§ 18. _On Induction._ ¶ i. _Induction an action of contiguous particles._
¶ ii. _Absolute charge of matter._ ¶ iii. _Electrometer and inductive
apparatus employed._ ¶ iv. _Induction in curved lines._ ¶ v. _Specific
inductive capacity._ ¶ vi. _General results as to induction._
Received November 30,--Read December 21, 1837.
¶ i. _Induction an action of contiguous particles._
1161. The science of electricity is in that state in which every part of it
requires experimental investigation; not merely for the discovery of new
effects, but what is just now of far more importance, the development of
the means by which the old effects are produced, and the consequent more
accurate determination of the first principles of action of the most
extraordinary and universal power in nature:--and to those philosophers who
pursue the inquiry zealously yet cautiously, combining experiment with
analogy, suspicious of their preconceived notions, paying more respect to a
fact than a theory, not too hasty to generalize, and above all things,
willing at every step to cross-examine their own opinions, both by
reasoning and experiment, no branch of knowledge can afford so fine and
ready a field for discovery as this. Such is most abundantly shown to be
the case by the progress which electricity has made in the last thirty
years: Chemistry and Magnetism have successively acknowledged its
over-ruling influence; and it is probable that every effect depending upon
the powers of inorganic matter, and perhaps most of those related to
vegetable and animal life, will ultimately be found subordinate to it.
1162. Amongst the actions of different kinds into which electricity has
conventionally been subdivided, there is, I think, none which excels, or
even equals in importance, that called _Induction_. It is of the most
general influence in electrical phenomena, appearing to be concerned in
every one of them, and has in reality the character of a first, essential,
and fundamental principle. Its comprehension is so important, that I think
we cannot proceed much further in the investigation of the laws of
electricity without a more thorough understanding of its nature; how
otherwise can we hope to comprehend the harmony and even unity of action
which doubtless governs electrical excitement by friction, by chemical
means, by heat, by magnetic influence, by evaporation, and even by the
living being?
1163. In the long-continued course of experimental inquiry in which I have
been engaged, this general result has pressed upon me constantly, namely,
the necessity of admitting two forces, or two forms or directions of a
force (516. 517.), combined with the impossibility of separating these two
forces (or electricities) from each other, either in the phenomena of
statical electricity or those of the current. In association with this, the
impossibility under any circumstances, as yet, of absolutely charging
matter of any kind with one or the other electricity only, dwelt on my
mind, and made me wish and search for a clearer view than any that I was
acquainted with, of the way in which electrical powers and the particles of
matter are related; especially in inductive actions, upon which almost all
others appeared to rest.
1164. When I discovered the general fact that electrolytes refused to yield
their elements to a current when in the solid state, though they gave them
forth freely if in the liquid condition (380. 394. 402.), I thought I saw
an opening to the elucidation of inductive action, and the possible
subjugation of many dissimilar phenomena to one law. For let the
electrolyte be water, a plate of ice being coated with platina foil on its
two surfaces, and these coatings connected with any continued source of the
two electrical powers, the ice will charge like a Leyden arrangement,
presenting a case of common induction, but no current will pass. If the ice
be liquefied, the induction will fall to a certain degree, because a
current can now pass; but its passing is dependent upon a _peculiar
molecular arrangement_ of the particles consistent with the transfer of the
elements of the electrolyte in opposite directions, the degree of discharge
and the quantity of elements evolved being exactly proportioned to each
other (377. 783.). Whether the charging of the metallic coating be effected
by a powerful electrical machine, a strong and large voltaic battery, or a
single pair of plates, makes no difference in the principle, but only in
the degree of action (360). Common induction takes place in each case if
the electrolyte be solid, or if fluid, chemical action and decomposition
ensue, provided opposing actions do not interfere; and it is of high
importance occasionally thus to compare effects in their extreme degrees,
for the purpose of enabling us to comprehend the nature of an action in its
weak state, which may be only sufficiently evident to us in its stronger
condition (451.). As, therefore, in the electrolytic action, _induction_
appeared to be the _first_ step, and _decomposition_ the _second_ (the
power of separating these steps from each other by giving the solid or
fluid condition to the electrolyte being in our hands); as the induction
was the same in its nature as that through air, glass, wax, &c. produced by
any of the ordinary means; and as the whole effect in the electrolyte
appeared to be an action of the particles thrown into a peculiar or
polarized state, I was led to suspect that common induction itself was in
all cases an _action of contiguous particles_[A], and that electrical
action at a distance (i.e. ordinary inductive action) never occurred except
through the influence of the intervening matter.
[A] The word _contiguous_ is perhaps not the best that might have been
used here and elsewhere; for as particles do not touch each other it
is not strictly correct. I was induced to employ it, because in its
common acceptation it enabled me to state the theory plainly and with
facility. By contiguous particles I mean those which are next.--_Dec.
1838._
1165. The respect which I entertain towards the names of Epinus, Cavendish,
Poisson, and other most eminent men, all of whose theories I believe
consider induction as an action at a distance and in straight lines, long
indisposed me to the view I have just stated; and though I always watched
for opportunities to prove the opposite opinion, and made such experiments
occasionally as seemed to bear directly on the point, as, for instance, the
examination of electrolytes, solid and fluid, whilst under induction by
polarized light (951. 955.), it is only of late, and by degrees, that the
extreme generality of the subject has urged me still further to extend my
experiments and publish my view. At present I believe ordinary induction in
all cases to be an action of contiguous particles consisting in a species
of polarity, instead of being an action of either particles or masses at
sensible distances; and if this be true, the distinction and establishment
of such a truth must be of the greatest consequence to our further progress
in the investigation of the nature of electric forces. The linked condition
of electrical induction with chemical decomposition; of voltaic excitement
with chemical action; the transfer of elements in an electrolyte; the
original cause of excitement in all cases; the nature and relation of
conduction and insulation of the direct and lateral or transverse action
constituting electricity and magnetism; with many other things more or less
incomprehensible at present, would all be affected by it, and perhaps
receive a full explication in their reduction under one general law.
1166. I searched for an unexceptionable test of my view, not merely in the
accordance of known facts with it, but in the consequences which would flow
from it if true; especially in those which would not be consistent with the
theory of action at a distance. Such a consequence seemed to me to present
itself in the direction in which inductive action could be exerted. If in
straight lines only, though not perhaps decisive, it would be against my
view; but if in curved lines also, that would be a natural result of the
action of contiguous particles, but, as I think, utterly incompatible with
action at a distance, as assumed by the received theories, which, according
to every fact and analogy we are acquainted with, is always in straight
lines.
1167. Again, if induction be an action of contiguous particles, and also
the first step in the process of electrolyzation (1164. 919.), there seemed
reason to expect some particular relation of it to the different kinds of
matter through which it would be exerted, or something equivalent to a
_specific electric induction_ for different bodies, which, if it existed,
would unequivocally prove the dependence of induction on the particles; and
though this, in the theory of Poisson and others, has never been supposed
to be the case, I was soon led to doubt the received opinion, and have
taken great pains in subjecting this point to close experimental
examination.
1168. Another ever-present question on my mind has been, whether
electricity has an actual and independent existence as a fluid or fluids,
or was a mere power of matter, like what we conceive of the attraction of
gravitation. If determined either way it would be an enormous advance in
our knowledge; and as having the most direct and influential bearing on my
notions, I have always sought for experiments which would in any way tend
to elucidate that great inquiry. It was in attempts to prove the existence
of electricity separate from matter, by giving an independent charge of
either positive or negative power only, to some one substance, and the
utter failure of all such attempts, whatever substance was used or whatever
means of exciting or _evolving_ electricity were employed, that first drove
me to look upon induction as an action of the particles of matter, each
having _both_ forces developed in it in exactly equal amount. It is this
circumstance, in connection with others, which makes me desirous of placing
the remarks on absolute charge first, in the order of proof and argument,
which I am about to adduce in favour of my view, that electric induction is
an action of the contiguous particles of the insulating medium or
_dielectric_[A].
[A] I use the word _dielectric_ to express that substance through or
across which the electric forces are acting.--_Dec. 1838._
¶ ii. _On the absolute charge of matter._
1169. Can matter, either conducting or non-conducting, be charged with one
electric force independently of the other, in any degree, either in a
sensible or latent state?
1170. The beautiful experiments of Coulomb upon the equality of action of
_conductors_, whatever their substance, and the residence of _all_ the
electricity upon their surfaces[A], are sufficient, if properly viewed, to
prove that _conductors cannot be bodily charged_; and as yet no means of
communicating electricity to a conductor so as to place its particles in
relation to one electricity, and not at the same time to the other in
exactly equal amount, has been discovered.
[A] Mémoires de l'Académie, 1786, pp. 67. 69. 72; 1787, p. 452.
1171. With regard to electrics or non-conductors, the conclusion does not
at first seem so clear. They may easily be electrified bodily, either by
communication (1247.) or excitement; but being so charged, every case in
succession, when examined, came out to be a case of induction, and not of
absolute charge. Thus, glass within conductors could easily have parts not
in contact with the conductor brought into an excited state; but it was
always found that a portion of the inner surface of the conductor was in an
opposite and equivalent state, or that another part of the glass itself was
in an equally opposite state, an _inductive_ charge and not an _absolute_
charge having been acquired.
1172. Well-purified oil of turpentine, which I find to be an excellent
liquid insulator for most purposes, was put into a metallic vessel, and,
being insulated, an endeavour was made to charge its particles, sometimes
by contact of the metal with the electrical machine, and at others by a
wire dipping into the fluid within; but whatever the mode of communication,
no electricity of one kind only was retained by the arrangement, except
what appeared on the exterior surface of the metal, that portion being
present there only by an inductive action through the air to the
surrounding conductors. When the oil of turpentine was confined in glass
vessels, there were at first some appearances as if the fluid did receive
an absolute charge of electricity from the charging wire, but these were
quickly reduced to cases of common induction jointly through the fluid, the
glass, and the surrounding air.
1173. I carried these experiments on with air to a very great extent. I had
a chamber built, being a cube of twelve feet. A slight cubical wooden frame
was constructed, and copper wire passed along and across it in various
directions, so as to make the sides a large net-work, and then all was
covered in with paper, placed in close connexion with the wires, and
supplied in every direction with bands of tin foil, that the whole might be
brought into good metallic communication, and rendered a free conductor in
every part. This chamber was insulated in the lecture-room of the Royal
Institution; a glass tube about six feet in length was passed through its
side, leaving about four feet within and two feet on the outside, and
through this a wire passed from the large electrical machine (290.) to the
air within. By working the machine, the air in this chamber could be
brought into what is considered a highly electrified state (being, in fact,
the same state as that of the air of a room in which a powerful machine is
in operation), and at the same time the outside of the insulated cube was
everywhere strongly charged. But putting the chamber in communication with
the perfect discharging train described in a former series (292.), and
working the machine so as to bring the air within to its utmost degree of
charge if I quickly cut off the connexion with the machine, and at the same
moment or instantly after insulated the cube, the air within had not the
least power to communicate a further charge to it. If any portion of the
air was electrified, as glass or other insulators may be charged (1171.),
it was accompanied by a corresponding opposite action _within_ the cube,
the whole effect being merely a case of induction. Every attempt to charge
air bodily and independently with the least portion of either electricity
failed.
1174 I put a delicate gold-leaf electrometer within the cube, and then
charged the whole by an _outside_ communication, very strongly, for some
time together; but neither during the charge or after the discharge did the
electrometer or air within show the least signs of electricity. I charged
and discharged the whole arrangement in various ways, but in no case could
I obtain the least indication of an absolute charge; or of one by induction
in which the electricity of one kind had the smallest superiority in
quantity over the other. I went into the cube and lived in it, and using
lighted candles, electrometers, and all other tests of electrical states, I
could not find the least influence upon them, or indication of any thing
particular given by them, though all the time the outside of the cube was
powerfully charged, and large sparks and brushes were darting off from
every part of its outer surface. The conclusion I have come to is, that
non-conductors, as well as conductors, have never yet had an absolute and
independent charge of one electricity communicated to them, and that to all
appearance such a state of matter is impossible.
1175. There is another view of this question which may be taken under the
supposition of the existence of an electric fluid or fluids. It may be
impossible to have one fluid or state in a free condition without its
producing by induction the other, and yet possible to have cases in which
an isolated portion of matter in one condition being uncharged, shall, by a
change of state, evolve one electricity or the other: and though such
evolved electricity might immediately induce the opposite state in its
neighbourhood, yet the mere evolution of one electricity without the other
in the _first instance_, would be a very important fact in the theories
which assume a fluid or fluids; these theories as I understand them
assigning not the slightest reason why such an effect should not occur.
1176. But on searching for such cases I cannot find one. Evolution by
friction, as is well known, gives both powers in equal proportion. So does
evolution by chemical action, notwithstanding the great diversity of bodies
which may be employed, and the enormous quantity of electricity which can
in this manner be evolved (371. 376. 861. 868. 961.). The more promising
cases of change of state, whether by evaporation, fusion, or the reverse
processes, still give both forms of the power in _equal_ proportion; and
the cases of splitting of mica and other crystals, the breaking of sulphur,
&c., are subject to the same law of limitation.
1177. As far as experiment has proceeded, it appears, therefore, impossible
either to evolve or make disappear one electric force without equal and
corresponding change in the other. It is also equally impossible
experimentally to charge a portion of matter with one electric force
independently of the other. Charge always implies _induction_, for it can
in no instance be effected without; and also the presence of the _two_
forms of power, equally at the moment of the development and afterwards.
There is no _absolute_ charge of matter with one fluid; no latency of a
single electricity. This though a negative result is an exceedingly
important one, being probably the consequence of a natural impossibility,
which will become clear to us when we understand the true condition and
theory of the electric power.
1178. The preceding considerations already point to the following
conclusions: bodies cannot be charged absolutely, but only relatively, and
by a principle which is the same with that of _induction_. All _charge_ is
sustained by induction. All phenomena of _intensity_ include the principle
of induction. All _excitation_ is dependent on or directly related to
induction. All _currents_ involve previous intensity and therefore previous
induction. INDUCTION appears to be the essential function both the first
development and the consequent phenomena of electricity.
¶ iii. _Electrometer and inductive apparatus employed._
1179. Leaving for a time the further consideration of the preceding facts
until they can be collated with other results bearing directly on the great
question of the nature of induction, I will now describe the apparatus I
have had occasion to use; and in proportion to the importance of the
principles sought to be established is the necessity of doing this so
clearly, as to leave no doubt of the results behind.
1180. _Electrometer._--The measuring instrument I have employed has been
the torsion balance electrometer of Coulomb, constructed, generally,
according to his directions[A], but with certain variations and additions,
which I will briefly describe. The lower part was a glass cylinder eight
inches in height and eight inches in diameter; the tube for the torsion
thread was seventeen inches in length. The torsion thread itself was not of
metal, but glass, according to the excellent suggestion of the late Dr.
Ritchie[B]. It was twenty inches in length, and of such tenuity that when
the shell-lac lever and attached ball, &c. were connected with it, they
made about ten vibrations in a minute. It would bear torsion through four
revolutions or 1440°, and yet, when released, return accurately to its
position; probably it would have borne considerably more than this without
injury. The repelled ball was of pith, gilt, and was 0.3 of an inch in
diameter. The horizontal stem or lever supporting it was of shell-lac,
according to Coulomb's direction, the arm carrying the ball being 2.4
inches long, and the other only 1.2 inches: to this was attached the vane,
also described by Coulomb, which I found to answer admirably its purpose of
quickly destroying vibrations. That the inductive action within the
electrometer might be uniform in all positions of the repelled ball and in
all states of the apparatus, two bands of tin foil, about an inch wide
each, were attached to the inner surface of the glass cylinder, going
entirely round it, at the distance of 0.4 of an inch from each other, and
at such a height that the intermediate clear surface was in the same
horizontal plane with the lever and ball. These bands were connected with
each other and with the earth, and, being perfect conductors, always
exerted a uniform influence on the electrified balls within, which the
glass surface, from its irregularity of condition at different times, I
found, did not. For the purpose of keeping the air within the electrometer
in a constant state as to dryness, a glass dish, of such size as to enter
easily within the cylinder, had a layer of fused potash placed within it,
and this being covered with a disc of fine wire-gauze to render its
inductive action uniform at all parts, was placed within the instrument at
the bottom and left there.
[A] Mémoires de l'Académie, 1785, p. 570.
[B] Philosophical Transactions, 1830.
1181. The moveable ball used to take and measure the portion of electricity
under examination, and which may be called the _repelling_, or the
_carrier_, ball, was of soft alder wood, well and smoothly gilt. It was
attached to a fine shell-lac stem, and introduced through a hole into the
electrometer according to Coulomb's method: the stem was fixed at its upper
end in a block or vice, supported on three short feet; and on the surface
of the glass cover above was a plate of lead with stops on it, so that when
the carrier ball was adjusted in its right position, with the vice above
bearing at the same time against these stops, it was perfectly easy to
bring away the carrier-ball and restore it to its place again very
accurately, without any loss of time.
1182. It is quite necessary to attend to certain precautions respecting
these balls. If of pith alone they are bad; for when very dry, that
substance is so imperfect a conductor that it neither receives nor gives a
charge freely, and so, after contact with a charged conductor, it is liable
to be in an uncertain condition. Again, it is difficult to turn pith so
smooth as to leave the ball, even when gilt, so free from irregularities of
form, as to retain its charge undiminished for a considerable length of
time. When, therefore, the balls are finally prepared and gilt they should
be examined; and being electrified, unless they can hold their charge with
very little diminution for a considerable time, and yet be discharged
instantly and perfectly by the touch of an uninsulated conductor, they
should be dismissed.
1183. It is, perhaps, unnecessary to refer to the graduation of the
instrument, further than to explain how the observations were made. On a
circle or ring of paper on the outside of the glass cylinder, fixed so as
to cover the internal lower ring of tinfoil, were marked four points
corresponding to angles of 90°; four other points exactly corresponding to
these points being marked on the upper ring of tinfoil within. By these and
the adjusting screws on which the whole instrument stands, the glass
torsion thread could be brought accurately into the centre of the
instrument and of the graduations on it. From one of the four points on the
exterior of the cylinder a graduation of 90° was set off, and a
corresponding graduation was placed upon the upper tinfoil on the opposite
side of the cylinder within; and a dot being marked on that point of the
surface of the repelled ball nearest to the side of the electrometer, it
was easy, by observing the line which this dot made with the lines of the
two graduations just referred to, to ascertain accurately the position of
the ball. The upper end of the glass thread was attached, as in Coulomb's
original electrometer, to an index, which had its appropriate graduated
circle, upon which the degree of torsion was ultimately to be read off.
1184. After the levelling of the instrument and adjustment of the glass
thread, the blocks which determine the place of the _carrier ball_ are to
be regulated (1181.) so that, when the carrier arrangement is placed
against them, the centre of the ball may be in the radius of the instrument
corresponding to 0° on the lower graduation or that on the side of the
electrometer, and at the same level and distance from the centre as the
_repelled ball_ on the suspended torsion lever. Then the torsion index is
to be turned until the ball connected with it (the repelled ball) is
accurately at 30°, and finally the graduated arc belonging to the torsion
index is to be adjusted so as to bring 0° upon it to the index. This state
of the instrument was adopted as that which gave the most direct expression
of the experimental results, and in the form having fewest variable errors;
the angular distance of 30° being always retained as the standard distance
to which the balls were in every case to be brought, and the whole of the
torsion being read off at once on the graduated circle above. Under these
circumstances the distance of the balls from each other was not merely the
same in degree, but their position in the instrument, and in relation to
every part of it, was actually the same every time that a measurement was
made; so that all irregularities arising from slight difference of form and
action in the instrument and the bodies around were avoided. The only
difference which could occur in the position of anything within, consisted
in the deflexion of the torsion thread from a vertical position, more or
less, according to the force of repulsion of the balls; but this was so
slight as to cause no interfering difference in the symmetry of form within
the instrument, and gave no error in the amount of torsion force indicated
on the graduation above.
1185. Although the constant angular distance of 30° between the centres of
the balls was adopted, and found abundantly sensible, for all ordinary
purposes, yet the facility of rendering the instrument far more sensible by
diminishing this distance was at perfect command; the results at different
distances being very easily compared with each other either by experiment,
or, as they are inversely as the squares of the distances, by calculation.
1186. The Coulomb balance electrometer requires experience to be
understood; but I think it a very valuable instrument in the hands of those
who will take pains by practice and attention to learn the precautions
needful in its use. Its insulating condition varies with circumstances, and
should be examined before it is employed in experiments. In an ordinary and
fair condition, when the balls were so electrified as to give a repulsive
torsion force of 100° at the standard distance of 30°, it took nearly four
hours to sink to 50° at the same distance; the average loss from 400° to
300° being at the rate of 2°.7 per minute, from 300° to 200° of 1°.7 per
minute, from 200° to 100° of 1°.3 per minute, and from 100° to 50° of 0°.87
per minute. As a complete measurement by the instrument may be made in much
less than a minute, the amount of loss in that time is but small, and can
easily be taken into account.
1187. _The inductive apparatus._--My object was to examine inductive action
carefully when taking place through different media, for which purpose it
was necessary to subject these media to it in exactly similar
circumstances, and in such quantities as should suffice to eliminate any
variations they might present. The requisites of the apparatus to be
constructed were, therefore, that the inducing surfaces of the conductors
should have a constant form and state, and be at a constant distance from
each other; and that either solids, fluids, or gases might be placed and
retained between these surfaces with readiness and certainty, and for any
length of time.
1188. The apparatus used may be described in general terms as consisting of
two metallic spheres of unequal diameter, placed, the smaller within the
larger, and concentric with it; the interval between the two being the
space through which the induction was to take place. A section of it is
given (Plate VII. fig. 104.) on a scale of one-half: _a, a_ are the two
halves of a brass sphere, with an air-tight joint at _b_, like that of the
Magdeburg hemispheres, made perfectly flush and smooth inside so as to
present no irregularity; _c_ is a connecting piece by which the apparatus
is joined to a good stop-cock _d_, which is itself attached either to the
metallic foot _e_, or to an air-pump. The aperture within the hemisphere at
_f_ is very small: _g_ is a brass collar fitted to the upper hemisphere,
through which the shell-lac support of the inner ball and its stem passes;
_h_ is the inner ball, also of brass; it screws on to a brass stem _i_,
terminated above by a brass ball B, _l, l_ is a mass of shell-lac, moulded
carefully on to _i_, and serving both to support and insulate it and its
balls _h_, B. The shell-lac stem _l_ is fitted into the socket _g_, by a
little ordinary resinous cement, more fusible than shell-lac, applied at
_mm_ in such a way as to give sufficient strength and render the apparatus
air-tight there, yet leave as much as possible of the lower part of the
shell-lac stem untouched, as an insulation between the ball _h_ and the
surrounding sphere _a, a_. The ball _h_ has a small aperture at _n_, so
that when the apparatus is exhausted of one gas and filled with another,
the ball _h_ may itself also be exhausted and filled, that no variation of
the gas in the interval _o_ may occur during the course of an experiment.
1189. It will be unnecessary to give the dimensions of all the parts, since
the drawing is to a scale of one-half: the inner ball has a diameter 2.33
inches, and the surrounding sphere an internal diameter of 3.57 inches.
Hence the width of the intervening space, through which the induction is to
take place, is 0.62 of an inch; and the extent of this place or plate, i.e.
the surface of a medium sphere, may be taken as twenty-seven square inches,
a quantity considered as sufficiently large for the comparison of different
substances. Great care was taken in finishing well the inducing surfaces of
the ball _h_ and sphere _a, a_; and no varnish or lacquer was applied to
them, or to any part of the metal of the apparatus.
1190. The attachment and adjustment of the shell-lac stem was a matter
requiring considerable care, especially as, in consequence of its cracking,
it had frequently to be renewed. The best lac was chosen and applied to the
wire _i_, so as to be in good contact with it everywhere, and in perfect
continuity throughout its own mass. It was not smaller than is given by
scale in the drawing, for when less it frequently cracked within a few
hours after it was cold. I think that very slow cooling or annealing
improved its quality in this respect. The collar _g_ was made as thin as
could be, that the lac might be as wide there as possible. In order that at
every re-attachment of the stem to the upper hemisphere the ball _h_ might
have the same relative position, a gauge _p_ (fig. 105.) was made of wood,
and this being applied to the ball and hemisphere whilst the cement at _m_
was still soft, the bearings of the ball at _qq_, and the hemisphere at
_rr_, were forced home, and the whole left until cold. Thus all difficulty
in the adjustment of the ball in the sphere was avoided.
1191. I had occasion at first to attach the stem to the socket by other
means, as a band of paper or a plugging of white silk thread; but these
were very inferior to the cement, interfering much with the insulating
power of the apparatus.
1192. The retentive power of this apparatus was, when in good condition,
better than that of the electrometer (1186.), i.e. the proportion of loss
of power was less. Thus when the apparatus was electrified, and also the
balls in the electrometer, to such a degree, that after the inner ball had
been in contact with the top _k_ of the ball of the apparatus, it caused a
repulsion indicated by 600° of torsion force, then in falling from 600° to
400° the average loss was 8°.6 per minute; from 400° to 300° the average
loss was 2°.6 per minute; from 300° to 200° it was 1°.7 per minute; from
200° to 170° it was 1° per minute. This was after the apparatus had been
charged for a short time; at the first instant of charging there is an
apparent loss of electricity, which can only be comprehended hereafter
(1207. 1250.).
1193. When the apparatus loses its insulating power suddenly, it is almost
always from a crack near to or within the brass socket. These cracks are
usually transverse to the stem. If they occur at the part attached by
common cement to the socket, the air cannot enter, and thus constituting
vacua, they conduct away the electricity and lower the charge, as fast
almost as if a piece of metal had been introduced there. Occasionally stems
in this state, being taken out and cleared from the common cement, may, by
the careful application of the heat of a spirit-lamp, be so far softened
and melted as to restore the perfect continuity of the parts; but if that
does not succeed in replacing things in a good condition, the remedy is a
new shell-lac stem.
1194. The apparatus when in order could easily be exhausted of air and
filled with any given gas; but when that gas was acid or alkaline, it could
not properly be removed by the air-pump, and yet required to be perfectly
cleared away. In such cases the apparatus was opened and emptied of gas;
and with respect to the inner ball _h_, it was washed out two or three
times with distilled water introduced at the screw-hole, and then being
heated above 212°, air was blown through to render the interior perfectly
dry.
1195. The inductive apparatus described is evidently a Leyden phial, with
the advantage, however, of having the _dielectric_ or insulating medium
changed at pleasure. The balls _h_ and B, with the connecting wire _i_,
constitute the charged conductor, upon the surface of which all the
electric force is resident by virtue of induction (1178.). Now though the
largest portion of this induction is between the ball _h_ and the
surrounding sphere _aa_, yet the wire _i_ and the ball B determine a part
of the induction from their surfaces towards the external surrounding
conductors. Still, as all things in that respect remain the same, whilst
the medium within at _oo_, may be varied, any changes exhibited by the
whole apparatus will in such cases depend upon the variations made in the
interior; and these were the changes I was in search of, the negation or
establishment of such differences being the great object of my inquiry. I
considered that these differences, if they existed, would be most
distinctly set forth by having two apparatus of the kind described,
precisely similar in every respect; and then, _different insulating media_
being within, to charge one and measure it, and after dividing the charge
with the other, to observe what the ultimate conditions of both were. If
insulating media really had any specific differences in favouring or
opposing inductive action through them, such differences, I conceived,
could not fail of being developed by such a process.
1196. I will wind up this description of the apparatus, and explain the
precautions necessary to their use, by describing the form and order of the
experiments made to prove their equality when both contained common air. In
order to facilitate reference I will distinguish the two by the terms App.
i. and App. ii.
1197. The electrometer is first to be adjusted and examined (1184.), and
the app. i. and ii. are to be perfectly discharged. A Leyden phial is to be
charged to such a degree that it would give a spark of about one-sixteenth
or one-twentieth of an inch in length between two balls of half an inch
diameter; and the carrier ball of the electrometer being charged by this
phial, is to be introduced into the electrometer, and the lever ball
brought by the motion of the torsion index against it; the charge is thus
divided between the balls, and repulsion ensues. It is useful then to bring
the repelled ball to the standard distance of 30° by the motion of the
torsion index, and observe the force in degrees required for this purpose;
this force will in future experiments be called _repulsion of the balls_.
1198. One of the inductive apparatus, as, for instance, app. i., is now to
be charged from the Leyden phial, the latter being in the state it was in
when used to charge the balls; the carrier ball is to be brought into
contact with the top of its upper ball (_k_, fig. 104.), then introduced
into the electrometer, and the repulsive force (at the distance of 30°)
measured. Again, the carrier should be applied to the app. i. and the
measurement repeated; the apparatus i. and ii. are then to be joined, so as
to _divide_ the charge, and afterwards the force of each measured by the
carrier ball, applied as before, and the results carefully noted. After
this both i. and ii. are to be discharged; then app. ii. charged, measured,
divided with app. i., and the force of each again measured and noted. If in
each case the half charges of app. i. and ii. are equal, and are together
equal to the whole charge before division, then it may be considered as
proved that the two apparatus are precisely equal in power, and fit to be
used in cases of comparison between different insulating media or
_dielectrics_.
1199. But the _precautions_ necessary to obtain accurate results are
numerous. The apparatus i. and ii. must always be placed on a thoroughly
uninsulating medium. A mahogany table, for instance, is far from
satisfactory in this respect, and therefore a sheet of tinfoil, connected
with an extensive discharging train (292.), is what I have used. They must
be so placed also as not to be too near each other, and yet equally exposed
to the inductive influence of surrounding objects; and these objects,
again, should not be disturbed in their position during an experiment, or
else variations of induction upon the external ball B of the apparatus may
occur, and so errors be introduced into the results. The carrier ball, when
receiving its portion of electricity from the apparatus, should always be
applied at the same part of the ball, as, for instance, the summit _k_, and
always in the same way; variable induction from the vicinity of the head,
hands, &c. being avoided, and the ball after contact being withdrawn
upwards in a regular and constant manner.
1200. As the stem had occasionally to be changed (1190.), and the change
might occasion slight variations in the position of the ball within, I made
such a variation purposely, to the amount of an eighth of an inch (which is
far more than ever could occur in practice), but did not find that it
sensibly altered the relation of the apparatus, or its inductive condition
_as a whole_. Another trial of the apparatus was made as to the effect of
dampness in the air, one being filled with very dry air, and the other with
air from over water. Though this produced no change in the result, except
an occasional tendency to more rapid dissipation, yet the precaution was
always taken when working with gases (1290.) to dry them perfectly.
1201. It is essential that the interior of the apparatus should be
perfectly free from _dust or small loose particles_, for these very rapidly
lower the charge and interfere on occasions when their presence and action
would hardly be expected. To breathe on the interior of the apparatus and
wipe it out quietly with a clean silk handkerchief, is an effectual way of
removing them; but then the intrusion of other particles should be
carefully guarded against, and a dusty atmosphere should for this and
several other reasons be avoided.
1202. The shell-lac stem requires occasionally to be well-wiped, to remove,
in the first instance, the film of wax and adhering matter which is upon
it; and afterwards to displace dirt and dust which will gradually attach to
it in the course of experiments. I have found much to depend upon this
precaution, and a silk handkerchief is the best wiper.
1203. But wiping and some other circumstances tend to give a charge to the
surface of the shell-lac stem. This should be removed, for, if allowed to
remain, it very seriously affects the degree of charge given to the carrier
ball by the apparatus (1232.). This condition of the stem is best observed
by discharging the apparatus, applying the carrier ball to the stem,
touching it with the finger, insulating and removing it, and examining
whether it has received any charge (by induction) from the stem; if it has,
the stem itself is in a charged state. The best method of removing the
charge I have found to be, to cover the finger with a single fold of a silk
handkerchief, and breathing on the stem, to wipe it immediately after with
the finger; the ball B and its connected wire, &c. being at the same time
_uninsulated_: the wiping place of the silk must not be changed; it then
becomes sufficiently damp not to excite the stem, and is yet dry enough to
leave it in a clean and excellent insulating condition. If the air be
dusty, it will be found that a single charge of the apparatus will bring on
an electric state of the outside of the stem, in consequence of the
carrying power of the particles of dust; whereas in the morning, and in a
room which has been left quiet, several experiments can be made in
succession without the stem assuming the least degree of charge.
1204. Experiments should not be made by candle or lamp light except with
much care, for flames have great and yet unsteady powers of affecting and
dissipating electrical charges.
1205. As a final observation on the state of the apparatus, they should
retain their charges well and uniformly, and alike for both, and at the
same time allow of a perfect and instantaneous discharge, giving afterwards
no charge to the carrier ball, whatever part of the ball B it may be
applied to (1218.).
1206. With respect to the balance electrometer, all the precautions that
need be mentioned, are, that the carrier ball is to be preserved during the
first part of an experiment in its electrified state, the loss of
electricity which would follow upon its discharge being avoided; and that
in introducing it into the electrometer through the hole in the glass plate
above, care should be taken that it do not touch, or even come near to, the
edge of the glass.
1207. When the whole charge in one apparatus is divided between the two,
the gradual fall, apparently from dissipation, in the apparatus which has
_received_ the half charge is greater than in the one _originally_ charged.
This is due to a peculiar effect to be described hereafter (1250. 1251.),
the interfering influence of which may be avoided to a great extent by
going through the steps of the process regularly and quickly; therefore,
after the original charge has been measured, in app. i. for instance, i.
and ii. are to be symmetrically joined by their balls B, the carrier
touching one of these balls at the same time; it is first to be removed,
and then the apparatus separated from each other; app. ii. is next quickly
to be measured by the carrier, then app. i.; lastly, ii. is to be
discharged, and the discharged carrier applied to it to ascertain whether
any residual effect is present (1205.), and app. i. being discharged is
also to be examined in the same manner and for the same purpose.
1208. The following is an example of the division of a charge by the two
apparatus, air being the dielectric in both of them. The observations are
set down one under the other in the order in which they were taken, the
left-hand numbers representing the observations made on app. i., and the
right-hand numbers those on app. ii. App. i. is that which was originally
charged, and after two measurements, the charge was divided with app. ii.
App. i. App. ii.
Balls 160°
. . . . 0°
254° . . . .
250 . . . .
divided and instantly taken
. . . . 122
124 . . . .
1 . . . . after being discharged.
. . . . 2 after being discharged.
1209. Without endeavouring to allow for the loss which must have been
gradually going on during the time of the experiment, let us observe the
results of the numbers as they stand. As 1° remained in app. i. in an
undischargeable state, 249° may be taken as the utmost amount of the
transferable or divisible charge, the half of which is 124°.5. As app. ii.
was free of charge in the first instance, and immediately after the
division was found with 122°, this amount _at least_ may be taken as what
it had received. On the other hand 124° minus 1°, or 123°, may be taken as
the half of the transferable charge retained by app. i. Now these do not
differ much from each other, or from 124°.5, the half of the full amount of
transferable charge; and when the gradual loss of charge evident in the
difference between 254° and 250° of app. i. is also taken into account,
there is every reason to admit the result as showing an equal division of
charge, _unattended by any disappearance of power_ except that due to
dissipation.
1210. I will give another result, in which app. ii. was first charged, and
where the residual action of that apparatus was greater than in the former
case.
App. i. App. ii.
Balls 150°
. . . . 152°
. . . . 148
divided and instantly taken
70° . . . .
. . . . 78
. . . . 5 immediately after discharge.
0 . . . . immediately after discharge.
1211. The transferable charge being 148° - 5°, its half is 71°.5, which is
not far removed from 70°, the half charge of i.; or from 73°, the half
charge of ii.: these half charges again making up the sum of 143°, or just
the amount of the whole transferable charge. Considering the errors of
experiment, therefore, these results may again be received as showing that
the apparatus were equal in inductive capacity, or in their powers of
receiving charges.
1212. The experiments were repeated with charges of negative electricity
with the same general results.
1213. That I might be sure of the sensibility and action of the apparatus,
I made such a change in one as ought upon principle to increase its
inductive force, i.e. I put a metallic lining into the lower hemisphere of
app. i., so as to diminish the thickness of the intervening air in that
part, from 0.62 to 0.435 of an inch: this lining was carefully shaped and
rounded so that it should not present a sudden projection within at its
edge, but a gradual transition from the reduced interval in the lower part
of the sphere to the larger one in the upper.
1214. This change immediately caused app. i. to produce effects indicating
that it had a greater aptness or capacity for induction than app. ii. Thus,
when a transferable charge in app. ii. of 469° was divided with app. i.,
the former retained a charge of 225°, whilst the latter showed one of 227°,
i.e. the former had lost 244° in communicating 227° to the latter: on the
other hand, when app. i. had a transferable charge in it of 381° divided by
contact with app. ii., it lost 181° only, whilst it gave to app. ii. as
many as 194:--the sum of the divided forces being in the first instance
_less_, and in the second instance _greater_ than the original undivided
charge. These results are the more striking, as only one-half of the
interior of app. i. was modified, and they show that the instruments are
capable of bringing out differences in inductive force from amongst the
errors of experiment, when these differences are much less than that
produced by the alteration made in the present instance.
¶ iv. _Induction in curved lines._
1215. Amongst those results deduced from the molecular view of induction
(1166.), which, being of a peculiar nature, are the best tests of the truth
or error of the theory, the expected action in curved lines is, I think,
the most important at present; for, if shown to take place in an
unexceptionable manner, I do not see how the old theory of action at a
distance and in straight lines can stand, or how the conclusion that
ordinary induction is an action of contiguous particles can be resisted.
1216. There are many forms of old experiments which might be quoted as
favourable to, and consistent with the view I have adopted. Such are most
cases of electro-chemical decomposition, electrical brushes, auras, sparks,
&c.; but as these might be considered equivocal evidence, inasmuch as they
include a current and discharge, (though they have long been to me
indications of prior molecular action (1230.)) I endeavoured to devise such
experiments for first proofs as should not include transfer, but relate
altogether to the pure simple inductive action of statical electricity.
1217. It was also of importance to make these experiments in the simplest
possible manner, using not more than one insulating medium or dielectric at
a time, lest differences of slow conduction should produce effects which
might erroneously be supposed to result from induction in curved lines. It
will be unnecessary to describe the steps of the investigation minutely; I
will at once proceed to the simplest mode of proving the facts, first in
air and then in other insulating media.
1218. A cylinder of solid shell-lac, 0.9 of an inch in diameter and seven
inches in length, was fixed upright in a wooden foot (fig. 106.): it was
made concave or cupped at its upper extremity so that a brass ball or other
small arrangement could stand upon it. The upper half of the stem having
been excited _negatively_ by friction with warm flannel, a brass ball, B, 1
inch in diameter, was placed on the top, and then the whole arrangement
examined by the carrier ball and Coulomb's electrometer (1180. &c.). For
this purpose the balls of the electrometer were charged _positively_ to
about 360°, and then the carrier being applied to various parts of the ball
B, the two were uninsulated whilst in contact or in position, then
insulated[A], separated, and the charge of the carrier examined as to its
nature and force. Its electricity was always positive, and its force at the
different positions _a, b, c, d,_ &c. (figs. 106. and 107.) observed in
succession, was as follows:
at _a_ above 1000°
_b_ it was 149
_c_ 270
_d_ 512
_b_ 130
[A] It can hardly be necessary for me to say here, that whatever
general state the carrier ball acquired in any place where it was
uninsulated and then insulated, it retained on removal from that
place, notwithstanding that it might pass through other places that
would have given to it, if uninsulated, a different condition.
1219. To comprehend the full force of these results, it must first be
understood, that all the charges of the ball B and the carrier are charges
by induction, from the action of the excited surface of the shell-lac
cylinder; for whatever electricity the ball B received by _communication_
from the shell-lac, either in the first instance or afterwards, was removed
by the uninsulating contacts, only that due to induction remaining; and
this is shown by the charges taken from the ball in this its uninsulated
state being always positive, or of the contrary character to the
electricity of the shell-lac. In the next place, the charges at _a_, _c_,
and _d_ were of such a nature as might be expected from an inductive action
in straight lines, but that obtained at _b_ is _not so_: it is clearly a
charge by induction, but _induction_ in _a curved line_; for the carrier
ball whilst applied to _b_, and after its removal to a distance of six
inches or more from B, could not, in consequence of the size of B, be
connected by a straight line with any part of the excited and inducing
shell-lac.
1220. To suppose that the upper part of the _uninsulated_ ball B, should in
some way be retained in an electrified state by that portion of the surface
of the ball which is in sight of the shell-lac, would be in opposition to
what we know already of the subject. Electricity is retained upon the
surface of conductors only by induction (1178.); and though some persons
may not be prepared as yet to admit this with respect to insulated
conductors, all will as regards uninsulated conductors like the ball B; and
to decide the matter we have only to place the carrier ball at _e_ (fig.
107.), so that it shall not come in contact with B, uninsulate it by a
metallic rod descending perpendicularly, insulate it, remove it, and
examine its state; it will be found charged with the same kind of
electricity as, and even to a _higher degree_ (1224.) than, if it had been
in contact with the summit of B.
1221. To suppose, again, that induction acts in some way _through or
across_ the metal of the ball, is negatived by the simplest considerations;
but a fact in proof will be better. If instead of the ball B a small disc
of metal be used, the carrier may be charged at, or above the middle of its
upper surface: but if the plate be enlarged to about 1-1/2 or 2 inches in
diameter, C (fig. 108.), then no charge will be given to the carrier at
_f_, though when applied nearer to the edge at _g_, or even _above the
middle_ at _h_, a charge will be obtained; and this is true though the
plate may be a mere thin film of gold-leaf. Hence it is clear that the
induction is not _through_ the metal, but through the surrounding air or
_dielectric_, and that in curved lines.
1222. I had another arrangement, in which a wire passing downwards through
the middle of the shell-lac cylinder to the earth, was connected with the
ball B (fig. 109.) so as to keep it in a constantly uninsulated state. This
was a very convenient form of apparatus, and the results with it were the
same as those just described.
1223. In another case the ball B was supported by a shell-lac stem,
independently of the excited cylinder of shell-lac, and at half an inch
distance from it; but the effects were the same. Then the brass ball of a
charged Leyden jar was used in place of the excited shell-lac to produce
induction; but this caused no alteration of the phenomena. Both positive
and negative inducing charges were tried with the same general results.
Finally, the arrangement was inverted in the air for the purpose of
removing every possible objection to the conclusions, but they came out
exactly the same.
1224. Some results obtained with a brass hemisphere instead of the ball B
were exceedingly interesting, It was 1.36 of an inch in diameter, (fig.
110.), and being placed on the top of the excited shell-lac cylinder, the
carrier ball was applied, as in the former experiments (1218.), at the
respective positions delineated in the figure. At _i_ the force was 112°,
at _k_ 108°, at _l_ 65°, at _m_ 35°; the inductive force gradually
diminishing, as might have been expected, to this point. But on raising the
carrier to the position _n_, the charge increased to 87°; and on raising it
still higher to _o_, the charge still further increased to 105°: at a
higher point still, _p_, the charge taken was smaller in amount, being 98°,
and continued to diminish for more elevated positions. Here the induction
fairly turned a corner. Nothing, in fact, can better show both the curved
lines or courses of the inductive action, disturbed as they are from their
rectilineal form by the shape, position, and condition of the metallic
hemisphere; and also a _lateral tension,_ so to speak, of these lines on
one another:--all depending, as I conceive, on induction being an action of
the contiguous particles of the dielectric, which being thrown into a state
of polarity and tension, are in mutual relation by their forces in all
directions.
1225. As another proof that the whole of these actions were inductive I may
state a result which was exactly what might be expected, namely, that if
uninsulated conducting matter was brought round and near to the excited
shell-lac stem, then the inductive force was directed towards it, and could
not be found on the top of the hemisphere. Removing this matter the lines
of force resumed their former direction. The experiment affords proofs of
the lateral tension of these lines, and supplies a warning to remove such
matter in repeating the above investigation.
1226. After these results on curved inductive action in air I extended the
experiments to other gases, using first carbonic acid and then hydrogen:
the phenomena were precisely those already described. In these experiments
I found that if the gases were confined in vessels they required to be very
large, for whether of glass or earthenware, the conducting power of such
materials is so great that the induction of the excited shell-lac cylinder
towards them is as much as if they were metal; and if the vessels be small,
so great a portion of the inductive force is determined towards them that
the lateral tension or mutual repulsion of the lines of force before spoken
of, (1224.) by which their inflexion is caused, is so much relieved in
other directions, that no inductive charge will be given to the carrier
ball in the positions _k, l, m, n, o, p_ (fig. 110.). A very good mode of
making the experiment is to let large currents of the gases ascend or
descend through the air, and carry on the experiments in these currents.
1227. These experiments were then varied by the substitution of a liquid
dielectric, namely, _oil of turpentine_, in place of air and gases. A dish
of thin glass well-covered with a film of shell-lac (1272.), which was
found by trial to insulate well, had some highly rectified oil of
turpentine put into it to the depth of half an inch, and being then placed
upon the top of the brass hemisphere (fig. 110.), observations were made
with the carrier ball as before (1224.). The results were the same, and the
circumstance of some of the positions being within the fluid and some
without, made no sensible difference.
1228. Lastly, I used a few solid dielectrics for the same purpose, and with
the same results. These were shell-lac, sulphur, fused and cast borate of
lead, flint glass well-covered with a film of lac, and spermaceti. The
following was the form of experiment with sulphur, and all were of the same
kind. A square plate of the substance, two inches in extent and 0.6 of an
inch in thickness, was cast with a small hole or depression in the middle
of one surface to receive the carrier ball. This was placed upon the
surface of the metal hemisphere (fig. 112.) arranged on the excited lac as
in former cases, and observations were made at _n, o, p_, and _q_. Great
care was required in these experiments to free the sulphur or other solid
substance from any charge it might previously have received. This was done
by breathing and wiping (1203.), and the substance being found free from
all electrical excitement, was then used in the experiment; after which it
was removed and again examined, to ascertain that it had received no
charge, but had acted really as a dielectric. With all these precautions
the results were the same: and it is thus very satisfactory to obtain the
curved inductive action through _solid bodies_, as any possible effect from
the translation of charged particles in fluids or gases, which some persons
might imagine to be the case, is here entirely negatived.
1229. In these experiments with solid dielectrics, the degree of charge
assumed by the carrier ball at the situations _n, o, p_ (fig. 112.), was
decidedly greater than that given to the ball at the same places when air
only intervened between it and the metal hemisphere. This effect is
consistent with what will hereafter be found to be the respective relations
of these bodies, as to their power of facilitating induction through them
(1269. 1273. 1277.).
1230. I might quote _many_ other forms of experiment, some old and some
new, in which induction in curved or contorted lines takes place, but think
it unnecessary after the preceding results; I shall therefore mention but
two. If a conductor A, (fig. 111.) be electrified, and an uninsulated
metallic ball B, or even a plate, provided the edges be not too thin, be
held before it, a small electrometer at _c_ or at _d_, uninsulated, will
give signs of electricity, opposite in its nature to that of A, and
therefore caused by induction, although the influencing and influenced
bodies cannot be joined by a right line passing through the air. Or if, the
electrometers being removed, a point be fixed at the back of the ball in
its uninsulated state as at C, this point will become luminous and
discharge the conductor A. The latter experiment is described by
Nicholson[A], who, however, reasons erroneously upon it. As to its
introduction here, though it is a case of discharge, the discharge is
preceded by induction, and that induction must be in curved lines.
[A] Encyclopædia Britannica, vol. vi. p. 504.
1231. As argument against the received theory of induction and in favour of
that which I have ventured to put forth, I cannot see how the preceding
results can be avoided. The effects are clearly inductive effects produced
by electricity, not in currents but in its statical state, and this
induction is exerted in lines of force which, though in many experiments
they may be straight, are here curved more or less according to
circumstances. I use the term _line of inductive force_ merely as a
temporary conventional mode of expressing the direction of the power in
cases of induction; and in the experiments with the hemisphere (1224.), it
is curious to see how, when certain lines have terminated on the under
surface and edge of the metal, those which were before lateral to them
_expand and open out from each other_, some bending round and terminating
their action on the upper surface of the hemisphere, and others meeting, as
it were, above in their progress outwards, uniting their forces to give an
increased charge to the carrier ball, at an _increased distance_ from the
source of power, and influencing each other so as to cause a second flexure
in the contrary direction from the first one. All this appears to me to
prove that the whole action is one of contiguous particles, related to each
other, not merely in the lines which they may be conceived to form through
the dielectric, between the _inductric_ and the _inducteous_ surfaces
(1483.), but in other lateral directions also. It is this which gives an
effect equivalent to a lateral repulsion or expansion in the lines of force
I have spoken of, and enables induction to turn a corner (1304.). The
power, instead of being like that of gravity, which causes particles to act
on each other through straight lines, whatever other particles may be
between them, is more analogous to that of a series of magnetic needles, or
to the condition of the particles considered as forming the whole of a
straight or a curved magnet. So that in whatever way I view it, and with
great suspicion of the influence of favourite notions over myself, I cannot
perceive how the ordinary theory applied to explain induction can be a
correct representation of that great natural principle of electrical
action.
1232. I have had occasion in describing the precautions necessary in the
use of the inductive apparatus, to refer to one founded on induction in
curved lines (1203.); and after the experiments already described, it will
easily be seen how great an influence the shell-lac stem may exert upon the
charge of the carrier ball when applied to the apparatus (1218.), unless
that precaution be attended to.
1233. I think it expedient, next in the course of these experimental
researches, to describe some effects due to _conduction_, obtained with
such bodies as glass, lac, sulphur, &c., which had not been anticipated.
Being understood, they will make us acquainted with certain precautions
necessary in investigating the great question of specific inductive
capacity.
1234. One of the inductive apparatus already described (1187, &c.) had a
hemispherical cup of shell-lac introduced, which being in the interval
between the inner bull and the lower hemisphere, nearly occupied the space
there; consequently when the apparatus was charged, the lac was the
dielectric or insulating medium through which the induction took place in
that part. When this apparatus was first charged with electricity (1198.)
up to a certain intensity, as 400°, measured by the COULOMB'S electrometer
(1180.), it sank much faster from that degree than if it had been
previously charged to a higher point, and had gradually fallen to 400°; or
than it would do if the charge were, by a second application, raised up
again to 400°; all other things remaining the same. Again, if after having
been charged for some time, as fifteen or twenty minutes, it was suddenly
and perfectly discharged, even the stem having all electricity removed from
it (1203.), then the apparatus being left to itself, would gradually
recover a charge, which in nine or ten minutes would rise up to 50° or 60°,
and in one instance to 80°.
1235. The electricity, which in these cases returned from an apparently
latent to a sensible state, was always of the same kind as that which had
been given by the charge. The return took place at both the inducing
surfaces; for if after the perfect discharge of the apparatus the whole was
insulated, as the inner ball resumed a positive state the outer sphere
acquired a negative condition.
1236. This effect was at once distinguished from that produced by the
excited stem acting in curved lines of induction (1203. 1232.), by the
circumstance that all the returned electricity could be perfectly and
instantly discharged. It appeared to depend upon the shell-lac within, and
to be, in some way, due to electricity evolved from it in consequence of a
previous condition into which it had been brought by the charge of the
metallic coatings or balls.
1237. To examine this state more accurately, the apparatus, with the
hemispherical cup of shell-lac in it, was charged for about forty-five
minutes to above 600° with positive electricity at the balls _h_ and B.
(fig. 104.) above and within. It was then discharged, opened, the shell-lac
taken out, and its state examined; this was done by bringing the carrier
ball near the shell-lac, uninsulating it, insulating it, and then observing
what charge it had acquired. As it would be a charge by induction, the
state of the ball would indicate the opposite state of electricity in that
surface of the shell-lac which had produced it. At first the lac appeared
quite free from any charge; but gradually its two surfaces assumed opposite
states of electricity, the concave surface, which had been next the inner
and positive ball; assuming a positive state, and the convex surface, which
had been in contact with the negative coating, acquiring a negative state;
these states gradually increased in intensity for some time.