Michael Faraday Eye-Popping Experiments

¶ ix. _Disruptive discharge and insulation._

Vol -2

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1359. The next form of discharge has been distinguished by the adjective
_disruptive_ (1319.), as it in every case displaces more or less the
particles amongst and across which it suddenly breaks. I include under it,
discharge in the form of sparks, brushes, and glow (1405.), but exclude the
cases of currents of air, fluids, &c., which, though frequently
accompanying the former, are essentially distinct in their nature.

_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. 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.

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, 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.

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.

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.).