfaces, is
accompanied by a lateral or transverse force equivalent to a dilatation or
repulsion of these representative lines (1224.); or the attractive force
which exists amongst the particles of the dielectric in the direction of
the induction is accompanied by a repulsive or a diverging force in the
transverse direction (1304.).
1298. Induction appears to consist in a certain polarized state of the
particles, into which they are thrown by the electrified body sustaining
the action, the particles assuming positive and negative points or parts,
which are symmetrically arranged with respect to each other and the
inducting surfaces or particles[A]. The state must be a forced one, for it
is originated and sustained only by force, and sinks to the normal or
quiescent state when that force is removed. It can be _continued_ only in
insulators by the same portion of electricity, because they only can retain
this state of the particles (1304).
[A] The theory of induction which I am stating does not pretend to
decide whether electricity be a fluid or fluids, or a mere power or
condition of recognized matter. That is a question which I may be
induced to consider in the next or following series of these
researches.
1299. The principle of induction is of the utmost generality in electric
action. It constitutes charge in every ordinary case, and probably in every
case; it appears to be the cause of all excitement, and to precede every
current. The degree to which the particles are affected in this their
forced state, before discharge of one kind or another supervenes, appears
to constitute what we call _intensity_.
1300. When a Leyden jar is _charged_, the particles of the glass are forced
into this polarized and constrained condition by the electricity of the
charging apparatus. _Discharge_ is the return of these particles to their
natural state from their state of tension, whenever the two electric forces
are allowed to be disposed of in some other direction.
1301. All charge of conductors is on their surface, because being
essentially inductive, it is there only that the medium capable of
sustaining the necessary inductive state begins. If the conductors are
hollow and contain air or any other dielectric, still no _charge_ can
appear upon that internal surface, because the dielectric there cannot
assume the polarized state throughout, in consequence of the opposing
actions in different directions.
1302. The known influence of _form_ is perfectly consistent with the
corpuscular view of induction set forth. An electrified cylinder is more
affected by the influence of the surrounding conductors (which complete the
condition of charge) at the ends than at the middle, because the ends are
exposed to a greater sum of inductive forces than the middle; and a point
is brought to a higher condition than a ball, because by relation to the
conductors around, more inductive force terminates on its surface than on
an equal surface of the ball with which it is compared. Here too,
especially, can be perceived the influence of the lateral or transverse
force (1297.), which, being a power of the nature of or equivalent to
repulsion, causes such a disposition of the lines of inductive force in
their course across the dielectric, that they must accumulate upon the
point, the end of the cylinder, or any projecting part.
1303. The influence of _distance_ is also in harmony with the same view.
There is perhaps no distance so great that induction cannot take place
through it[A]; but with the same constraining force (1298.) it takes place
the more easily, according as the extent of dielectric through which it is
exerted is lessened. And as it is assumed by the theory that the particles
of the dielectric, though tending to remain in a normal state, are thrown
into a forced condition during the induction; so it would seem to follow
that the fewer there are of these intervening particles opposing their
tendency to the assumption of the new state, the greater degree of change
will they suffer, i.e. the higher will be the condition they assume, and
the larger the amount of inductive action exerted through them.
[A] I have traced it experimentally from a ball placed in the middle
of the large cube formerly described (1173.) to the sides of the cube
six feet distant, and also from the same ball placed in the middle of
our large lecture-room to the walls of the room at twenty-six feet
distance, the charge sustained upon the ball in these cases being
solely due to induction through these distances.
1304. I have used the phrases _lines of inductive force_ and _curved lines_
of force (1231. 1297. 1298. 1302.) in a general sense only, just as we
speak of the lines of magnetic force. The lines are imaginary, and the
force in any part of them is of course the resultant of compound forces,
every molecule being related to every other molecule in _all_ directions by
the tension and reaction of those which are contiguous. The transverse
force is merely this relation considered in a direction oblique to the
lines of inductive force, and at present I mean no more than that by the
phrase. With respect to the term _polarity_ also, I mean at present only a
disposition of force by which the same molecule acquires opposite powers on
different parts. The particular way in which this disposition is made will
come into consideration hereafter, and probably varies in different bodies,
and so produces variety of electrical relation[A]. All I am anxious about
at present is, that a more particular meaning should not be attached to the
expressions used than I contemplate. Further inquiry, I trust, will enable
us by degrees to restrict the sense more and more, and so render the
explanation of electrical phenomena day by day more and more definite.
[A] See now 1685. &c.--_Dec. 1838._
1305. As a test of the probable accuracy of my views, I have throughout
this experimental examination compared them with the conclusions drawn by
M. Poisson from his beautiful mathematical inquiries[A]. I am quite unfit
to form a judgment of these admirable papers; but as far as I can perceive,
the theory I have set forth and the results I have obtained are not in
opposition to such of those conclusions as represent the final disposition
and state of the forces in the limited number of cases be has considered.
His theory assumes a very different mode of action in induction to that
which I have ventured to support, and would probably find its mathematical
test in the endeavour to apply it to cases of induction in curved lines. To
my feeling it is insufficient in accounting for the retention of
electricity upon the surface of conductors by the pressure of the air, an
effect which I hope to show is simple and consistent according to the
present view[B]; and it does not touch voltaic electricity, or in any way
associate it and what is called ordinary electricity under one common
principle.
[A] Mémoires de L'Institut, 1811, tom. xii. the first page 1, and the
second paging 163.
[B] Refer to 1377, 1378, 1379, 1398.--_Dec. 1838._
I have also looked with some anxiety to the results which that
indefatigable philosopher Harris has obtained in his investigation of the
laws of induction[A], knowing that they were experimental, and having a
full conviction of their exactness; but I am happy in perceiving no
collision at present between them and the views I have taken.
[A] Philosophical Transactions, 1834, p. 213.
1306. Finally, I beg to say that I put forth my particular view with doubt
and fear, lest it should not bear the test of general examination, for
unless true it will only embarrass the progress of electrical science. It
has long been on my mind, but I hesitated to publish it until the
increasing persuasion of its accordance with all known facts, and the
manner in which it linked together effects apparently very different in
kind, urged me to write the present paper. I as yet see no inconsistency
between it and nature, but, on the contrary, think I perceive much new
light thrown by it on her operations; and my next papers will be devoted to
a review of the phenomena of conduction, electrolyzation, current,
magnetism, retention, discharge, and some other points, with an application
of the theory to these effects, and an examination of it by them.
_Royal Institution,
November 16, 1837._
* * * * *
_Supplementary Note to Experimental Researches in Electricity._
_Eleventh Series._
Received March 29, 1838.
1307. I have recently put into an experimental form that general statement
of the question of _specific inductive capacity_ which is given at No. 1252
of Series XI., and the result is such as to lead me to hope the Council of
the Royal Society will authorize its addition to the paper in the form of a
supplementary note. Three circular brass plates, about five inches in
diameter, were mounted side by side upon insulating pillars; the middle
one, A, was a fixture, but the outer plates B and C were moveable on
slides, so that all three could be brought with their sides almost into
contact, or separated to any required distance. Two gold leaves were
suspended in a glass jar from insulated wires; one of the outer plates B
was connected with one of the gold leaves, and the other outer plate with
the other leaf. The outer plates B and C were adjusted at the distance of
an inch and a quarter from the middle plate A, and the gold leaves were
fixed at two inches apart; A was then slightly charged with electricity,
and the plates B and C, with their gold leaves, thrown out of insulation
_at the same time_, and then left insulated. In this state of things A was
charged positive inductrically, and B and C negative inducteously; the same
dielectric, air, being in the two intervals, and the gold leaves hanging,
of course, parallel to each other in a relatively unelectrified state.
1308. A plate of shell-lac three-quarters of an inch in thickness, and four
inches square, suspended by clean white silk thread, was very carefully
deprived of all charge (1203.) (so that it produced no effect on the gold
leaves if A were uncharged) and then introduced between plates A and B; the
electric relation of the three plates was immediately altered, and the gold
leaves attracted each other. On removing the shell-lac this attraction
ceased; on introducing it between A and C it was renewed; on removing it
the attraction again ceased; and the shell-lac when examined by a delicate
Coulomb electrometer was still without charge.
1309. As A was positive, B and C were of course negative; but as the
specific inductive capacity of shell-lac is about twice that of air
(1270.), it was expected that when the lac was introduced between A and B,
A would induce more towards B than towards C; that therefore B would become
more negative than before towards A, and consequently, because of its
insulated condition, be positive externally, as at its back or at the gold
leaves; whilst C would be less negative towards A, and therefore negative
outwards or at the gold leaves. This was found to be the case; for on
whichever side of A the shell-lac was introduced the external plate at that
side was positive, and the external plate on the other side negative
towards each other, and also to uninsulated external bodies.
1310. On employing a plate of sulphur instead of shell-lac, the same
results were obtained; consistent with the conclusions drawn regarding the
high specific inductive capacity of that body already given (1276.).
1311. These effects of specific inductive capacity can be exalted in
various ways, and it is this capability which makes the great value of the
apparatus. Thus I introduced the shell-lac between A and B, and then for a
moment connected B and C, uninsulated them, and finally left them in the
insulated state; the gold leaves were of course hanging parallel to each
other. On removing the shell-lac the gold leaves attracted each other; on
introducing the shell-lac between A and C this attraction was _increased_,
(as had been anticipated from theory,) and the leaves came together, though
not more than four inches long, and hanging three inches apart.
1312. By simply bringing the gold leaves nearer to each other I was able to
show the difference of specific inductive capacity when only thin plates of
shell-lac were used, the rest of the dielectric space being filled with
air. By bringing B and C nearer to A another great increase of sensibility
was made. By enlarging the size of the plates still further power was
gained. By diminishing the extent of the wires, &c. connected with the gold
leaves, another improvement resulted. So that in fact the gold leaves
became, in this manner, as delicate a test of _specific inductive action_
as they are, in Bennet's and Singer's electrometers, of ordinary electrical
charge.
1313. It is evident that by making the three plates the sides of cells,
with proper precautions as regards insulation, &c., this apparatus may be
used in the examination of gases, with far more effect than the former
apparatus (1187. 1290), and may, perhaps, bring out differences which have
as yet escaped me (1292. 1293.)
1314. It is also evident that two metal plates are quite sufficient to form
the instrument; the state of the single inducteous plate when the
dielectric is changed, being examined either by bringing a body excited in
a known manner towards its gold leaves, or, what I think will be better,
employing a carrier ball in place of the leaf, and examining that ball by
the Coulomb electrometer (1180.). The inductive and inducteous surfaces may
even be balls; the latter being itself the carrier ball of the Coulomb's
electrometer (1181. 1229.).
1315. To increase the effect, a small condenser may be used with great
advantage. Thus if, when two inducteous plates are used, a little condenser
were put in the place of the gold leaves, I have no doubt the three
principal plates might be reduced to an inch or even half an inch in
diameter. Even the gold leaves act to each other for the time as the plates
of a condenser. If only two plates were used, by the proper application of
the condenser the same reduction might take place. This expectation is
fully justified by an effect already observed and described (1229.).
1316. In that case the application of the instrument to very extensive
research is evident. Comparatively small masses of dielectrics could be
examined, as diamonds and crystals. An expectation, that the specific
inductive capacity of crystals will vary in different directions, according
as the lines of inductive force (1304.) are parallel to, or in other
positions in relation to the axes of the crystals, can be tested[A]: I
purpose that these and many other thoughts which arise respecting specific
inductive action and the polarity of the particles of dielectric matter,
shall be put to the proof as soon as I can find time.
[A] Refer for this investigation to 1680-1698.--_Dec. 1838._
1317. Hoping that this apparatus will form an instrument of considerable
use, I beg to propose for it (at the suggestion of a friend) the name of
_Differential Inductometer_.
_Royal Institution,
March 29, 1838._