§ 18. _On Induction (continued)._ ¶ ix. _Disruptive discharge
(continued)--Peculiarities of positive and negative discharge either as
spark or brush--Glow discharge--Dark discharge._ ¶ x. _Convection, or
carrying discharge._ ¶ xi. _Relation of a vacuum to electrical phenomena._
§ 19. _Nature of the electrical current._
Received February 22,--Read March 15, 1838.
¶ ix. _Disruptive discharge (continued)._
1480. Let us now direct our attention to the general difference of the
positive and negative disruptive discharge, with the object of tracing, as
far as possible, the cause of that difference, and whether it depends on
the charged conductors principally, or on the interposed dielectric; and as
it appears to be great in air and nitrogen (1476.), let us observe the
phenomena in air first.
1481. The general case is best understood by a reference to surfaces of
considerable size rather than to points, which involve (as a secondary
effect) the formation of currents (1562). My investigation, therefore, was
carried on with balls and terminations of different diameters, and the
following are some of the principal results.
1482. If two balls of very different dimensions, as for instance one-half
an inch, and the other three inches in diameter, be arranged at the ends of
rods so that either can be electrified by a machine and made to discharge
by sparks to the other, which is at the same time uninsulated; then, as is
well known, far longer sparks are obtained when the small ball is positive
and the large ball negative, than when the small ball is negative and the
large ball positive. In the former case, the sparks are 10 or 12 inches in
length; in the latter, an inch or an inch and a half only.
* * * * *
1483. But previous to the description of further experiments, I will
mention two words, for which with many others I am indebted to a friend,
and which I think it would be expedient to introduce and use. It is
important in ordinary inductive action, to distinguish at which charged
surface the induction originates and is sustained: i.e. if two or more
metallic balls, or other masses of matter, are in inductive relation, to
express which are charged originally, and which are brought by them into
the opposite electrical condition. I propose to call those bodies which are
originally charged, _inductric_ bodies; and those which assume the opposite
state, in consequence of the induction, _inducteous_ bodies. This
distinction is not needful because there is any difference between the sums
of the _inductric_ and the _inducteous_ forces; but principally because,
when a ball A is inductric, it not merely brings a ball B, which is
opposite to it, into an inducteous state, but also many other surrounding
conductors, though some of them may be a considerable distance off, and the
consequence is, that the balls do not bear the same precise relation to
each other when, first one, and then the other, is made the inductric ball;
though, in each case, the _same ball_ be made to assume the _same state._
1484, Another liberty which I may also occasionally take in language I will
explain and limit. It is that of calling a particular spark or brush,
_positive_ or _negative_, according as it may be considered as
_originating_ at a positive or a negative surface. We speak of the brush as
positive or negative when it shoots out from surfaces previously in those
states; and the experiments of Mr. Wheatstone go to prove that it _really
begins_ at the charged surface, and from thence extends into the air (1437.
1438.) or other dielectric. According to my view, _sparks_ also originate
or are determined at one particular spot (1370.), namely, that where the
tension first rises up to the maximum degree; and when this can be
determined, as in the simultaneous use of large and small balls, in which
case the discharge begins or is determined by the latter, I would call that
discharge which passes _at once_, a positive spark, if it was at the
positive surface that the maximum intensity was first obtained; or a
negative spark, if that necessary intensity was first obtained at the
negative surface.
* * * * *
1485. An apparatus was arranged, as in fig. 129. (Plate VIII.): A and B
were brass balls of very different diameters attached to metal rods, moving
through sockets on insulating pillars, so that the distance between the
balls could be varied at pleasure. The large ball A, 2 inches in diameter,
was connected with an insulated brass conductor, which could be rendered
positive or negative directly from a cylinder machine: the small ball B,
0.25 of an inch in diameter, was connected with a discharging train (292.)
and perfectly uninsulated. The brass rods sustaining the balls were 0.2 of
an inch in thickness.
1486. When the large ball was _positive_ and inductric (1483.), negative
sparks occurred until the interval was 0.49 of an inch; then mixed brush
and spark between that and 0.51; and from 0.52 and upwards, negative brush
alone. When the large ball was made _negative_ and inductric, then positive
spark alone occurred until the interval was as great as 1.15 inches; spark
and brush from that up to 1.55; and to have the positive brush alone, it
required an interval of at least 1.65 inches.
1487. The balls A and B were now changed for each other. Then making the
small ball B inductric _positively_, the positive sparks alone continued
only up to 0.67; spark and brush occurred from 0.68 up to 0.72; and
positive brush alone from 0.74 and upwards. Rendering the small ball B
inductric and _negative_, negative sparks alone occurred up to 0.40; then
spark and brush at 0.42; whilst from 0.44 and upwards the noisy negative
brush alone took place.
1488. We thus find a great difference as the balls are rendered inductric
or inducteous; the small ball rendered _positive_ inducteously giving a
spark nearly twice as long as that produced when it was charged positive
inductrically, and a corresponding difference, though not, under the
circumstances, to the same extent, was manifest, when it was rendered
_negative_[A].
[A] For similar experiments on different gases, see 1518.--_Dec. 1838._
1489. Other results are, that the small ball rendered positive gives a much
longer spark than when it is rendered negative, and that the small ball
rendered negative gives a brush more readily than when positive, in
relation to the effect produced by increasing the distance between the two
balls.
1490. When the interval was below 0.4 of an inch, so that the small ball
should give sparks, whether positive or negative, I could not observe that
there was any constant difference, either in their ready occurrence or the
number which passed in a given time. But when the interval was such that
the small ball when negative gave a brush, then the discharges from it, as
separate negative brushes, were far more numerous than the corresponding
discharges from it when rendered positive, whether those positive
discharges were as sparks or brushes.
1491. It is, therefore, evident that, when a ball is discharging
electricity in the form of brushes, the brushes are far more numerous, and
each contains or carries off far less electric force when the electricity
so discharged is negative, than when it is positive.
1492. In all such experiments as those described, the point of change from
spark to brush is very much governed by the working state of the electrical
machine and the size of the conductor connected with the discharging ball.
If the machine be in strong action and the conductor large, so that much
power is accumulated quickly for each discharge, then the interval is
greater at which the sparks are replaced by brushes; but the general effect
is the same[A].
[A] For similar experiments in different gases, see 1510-1517.--_Dec.
1838._
1493. These results, though indicative of very striking and peculiar
relations of the electric force or forces, do not show the relative degrees
of charge which the small ball acquires before discharge occurs, i.e. they
do not tell whether it acquires a higher condition in the negative, or in
the positive state, immediately preceding that discharge. To illustrate
this important point I arranged two places of discharge as represented, fig
130. A and D are brass balls 2 inches diameter, B and C are smaller brass
balls 0.25 of an inch in diameter; the forks L and R supporting them were
of brass wire 0.2 of an inch in diameter; the space between the large and
small ball on the same fork was 5 inches, that the two places of discharge
_n_ and _o_ might be sufficiently removed from each other's influence. The
fork L was connected with a projecting cylindrical conductor, which could
be rendered positive or negative at pleasure, by an electrical machine, and
the fork R was attached to another conductor, but thrown into an
uninsulated state by connection with a discharging train (292.). The two
intervals or places of discharge _n_ and _o_ could be varied at pleasure,
their extent being measured by the occasional introduction of a diagonal
scale. It is evident, that, as the balls A and B connected with the same
conductor are always charged at once, and that discharge may take place to
either of the balls connected with the discharging train, the intervals of
discharge _n_ and _o_ may be properly compared to each other, as respects
the influence of large and small balls when charged positively and
negatively in air.
1494. When the intervals _n_ and _o_ were each made = 0.9 of an inch, and
the balls A and B inductric _positively_, the discharge was all at _n_ from
the small ball of the conductor to the large ball of the discharging train,
and mostly by positive brush, though once by a spark. When the balls A and
B were made inductric _negatively_, the discharge was still from the same
small ball, at _n_, by a constant negative brush.
1495. I diminished the intervals _n_ and _o_ to 0.6 of an inch. When A and
B were inductric _positively_, all the discharge was at _n_ as a positive
brush: when A and B were inductric _negatively_, still all the discharge
was at _n_, as a negative brush.
1496. The facility of discharge at the positive and negative small balls,
therefore, did not appear to be very different. If a difference had
existed, there were always two small balls, one in each state, that the
discharge might happen at that most favourable to the effect. The only
difference was, that one was in the inductric, and the other in the
inducteous state, but whichsoever happened for the time to be in that
state, whether positive or negative, had the advantage.
1497. To counteract this interfering influence, I made the interval _n_ =
0.79 and interval _o_ = 0.58 of an inch. Then, when the balls A and B were
_inductric positive_, the discharge was about equal at both intervals.
When, on the other hand, the balls A and B were inductric _negative_, there
was discharge, still at both, but most at _n_, as if the small ball
_negative_ could discharge a little easier than the same ball _positive_.
1498. The small balls and terminations used in these and similar
experiments may very correctly be compared, in their action, to the same
balls and ends when electrified in free air at a much greater distance from
conductors, than they were in those cases from each other. In the first
place, the discharge, even when as a spark, is, according to my view,
determined, and, so to speak, begins at a spot on the surface of the small
ball (1374.), occurring when the intensity there has risen up to a certain
maximum degree (1370.); this determination of discharge at a particular
spot first, being easily traced from the spark into the brush, by
increasing the distance, so as, at last, even to render the time evident
which is necessary for the production of the effect (1436. 1438.). In the
next place, the large balls which I have used might be replaced by larger
balls at a still greater distance, and so, by successive degrees, may be
considered as passing into the sides of the rooms; these being under
general circumstances the inducteous bodies, whilst the small ball rendered
either positive or negative is the inductric body.
1499. But, as has long been recognised, the small ball is only a blunt end,
and, electrically speaking, a point only a small ball; so that when a point
or blunt end is throwing out its brushes into the air, it is acting exactly
as the small balls have acted in the experiments already described, and by
virtue of the same properties and relations.
1500. It may very properly be said with respect to the experiments, that
the large negative ball is as essential to the discharge as the small
positive ball, and also that the large negative ball shows as much
superiority over the large positive ball (which is inefficient in causing a
spark from its opposed small negative ball) as the small positive ball does
over the small negative ball; and probably when we understand the real
cause of the difference, and refer it rather to the condition of the
particles of the dielectric than to the sizes of the conducting balls, we
may find much importance in such an observation. But for the present, and
whilst engaged in investigating the point, we may admit, what is the fact,
that the forces are of higher intensity at the surfaces of the smaller
balls than at those of the larger (1372. 1374.); that the former,
therefore, determine the discharge, by first rising up to that exalted
condition which is necessary for it; and that, whether brought to this
condition by induction towards the walls of a room or the large balls I
have used, these may fairly be compared one with the other in their
influence and actions.
1501. The conclusions I arrive at are: first, that when two equal small
conducting surfaces equally placed in air are electrified, one positively
and the other negatively, that which is negative can discharge to the air
at a tension a little lower than that required for the positive ball:
second, that when discharge does take place, much more passes at each time
from the positive than from the negative surface (1491.). The last
conclusion is very abundantly proved by the optical analysis of the
positive and negative brushes already described (1468.), the latter set of
discharges being found to recur five or six times oftener than the
former[A].
[A] A very excellent mode of examining the relation of small positive
and negative surfaces would be by the use of drops of gum water,
solutions, or other liquids. See onwards (1581. 1593.).
1502. If, now, a small ball be made to give brushes or brushy sparks by a
powerful machine, we can, in some measure, understand and relate the
difference perceived when it is rendered positive or negative. It is known
to give when positive a much larger and more powerful spark than when
negative, and with greater facility (1482.): in fact, the spark, although
it takes away so much more electricity at once, commences at a tension
higher only in a small degree, if at all. On the other hand, if rendered
negative, though discharge may commence at a lower degree, it continues but
for a very short period, very little electricity passing away each time.
These circumstances are directly related; for the extent to which the
positive spark can reach, and the size and extent of the positive brush,
are consequences of the capability which exists of much electricity passing
off at one discharge from the positive surface (1468. 1501.).
1503. But to refer these effects only to the form and size of the
conductor, would, according to my notion of induction, be a very imperfect
mode of viewing the whole question (1523. 1600.). I apprehend that the
effects are due altogether to the mode in which the particles of the
interposed dielectric polarize, and I have already given some experimental
indications of the differences presented by different electrics in this
respect (1475. 1476.). The modes of polarization, as I shall have occasion
hereafter to show, may be very diverse in different dielectrics. With
respect to common air, what seems to be the consequence of a superiority in
the positive force at the surface of the small ball, may be due to the more
exalted condition of the negative polarity of the particles of air, or of
the nitrogen in it (the negative part being, perhaps, more compressed,
whilst the positive part is more diffuse, or _vice versa_ (1687. &c.)); for
such a condition could determine certain effects at the positive ball which
would not take place to the same degree at the negative ball, just as well
as if the positive ball had possessed some special and independent power of
its own.
1504. The opinion, that the effects are more likely to be dependent upon
the dielectric than the ball, is supported by the character of the two
discharges. If a small positive ball be throwing off brushes with
ramifications ten inches long, how can the ball affect that part of a
ramification which is five inches from it? Yet the portion beyond that
place has the same character as that preceding it, and no doubt has that
character impressed by the same general principle and law. Looking upon the
action of the contiguous particles of a dielectric as fully proved, I see,
in such a ramification, a propagation of discharge from particle to
particle, each doing for the one next it what was done for it by the
preceding particle, and what was done for the first particle by the charged
metal against which it was situated.
1505. With respect to the general condition and relations of the positive
and negative brushes in dense or rare air, or in other media and gases, if
they are produced at different times and places they are of course
independent of each other. But when they are produced from opposed ends or
balls at the same time, in the same vessel of gas (1470. 1477.), they are
frequently related; and circumstances may be so arranged that they shall be
isochronous, occurring in equal numbers in equal times; or shall occur in
multiples, i.e. with two or three negatives to one positive; or shall
alternate, or be quite irregular. All these variations I have witnessed;
and when it is considered that the air in the vessel, and also the glass of
the vessel, can take a momentary charge, it is easy to comprehend their
general nature and cause.
* * * * *
1506. Similar experiments to those in air (1485. 1493.) were made in
different gases, the results of which I will describe as briefly as
possible. The apparatus is represented fig. 131, consisting of a bell-glass
eleven inches in diameter at the widest part, and ten and a half inches
high up to the bottom of the neck. The balls are lettered, as in fig. 130,
and are in the same relation to each other; but A and B were on separate
sliding wires, which, however, were generally joined by a cross wire, _w_,
above, and that connected with the brass conductor, which received its
positive or negative charge from the machine. The rods of A and B were
graduated at the part moving through the stuffing-box, so that the
application of a diagonal scale applied there, told what was the distance
between these balls and those beneath them. As to the position of the balls
in the jar, and their relation to each other, C and D were three and a
quarter inches apart, their height above the pump plate five inches, and
the distance between any of the balls and the glass of the jar one inch and
three quarters at least, and generally more. The balls A and D were two
inches in diameter, as before (1493.); the balls B and C only 0.15 of an
inch in diameter.
Another apparatus was occasionally used in connection with that just
described, being an open discharger (fig. 132.), by which a comparison of
the discharge in air and that in gases could be obtained. The balls E and
F, each 0.6 of an inch in diameter, were connected with sliding rods and
other balls, and were insulated. When used for comparison, the brass
conductor was associated at the same time with the balls A and B of figure
131 and ball E of this apparatus (fig. 132.); whilst the balls C, D and F
were connected with the discharging train.
1507. I will first tabulate the results as to the _restraining power_ of
the gases over discharge. The balls A and C (fig. 131.) were thrown out of
action by distance, and the effects at B and D, or the interval _n_ in the
gas, compared with those at the interval _p_ in the air, between E and F
(fig. 132.). The Table sufficiently explains itself. It will be understood
that all discharge was in the air, when the interval there was less than
that expressed in the first or third columns of figures; and all the
discharge in the gas, when the interval in air was greater than that in the
second or fourth column of figures. At intermediate distances the discharge
was occasionally at both places, i.e. sometimes in the air, sometimes in
the gas.
_____________________________________________________________________
| | |
| | Interval _p_ in parts of an inch |
|_________________|___________________________________________________|
| | | |
| | When the small ball B | When the small ball B |
| Constant inter- | was inductric and | was inductric and |
| val _n_ between | _positive_ the | _negative_ the |
| B and D = 1 | discharge was all | discharge was all |
| inch | at _p_ in at _n_ in | at _p_ in at _n_ in |
| | air before the gas | air before the gas |
| | after | after |
|_________________|_________________________|_________________________|
| | _p_ = | _p_ = | _p_ = | _p_ = |
|In Air | 0.10 | 0.50 | 0.28 | 0.33 |
|In Nitrogen | 0.30 | 0.65 | 0.31 | 0.40 |
|In Oxygen | 0.33 | 0.52 | 0.27 | 0.30 |
|In Hydrogen | 0.20 | 0.10 | 0.22 | 0.24 |
|In Coal Gas | 0.20 | 0.90 | 0.20 | 0.27 |
|In Carbonic Acid | 0.61 | 1.30 | 0.30 | 0.15 |
|_________________|____________|____________|____________|____________|
1508. These results are the same generally, as far as they go, as those of
the like nature in the last series (1388.), and confirm the conclusion that
different gases restrain discharge in very different proportions. They are
probably not so good as the former ones, for the glass jar not being
varnished, acted irregularly, sometimes taking a certain degree of charge
as a non-conductor, and at other times acting as a conductor in the
conveyance and derangement of that charge. Another cause of difference in
the ratios is, no doubt, the relative sizes of the discharge balls in air;
in the former case they were of very different size, here they were alike.
1509. In future experiments intended to have the character of accuracy, the
influence of these circumstances ought to be ascertained, and, above all
things, the gases themselves ought to be contained in vessels of metal, and
not of glass.
* * * * *
1510. The next set of results are those obtained when the intervals _n_ and
_o_ (fig. 131.) were made equal to each other, and relate to the greater
facility of discharge at the small ball, when rendered positive or negative
(1493.).
1511. In _air_, with the intervals = 0.4 of an inch, A and B being
inductric and positive, discharge was nearly equal at _n_ and _o_; when A
and B were inductric and negative, the discharge was mostly at _n_ by
negative brush. When the intervals were = 0.8 of an inch, with A and B
inductric positively, all discharge was at _n_ by positive brush; with A
and B inductric negatively, all the discharge was at _n_ by a negative
brush. It is doubtful, therefore, from these results, whether the negative
ball has any greater facility than the positive.
1512. _Nitrogen._--Intervals _n_ and _o_ = 0.4 of an inch: A, B inductric
positive, discharge at both intervals, most at _n_, by positive sparks; A,
B inductric negative, discharge equal at _n_ and _o_. The intervals made =
0.8 of an inch: A, B inductric positive, discharge all at _n_ by positive
brush; A, B inductric negative, discharge most at _o_ by positive brush. In
this gas, therefore, though the difference is not decisive, it would seem
that the positive small ball caused the most ready discharge.
1513. _Oxygen._--Intervals _n_ and _o_ = 0.4 of an inch: A, B inductric
positive, discharge nearly equal; inductric negative, discharge mostly at
_n_ by negative brush. Made the intervals = 0.8 of an inch: A, B inductric
positive, discharge both at _n_ and _o_; inductric negative, discharge all
at _o_ by negative brush. So here the negative small ball seems to give the
most ready discharge.
1514. _Hydrogen._--Intervals _n_ and _o_ = 0.4 of an inch: A, B inductric
positive, discharge nearly equal: inductric negative, discharge mostly at
_o_. Intervals = 0.8 of an inch: A and B inductric positive, discharge
mostly at _n_, as positive brush; inductric negative, discharge mostly at
_o_, as positive brush. Here the positive discharge seems most facile.
1515. _Coal gas._--_n_ and _o_ = 0.4 of an inch: A, B inductric positive,
discharge nearly all at _o_ by negative spark: A, B inductric negative,
discharge nearly all at _n_ by negative spark. Intervals = 0.8 of an inch,
and A, B inductric positive, discharge mostly at _o_ by negative brush: A,
B inductric negative, discharge all at _n_ by negative brush. Here the
negative discharge most facile.
1516. _Carbonic acid gas._--_n_ and _o_ = 0.1 of an inch: A, B inductric
positive, discharge nearly all at _o_, or negative: A, B inductric
negative, discharge nearly all at _n_, or negative. Intervals = 0.8 of an
inch: A, B inductric positive, discharge mostly at _o_, or negative. A, B
inductric negative, discharge all at _n_, or negative. In this case the
negative had a decided advantage in facility of discharge.
1517. Thus, if we may trust this form of experiment, the negative small
ball has a decided advantage in facilitating disruptive discharge over the
positive small ball in some gases, as in carbonic acid gas and coal gas
(1399.), whilst in others that conclusion seems more doubtful; and in
others, again, there seems a probability that the positive small ball may
be superior. All these results were obtained at very nearly the same
pressure of the atmosphere.
* * * * *
1518. I made some experiments in these gases whilst in the air jar (fig.
131.), as to the change from spark to brush, analogous to those in the open
air already described (1486. 1487.). I will give, in a Table, the results
as to when brush began to appear mingled with the spark; but the after
results were so varied, and the nature of the discharge in different gases
so different, that to insert the results obtained without further
investigation, would be of little use. At intervals less than those
expressed the discharge was always by spark.
_______________________________________________________________________
| | | |
| | Discharge between | Discharge between |
| | balls B and D. | balls A and C. |
| |___________________________|___________________________|
| | | | | |
| | Small ball | Small ball | Large ball | Large ball |
| | B inductric | B inductric | A inductric | A inductric |
| | _pos_. | _neg_. | _pos_. | _neg_. |
|_______________|_____________|_____________|_____________|_____________|
| | | | | |
| Air | 0.55 | 0.30 | 0.40 | 0.75 |
| Nitrogen | 0.30 | 0.40 | 0.52 | 0.41 |
| Oxygen | 0.70 | 0.30 | 0.45 | 0.82 |
| Hydrogen | 0.20 | 0.10 | | |
| Coal gas | 0.13 | 0.30 | 0.30 | 0.44 |
| Carbonic acid | 0.82 | 0.43 | 1.60 | {above 1.80;|
| | | | | had not |
| | | | | space.) |
|_______________|_____________|_____________|_____________|_____________|
1519. It is to be understood that sparks occurred at much higher intervals
than these; the table only expresses that distance beneath which all
discharge was as spark. Some curious relations of the different gases to
discharge are already discernible, but it would be useless to consider them
until illustrated by further experiments.
* * * * *
1520. I ought not to omit noticing here, that Professor Belli of Milan has
published a very valuable set of experiments on the relative dissipation of
positive and negative electricity in the air[A]; he finds the latter far
more ready, in this respect, than the former.
[A] Bibliothèque Universelle, 1836, September, p. 152.
1521. I made some experiments of a similar kind, but with sustained high
charges; the results were less striking than those of Signore Belli, and I
did not consider them as satisfactory. I may be allowed to mention, in
connexion with the subject, an interfering effect which embarrassed me for
a long time. When I threw positive electricity from a given point into the
air, a certain intensity was indicated by an electrometer on the conductor
connected with the point, but as the operation continued this intensity
rose several degrees; then making the conductor negative with the same
point attached to it, and all other things remaining the same, a certain
degree of tension was observed in the first instance, which also gradually
rose as the operation proceeded. Returning the conductor to the positive
state, the tension was at first low, but rose as before; and so also when
again made negative.
1522. This result appeared to indicate that the point which had been giving
off one electricity, was, by that, more fitted for a short time to give off
the other. But on closer examination I found the whole depended upon the
inductive reaction of that air, which being charged by the point, and
gradually increasing in quantity before it, as the positive or negative
issue was continued, diverted and removed a part of the inductive action of
the surrounding wall, and thus apparently affected the powers of the point,
whilst really it was the dielectric itself that was causing the change of
tension.
* * * * *
1523. The results connected with the different conditions of positive and
negative discharge will have a far greater influence on the philosophy of
electrical science than we at present imagine, especially if, as I believe,
they depend on the peculiarity and degree of polarized condition which the
molecules of the dielectrics concerned acquire (1503. 1600.). Thus, for
instance, the relation of our atmosphere and the earth within it, to the
occurrence of spark or brush, must be especial and not accidental (1464.).
It would not else consist with other meteorological phenomena, also of
course dependent on the special properties of the air, and which being
themselves in harmony the most perfect with the functions of animal and
vegetable life, are yet restricted in their actions, not by loose
regulations, but by laws the most precise.
1524. Even in the passage through air of the voltaic current we see the
peculiarities of positive and negative discharge at the two charcoal
points; and if these discharges are made to take place simultaneously to
mercury, the distinction is still more remarkable, both as to the sound and
the quantity of vapour produced.
1525. It seems very possible that the remarkable difference recently
observed and described by my friend Professor Daniell[A], namely, that when
a zinc and a copper ball, the same in size, were placed respectively in
copper and zinc spheres, also the same in size, and excited by electrolytes
or dielectrics of the same strength and nature, the zinc ball far surpassed
the zinc sphere in action, may also be connected with these phenomena; for
it is not difficult to conceive how the polarity of the particles shall be
affected by the circumstance of the positive surface, namely the zinc,
being the larger or the smaller of the two inclosing the electrolyte. It is
even possible, that with different electrolytes or dielectrics the ratio
may be considerably varied, or in some cases even inverted.
[A] Philosophical Transactions, 1838, p. 47.
* * * * *
_Glow discharge._
1526. That form of disruptive discharge which appears as a _glow_ (1359.
1405.), is very peculiar and beautiful: it seems to depend on a quick and
almost continuous charging of the air close to, and in contact with, the
conductor.
1527. _Diminution of the charging surface_ will produce it. Thus, when a
rod 0.3 of an inch in diameter, with a rounded termination, was rendered
positive in free air, it gave fine brushes from the extremity, but
occasionally these disappeared, and a quiet phosphorescent continuous glow
took their place, covering the whole of the end of the wire, and extending
a very small distance from the metal into the air. With a rod 0.2 of an
inch in diameter the glow was more readily produced. With still smaller
rods, and also with blunt conical points, it occurred still more readily;
and with a fine point I could not obtain the brush in free air, but only
this glow. The positive glow and the positive star are, in fact, the same.
1528. _Increase of power in the machine_ tends to produce the glow; for
rounded terminations which will give only brushes when the machine is in
weak action, will readily give the glow when it is in good order.
1529. _Rarefaction of the air_ wonderfully favours the glow phenomena. A
brass ball, two and a half inches in diameter, being made positively
inductric in an air-pump receiver, became covered with glow over an area of
two inches in diameter, when the pressure was reduced to 4.4 inches of
mercury. By a little adjustment the ball could be covered all over with
this light. Using a brass ball 1.25 inches in diameter, and making it
inducteously positive by an inductric negative point, the phenomena, at
high degrees of rarefaction, were exceedingly beautiful. The glow came over
the positive ball, and gradually increased in brightness, until it was at
last very luminous; and it also stood up like a low flame, half an inch or
more in height. On touching the sides of the glass jar this lambent flame
was affected, assumed a ring form, like a crown on the top of the ball,
appeared flexible, and revolved with a comparatively slow motion, i.e.
about four or five times in a second. This ring-shape and revolution are
beautifully connected with the mechanical currents (1576.) taking place
within the receiver. These glows in rarefied air are often highly exalted
in beauty by a spark discharge at the conductor (1551. _Note_.).
1530. To obtain a _negative glow_ in air at common pressures is difficult.
I did not procure it on the rod 0.3 of an inch in diameter by my machine,
nor on much smaller rods; and it is questionable as yet, whether, even on
fine points, what is called the negative star is a very reduced and minute,
but still intermitting brush, or a glow similar to that obtained on a
positive point.
1531. In rarefied air the negative glow can easily be obtained. If the
rounded ends of two metal rods, about O.2 of an inch in diameter, are
introduced into a globe or jar (the air within being rarefied), and being
opposite to each other, are about four inches apart, the glow can be
obtained on both rods, covering not only the ends, but an inch or two of
the part behind. On using _balls_ in the air-pump jar, and adjusting the
distance and exhaustion, the negative ball could be covered with glow,
whether it were the inductric or the inducteous surface.
1532. When rods are used it is necessary to be aware that, if placed
concentrically in the jar or globe, the light on one rod is often reflected
by the sides of the vessel on to the other rod, and makes it apparently
luminous, when really it is not so. This effect may be detected by shifting
the eye at the time of observation, or avoided by using blackened rods.
1533. It is curious to observe the relation _of glow, brush_, and _spark_
to each other, as produced by positive or negative surfaces; thus,
beginning with spark discharge, it passes into brush much sooner when the
surface at which the discharge commences (1484.) is negative, than it does
when positive; but proceeding onwards in the order of change, we find that
the positive brush passes into _glow_ long before the negative brush does.
So that, though each presents the three conditions in the same general
order, the series are not precisely the same. It is probable, that, when
these points are minutely examined, as they must be shortly, we shall find
that each different gas or dielectric presents its own peculiar results,
dependent upon the mode in which its particles assume polar electric
condition.
1534. The glow occurs in all gases in which I have looked for it. These are
air, nitrogen, oxygen, hydrogen, coal gas, carbonic acid, muriatic acid,
sulphurous acid and ammonia. I thought also that I obtained it in oil of
turpentine, but if so it was very dull and small.
1535. The glow is always accompanied by a wind proceeding either directly
out from the glowing part, or directly towards it; the former being the
most general case. This takes place even when the glow occurs upon a ball
of considerable size: and if matters be so arranged that the ready and
regular access of air to a part exhibiting the glow be interfered with or
prevented, the glow then disappears.
1536. I have never been able to analyse or separate the glow into visible
elementary intermitting discharges (1427. 1433.), nor to obtain the other
evidence of intermitting action, namely an audible sound (1431.). The want
of success, as respects trials made by ocular means, may depend upon the
large size of the glow preventing the separation of the visible images:
and, indeed, if it does intermit, it is not likely that all parts intermit
at once with a simultaneous regularity.
1537. All the effects tend to show, that _glow_ is due to a continuous
charge or discharge of air; in the former case being accompanied by a
current from, and in the latter by one to, the place of the glow. As the
surrounding air comes up to the charged conductor, on attaining that spot
at which the tension of the particles is raised to the sufficient degree
(1370. 1410.), it becomes charged, and then moves off, by the joint action
of the forces to which it is subject; and, at the same time that it makes
way for other particles to come and be charged in turn, actually helps to
form that current by which they are brought into the necessary position.
Thus, through the regularity of the forces, a constant and quiet result is
produced; and that result is, the charging of successive portions of air,
the production of a current, and of a continuous glow.
1538. I have frequently been able to make the termination of a rod, which,
when left to itself, would produce a brush, produce in preference a glow,
simply by aiding the formation of a current of air at its extremity; and,
on the other hand, it is not at all difficult to convert the glow into
brushes, by affecting the current of air (1574. 1579.) or the inductive
action near it.
1539. The transition from glow, on the one hand, to brush and spark, on the
other, and, therefore, their connexion, may be established in various ways.
Those circumstances which tend to facilitate the charge of the air by the
excited conductor, and also those which tend to keep the tension at the
same degree notwithstanding the discharge, assist in producing the glow;
whereas those which tend to resist the charge of the air or other
dielectric, and those which favour the accumulation of electric force prior
to discharge, which, sinking by that act, has to be exalted before the
tension can again acquire the requisite degree, favour intermitting
discharge, and, therefore, the production of brush or spark. Thus,
rarefaction of the air, the removal of large conducting surfaces from the
neighbourhood of the glowing termination, the presentation of a sharp point
towards it, help to sustain or produce the glow: but the condensation of
the air, the presentation of the hand or other large surface, the gradual
approximation of a discharging ball, tend to convert the glow into brush or
even spark. All these circumstances may be traced and reduced, in a manner
easily comprehensible, to their relative power of assisting to produce,
either a _continuous_ discharge to the air, which gives the glow; or an
_interrupted_ one, which produces the brush, and, in a more exalted
condition, the spark.
1540. The rounded end of a brass rod, 0.3 of an inch in diameter, was
covered with a positive glow by the working of an electrical machine: on
stopping the machine, so that the charge of the connected conductor should
fall, the glow changed for a moment into brushes just before the discharge
ceased altogether, illustrating the necessity for a certain high continuous
charge, for a certain sized termination. Working the machine so that the
intensity should be just low enough to give continual brushes from the end
in free air, the approach of a fine point changed these brushes into a
glow. Working the machine so that the termination presented a continual
glow in free air, the gradual approach of the hand caused the glow to
contract at the very end of the wire, then to throw out a luminous point,
which, becoming a foot stalk (1426.), finally produced brushes with large
ramifications. All these results are in accordance with what is stated
above (1539.).
1541. Greasing the end of a rounded wire will immediately make it produce
brushes instead of glow. A ball having a blunt point which can be made to
project more or less beyond its surface, at pleasure, can be made to
produce every gradation from glow, through brush, to spark.
1542. It is also very interesting and instructive to trace the transition
from spark to glow, through the intermediate condition of stream, between
ends in a vessel containing air more or less rarefied; but I fear to be
prolix.
1543. All the effects show, that the glow is in its nature exactly the same
as the luminous part of a brush or ramification, namely a charging of air;
the only difference being, that the glow has a continuous appearance from
the constant renewal of the same action in the same place, whereas the
ramification is due to a momentary, independent and intermitting action of
the same kind.
* * * * *
_Dark discharge._
1544. I will now notice a very remarkable circumstance in the luminous
discharge accompanied by negative glow, which may, perhaps, be correctly
traced hereafter into discharges of much higher intensity. Two brass rods,
0.3 of an inch in diameter, entering a glass globe on opposite sides, had
their ends brought into contact, and the air about them very much rarefied.
A discharge of electricity from the machine was then made through them, and
whilst that was continued the ends were separated from each other. At the
moment of separation a continuous glow came over the end of the negative
rod, the positive termination remaining quite dark. As the distance was
increased, a purple stream or haze appeared on the end of the positive rod,
and proceeded directly outwards towards the negative rod; elongating as the
interval was enlarged, but never joining the negative glow, there being
always a short dark space between. This space, of about 1/16th or 1/20th of
an inch, was apparently invariable in its extent and its position, relative
to the negative rod; nor did the negative glow vary. Whether the negative
end were inductric or inducteous, the same effect was produced. It was
strange to see the positive purple haze diminish or lengthen as the ends
were separated, and yet this dark space and the negative glow remain
unaltered (fig. 133).
1545. Two balls were then used in a large air-pump receiver, and the air
rarefied. The usual transitions in the character of the discharge took
place; but whenever the luminous stream, which appears after the spark and
the brush have ceased, was itself changed into glow at the balls, the dark
space occurred, and that whether the one or the other ball was made
inductric, or positive, or negative.
1546. Sometimes when the negative ball was large, the machine in powerful
action, and the rarefaction high, the ball would be covered over half its
surface with glow, and then, upon a hasty observation, would seem to
exhibit no dark space: but this was a deception, arising from the
overlapping of the convex termination of the negative glow and the concave
termination of the positive stream. More careful observation and experiment
have convinced me, that when the negative glow occurs, it never visibly
touches the luminous part of the positive discharge, but that the dark
space is always there.
1547. This singular separation of the positive and negative discharge, as
far as concerns their luminous character, under circumstances which one
would have thought very favourable to their coalescence, is probably
connected with their differences when in the form of brush, and is perhaps
even dependent on the same cause. Further, there is every likelihood that
the dark parts which occur in feeble sparks are also connected with these
phenomena[A]. To understand them would be very important, for it is quite
clear that in many of the experiments, indeed in all that I have quoted,
discharge is taking place across the dark part of the dielectric to an
extent quite equal to what occurs in the luminous part. This difference in
the result would seem to imply a distinction in the modes by which the two
electric forces are brought into equilibrium in the respective parts; and
looking upon all the phenomena as giving additional proofs, that it is to
the condition of the particles of the dielectric we must refer for the
principles of induction and discharge, so it would be of great importance
if we could know accurately in what the difference of action in the dark
and the luminous parts consisted.
[A] See Professor Johnson's experiments. Silliman's Journal, xxv. p. 57.
1548. The dark discharge through air (1552.), which in the case mentioned
is very evident (1544.), leads to the inquiry, whether the particles of air
are generally capable of effecting discharge from one to another without
becoming luminous; and the inquiry is important, because it is connected
with that degree of tension which is necessary to originate discharge
(1368. 1370.). Discharge between _air and conductors_ without luminous
appearances are very common; and non-luminous discharges by carrying
currents of air and other fluids (1562. 1595.) are also common enough: but
these are not cases in point, for they are not discharges between
insulating particles.
1549. An arrangement was made for discharge between two balls (1485.) (fig.
129.) but, in place of connecting the inducteous ball directly with the
discharging train, it was put in communication with the inside coating of a
Leyden jar, and the discharging train with the outside coating. Then
working the machine, it was found that whenever sonorous and luminous
discharge occurred at the balls A B, the jar became charged; but that when
these did not occur, the jar acquired no charge: and such was the case when
small rounded terminations were used in place of the balls, and also in
whatever manner they were arranged. Under these circumstances, therefore,
discharge even between the air and conductors was always luminous.
1550. But in other cases, the phenomena are such as to make it almost
certain, that dark discharge can take place across air. If the rounded end
of a metal rod, 0.15 of an inch in diameter, be made to give a good
negative brush, the approach of a smaller end or a blunt point opposite to
it will, at a certain distance, cause a diminution of the brush, and a glow
will appear on the positive inducteous wire, accompanied by a current of
air passing from it. Now, as the air is being charged both at the positive
and negative surfaces, it seems a reasonable conclusion, that the charged
portions meet somewhere in the interval, and there discharge to each other,
without producing any luminous phenomena. It is possible, however, that the
air electrified positively at the glowing end may travel on towards the
negative surface, and actually form that atmosphere into which the visible
negative brushes dart, in which case dark discharge need not, of necessity,
occur. But I incline to the former opinion, and think, that the diminution
in size of the negative brush, as the positive glow comes on to the end of
the opposed wire, is in favour of that view.
1551. Using rarefied air as the dielectric, it is very easy to obtain
luminous phenomena as brushes, or glow, upon both conducting balls or
terminations, whilst the interval is dark, and that, when the action is so
momentary that I think we cannot consider currents as effecting discharge
across the dark part. Thus if two balls, about an inch in diameter, and 4
or more inches apart, have the air rarefied about them, and are then
interposed in the course of discharge, an interrupted or spark current
being produced at the machine[A], each termination may be made to show
luminous phenomena, whilst more or less of the interval is quite dark. The
discharge will pass as suddenly as a retarded spark (295. 334.), i.e. in an
interval of time almost inappreciably small, and in such a case, I think it
must have passed across the dark part as true disruptive discharge, and not
by convection.
[A] By spark current I mean one passing in a series of spark between
the conductor of the machine and the apparatus: by a continuous
current one that passes through metallic conductors, and in that
respect without interruption at the same place.
1552. Hence I conclude that dark disruptive discharge may occur (1547.
1550.); and also, that, in the luminous brush, the visible ramifications
may not show the full extent of the disruptive discharge (1444. 1452.), but
that each may have a dark outside, enveloping, as it were, every part
through which the discharge extends. It is probable, even, that there are
such things as dark discharges analogous in form to the brush and the
spark, but not luminous in any part (1445.).
1553. The occurrence of dark discharge in any case shows at how low a
tension disruptive discharge may occur (1548,), and indicates that the
light of the ultimate brush or spark is in no relation to the intensity
required (1368. 1370.). So to speak, the discharge begins in darkness, and
the light is a mere consequence of the quantity which, after discharge has
commenced, flows to that spot and there finds its most facile passage
(1418. 1435.). As an illustration of the growth generally of discharge, I
may remark that, in the experiments on the transition in oxygen of the
discharge from spark to brush (1518.), every spark was immediately preceded
by a short brush.
1554. The phenomena relative to dark discharge in other gases, though
differing in certain characters from those in air, confirm the conclusions
drawn above. The two rounded terminations (1544.) (fig. 133.), were placed
in _muriatic acid gas_ (1445. 1463.) at the pressure of 6.5 inches of
mercury, and a continuous machine current of electricity sent through the
apparatus: bright sparks occurred until the interval was about or above an
inch, when they were replaced by squat brushy intermitting glows upon both
terminations, with a dark part between. When the current at the machine was
in spark, then each spark caused a discharge across the muriatic acid gas,
which, with a certain interval, was bright; with a larger interval, was
straight across and flamy, like a very exhausted and sudden, but not a
dense sharp spark; and with a still larger interval, produced a feeble
brush on the inductric positive end, and a glow on the inducteous negative
end, the dark part being between (1544.); and at such times, the spark at
the conductor, instead of being sudden and sonorous, was dull and quiet
(334.).
1555. On introducing more muriatic acid gas, until the pressure was 29.97
inches, the same terminations gave bright sparks within at small distances;
but when they were about an inch or more apart, the discharge was generally
with very small brushes and glow, and frequently with no light at all,
though electricity had passed through the gas. Whenever the bright spark
did pass through the muriatic acid gas at this pressure, it was bright
throughout, presenting no dark or dull space.
1556. In _coal gas_, at common pressures, when the distance was about an
inch, the discharge was accompanied by short brushes on the ends, and a
dark interval of half an inch or more between them, notwithstanding the
discharge had the sharp quick sound of a dull spark, and could not have
depended in the dark part on _convection_ (1562.).
1557. This gas presents several curious points in relation to the bright
and dark parts of spark discharge. When bright sparks passed between the
rod ends 0.3 of an inch in diameter (1544.), very sudden dark parts would
occur next to the brightest portions of the spark. Again with these ends
and also with balls (1422.), the bright sparks would be sometimes red,
sometimes green, and occasionally green and red in different parts of the
same spark. Again, in the experiments described (1518.), at certain
intervals a very peculiar pale, dull, yet sudden discharge would pass,
which, though apparently weak, was very direct in its course, and
accompanied by a sharp snapping noise, as if quick in its occurrence.
1558. _Hydrogen_ frequently gave peculiar sparks, one part being bright
red, whilst the other was a dull pale gray, or else the whole spark was
dull and peculiar.
1559. _Nitrogen_ presents a very remarkable discharge, between two balls of
the respective diameters of 0.15 and 2 inches (1506. 1518.), the smaller
one being rendered negative either directly inducteously. The peculiar
discharge occurs at intervals between 0.42 and 0.68, and even at 1.4 inches
when the large ball was inductric positively; it consisted of a little
brushy part on the small negative ball, then a dark space, and lastly a
dull straight line on the large positive ball (fig. 134.). The position of
the dark space was very constant, and is probably in direct relation to the
dark space described when negative glow was produced (1544.). When by any
circumstance a bright spark was determined, the contrast with the peculiar
spark described was very striking; for it always had a faint purple part,
but the place of this part was constantly near the positive ball.
1560. Thus dark discharge appears to be decidedly established. But its
establishment is accompanied by proofs that it occurs in different degrees
and modes in different gases. Hence then another specific action, added to
the many (1296. 1398. 1399. 1423. 1454. 1503.) by which the electrical
relations of insulating dielectrics are distinguished and established, and
another argument in favour of that molecular theory of induction, which is
at present under examination[A].
[A] I cannot resist referring here by a note to Biot's philosophical
view of the nature of the light of the electric discharge, Annales de
Chimie, liii. p. 321.
* * * * *
1561. What I have had to say regarding disruptive discharge has extended to
some length, but I hope will be excused in consequence of the importance of
the subject. Before concluding my remarks, I will again intimate in the
form of a query, whether we have not reason to consider the tension or
retention and after discharge in air or other insulating dielectrics, as
the same thing with retardation and discharge in a metal wire, differing
only, but almost infinitely, in degree (1334. 1336.). In other words, can
we not, by a gradual chain of association, carry up discharge from its
occurrence in air, through spermaceti and water, to solutions, and then on
to chlorides, oxides and metals, without any essential change in its
character; and, at the same time, connecting the insensible conduction of
air, through muriatic acid gas and the dark discharge, with the better
conduction of spermaceti, water, and the all but perfect conduction of the
metals, associate the phenomena at both extremes? and may it not be, that
the retardation and ignition of a wire are effects exactly correspondent in
their nature to the retention of charge and spark in air? If so, here again
the two extremes in property amongst dielectrics will be found to be in
intimate relation, the whole difference probably depending upon the mode
and degree in which their particles polarize under the influence of
inductive actions (1338. 1603. 1610.).
* * * * *
¶ x. _Convection, or carrying discharge._
1562. The last kind of discharge which I have to consider is that effected
by the motion of charged particles from place to place. It is apparently
very different in its nature to any of the former modes of discharge
(1319.), but, as the result is the same, may be of great importance in
illustrating, not merely the nature of discharge itself, but also of what
we call the electric current. It often, as before observed, in cases of
brush and glow (1440. 1535.), joins its effect to that of disruptive
discharge, to complete the act of neutralization amongst the electric
forces.
1563. The particles which being charged, then travel, may be either of
insulating or conducting matter, large or small. The consideration in the
first place of a large particle of conducting matter may perhaps help our
conceptions.
1564. A copper boiler 3 feet in diameter was insulated and electrified, but
so feebly, that dissipation by brushes or disruptive discharge did not
occur at its edges or projecting parts in a sensible degree. A brass ball,
2 inches in diameter, suspended by a clean white silk thread, was brought
towards it, and it was found that, if the ball was held for a second or two
near any part of the charged surface of the boiler, at such distance (two
inches more or less) as not to receive any direct charge from it, it became
itself charged, although insulated the whole time; and its electricity was
the _reverse_ of that of the boiler.
1565. This effect was the strongest opposite the edges and projecting parts
of the boiler, and weaker opposite the sides, or those extended portions of
the surface which, according to Coulomb's results, have the weakest charge.
It was very strong opposite a rod projecting a little way from the boiler.
It occurred when the copper was charged negatively as well as positively:
it was produced also with small balls down to 0.2 of an inch and less in
diameter, and also with smaller charged conductors than the copper. It is,
indeed, hardly possible in some cases to carry an insulated ball within an
inch or two of a charged plane or convex surface without its receiving a
charge of the contrary kind to that of the surface.
1566. This effect is one of induction between the bodies, not of
communication. The ball, when related to the positive charged surface by
the intervening dielectric, has its opposite sides brought into contrary
states, that side towards the boiler being negative and the outer side
positive. More inductric action is directed towards it than would have
passed across the same place if the ball had not been there, for several
reasons; amongst others, because, being a conductor, the resistance of the
particles of the dielectric, which otherwise would have been there, is
removed (1298.); and also, because the reacting positive surface of the
ball being projected further out from the boiler than when there is no
introduction of conducting matter, is more free therefore to act through
the rest of the dielectric towards surrounding conductors, and so favours
the exaltation of that inductric polarity which is directed in its course.
It is, as to the exaltation of force upon its outer surface beyond that
upon the inductric surface of the boiler, as if the latter were itself
protuberant in that direction. Thus it acquires a state like, but higher
than, that of the surface of the boiler which causes it; and sufficiently
exalted to discharge at its positive surface to the air, or to affect small
particles, as it is itself affected by the boiler, and they flying to it,
take a charge and pass off; and so the ball, as a whole, is brought into
the contrary inducteous state. The consequence is, that, if free to move,
its tendency, under the influence of all the forces, to approach the boiler
is increased, whilst it at the same time becomes more and more exalted in
its condition, both of polarity and charge, until, at a certain distance,
discharge takes place, it acquires the same state as the boiler, is
repelled, and passing to that conductor most favourably circumstanced to
discharge it, there resumes its first indifferent condition.
1567. It seems to me, that the manner in which inductric bodies affect
uncharged floating or moveable conductors near them, is very frequently of
this nature, and generally so when it ends in a carrying operation (1562.
1602.). The manner in which, whilst the dominant inductric body cannot give
off its electricity to the air, the inducteous body _can_ effect the
discharge of the same kind of force, is curious, and, in the case of
elongated or irregularly shaped conductors, such as filaments or particles
of dust, the effect will often be very ready, and the consequent attraction
immediate.
1568. The effect described is also probably influential in causing those
variations in spark discharge referred to in the last series (1386. 1390.
1391.): for if a particle of dust were drawn towards the axis of induction
between the balls, it would tend, whilst at some distance from that axis,
to commence discharge at itself, in the manner described (1566.), and that
commencement might so far facilitate the act (1417. 1420.) as to make the
complete discharge, as spark, pass through the particle, though it might
not be the shortest course from ball to ball. So also, with equal balls at
equal distances, as in the experiments of comparison already described
(1493. 1506.), a particle being between one pair of balls would cause
discharge there in preference; or even if a particle were between each,
difference of size or shape would give one for the time a predominance over
the other.
1569. The power of particles of dust to carry off electricity in cases of
high tension is well known, and I have already mentioned some instances of
the kind in the use of the inductive apparatus (1201.). The general
operation is very well shown by large light objects, as the toy called the
electrical spider; or, if smaller ones are wanted for philosophical
investigation, by the smoke of a glowing green wax taper, which, presenting
a successive stream of such particles, makes their course visible.
1570. On using oil of turpentine as the dielectric, the action and course
of small conducting carrying particles in it can be well observed. A few
short pieces of thread will supply the place of carriers, and their
progressive action is exceedingly interesting.
1571. A very striking effect was produced on oil of turpentine, which,
whether it was due to the carrying power of the particles in it, or to any
other action of them, is perhaps as yet doubtful. A portion of that fluid
in a glass vessel had a large uninsulated silver dish at the bottom, and an
electrified metal rod with a round termination dipping into it at the top.
The insulation was very good, and the attraction and other phenomena
striking. The rod end, with a drop of gum water attached to it, was then
electrified in the fluid; the gum water soon spun off in fine threads, and
was quickly dissipated through the oil of turpentine. By the time that four
drops had in this way been commingled with a pint of the dielectric, the
latter had lost by far the greatest portion of its insulating power; no
sparks could be obtained in the fluid; and all the phenomena dependent upon
insulation had sunk to a low degree. The fluid was very slightly turbid.
Upon being filtered through paper only, it resumed its first clearness, and
now insulated as well as before. The water, therefore, was merely diffused
through the oil of turpentine, not combined with or dissolved in it: but
whether the minute particles acted as carriers, or whether they were not
rather gathered together in the line of highest inductive tension (1350.),
and there, being drawn into elongated forms by the electric forces,
combined their effects to produce a band of matter having considerable
conducting power, as compared with the oil of turpentine, is as yet
questionable.
1572. The analogy between the action of solid conducting carrying particles
and that of the charged particles of fluid insulating substances, acting as
dielectrics, is very evident and simple; but in the latter case the result
is, necessarily, currents in the mobile media. Particles are brought by
inductric action into a polar state; and the latter, after rising to a
certain tension (1370.), is followed by the communication of a part of the
force originally on the conductor; the particles consequently become
charged, and then, under the joint influence of the repellent and
attractive forces, are urged towards a discharging place, or to that spot
where these inductric forces are most easily compensated by the contrary
inducteous forces.
1573. Why a point should be so exceedingly favourable to the production of
currents in a fluid insulating dielectric, as air, is very evident. It is
at the extremity of the point that the intensity necessary to charge the
air is first acquired (1374.); it is from thence that the charged particle
recedes; and the mechanical force which it impresses on the air to form a
current is in every way favoured by the shape and position of the rod, of
which the point forms the termination. At the same time, the point, having
become the origin of an active mechanical force, does, by the very act of
causing that force, namely, by discharge, prevent any other part of the rod
from acquiring the same necessary condition, and so preserves and sustains
its own predominance.
1574. The very varied and beautiful phenomena produced by sheltering or
enclosing the point, illustrate the production of the current exceedingly
well, and justify the same conclusions; it being remembered that in such
cases the effect upon the discharge is of two kinds. For the current may be
interfered with by stopping the access of fresh uncharged air, or retarding
the removal of that which has been charged, as when a point is electrified
in a tube of insulating matter closed at one extremity; or the _electric
condition_ of the point itself may be altered by the relation of other
parts in its neighbourhood, also rendered electric, as when the point is in
a metal tube, by the metal itself, or when it is in the glass tube, by a
similar action of the charged parts of the glass, or even by the
surrounding air which has been charged, and which cannot escape.
1575. Whenever it is intended to observe such inductive phenomena in a
fluid dielectric as have a direct relation to, and dependence upon, the
fluidity of the medium, such, for instance, as discharge from points, or
attractions and repulsions, &c., then the mass of the fluid should be
great, and in such proportion to the distance between the inductric and
inducteous surfaces as to include all the _lines of inductive force_
(1369.) between them; otherwise, the effects of currents, attraction, &c.,
which are the resultants of all these forces, cannot be obtained. The
phenomena, which occur in the open air, or in the middle of a globe filled
with oil of turpentine, will not take place in the same media if confined
in tubes of glass, shell-lac, sulphur, or other such substances, though
they be excellent insulating dielectrics; nor can they be expected: for in
such cases, the polar forces, instead of being all dispersed amongst fluid
particles, which tend to move under their influence, are now associated in
many parts with particles that, notwithstanding their tendency to motion,
are constrained by their solidity to remain quiescent.
1576. The varied circumstances under which, with conductors differently
formed and constituted, currents can occur, all illustrate the same
simplicity of production. A _ball_, if the intensity be raised sufficiently
on its surface, and that intensity be greatest on a part consistent with
the production of a current of air up to and off from it, will produce the
effect like a point (1537); such is the case whenever the glow occurs upon
a ball, the current being essential to that phenomenon. If as large a
sphere as can well be employed with the production of glow be used, the
glow will appear at the place where the current leaves the ball, and that
will be the part directly opposite to the connection of the ball and rod
which supports it; but by increasing the tension elsewhere, so as to raise
it above the tension upon that spot, which can easily be effected
inductively, then the place of the glow and the direction of the current
will also change, and pass to that spot which for the time is most
favourable for their production (1591.).
1577. For instance, approaching the hand towards the ball will tend to
cause brush (1539.), but by increasing the supply of electricity the
condition of glow may be preserved; then on moving the hand about from side
to side the position of the glow will very evidently move with it.
1578. A point brought towards a glowing ball would at twelve or fourteen
inches distance make the glow break into brush, but when still nearer, glow
was reproduced, probably dependent upon the discharge of wind or air
passing from the point to the ball, and this glow was very obedient to the
motion of the point, following it in every direction.