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“On using a smaller ball, the general brush was smaller, and the sound, though weaker, more continuous. On resolving the brush into its elementary parts as before these were found to occur at much shorter intervals.
“Employing a wire with a round end, the brush was still smaller, but, as before, separable into successive discharges. The sound, though feebler, was higher in pitch, being a distinct musical note.”
The sound is in fact due to the recurrence of the noise of each separate discharge, and these happening at intervals nearly equal, under ordinary circumstances, cause a definite note to be head, whose pitch rises with the increased rapidity and regularity of the discharge.
“ By using wires with finer terminations, smaller brushes were obtained, until they could hardly be distinguished as brushes. But as long as sound was heard the discharge could be ascertained by the eye to be intermitting; and when the sound ceased the light became continuous as a glow.'
To those not accustomed to use the eye in the above-described manner, Wheatstone's apparatus with the revolving mirror is recommended. Another excellent process for analyzing the brush is to produce it on the end of a rod, held in the hand opposite to the prime conductor, and then move the rod rapidly from side to side, whilst the eye remains still.—(1428—1423.)
$ 83. The brush in various gases. The experiments on the brush in various gases Faraday made with brass rods, about one quarter of an inch thick, and whose rounded ends were placed opposite each other in a glass globe of seven inches diameter, containing the gas. One of these rods was connected with the prime conductor, the other with the ground.-(Pog. Ann., XLVII, 553.)
“ Air, Fine positive brushes are easily obtained in air, at common pressures, possessing the well known purplish light. When the air is rarefied the ramifications are very long, filling the globe; the light is greatly increased and is of a beautiful purple color, with an occasional rose tint in it.
“Oxygen. At common pressures the brush is very close and compressed, and of a dull whitish color. In rarefied oxygen the form and appearance are better; the color somewhat purplish, but all the characters very poor compared to those in air.'
“Nitrogen gives brushes with great facility at the positive surface, far beyond any other gas." “ They are almost always fine in form, light, and color, and in rarefied nitrogen are magnificent. They surpass the discharges in any other gas as to the quantity of light evolved."
“Hydrogen, at common pressures, gives a better brush than oxygen, but does not equal nitrogen ; the color was greenish gray.
In rarefied hydrogen the ramifications were very fine in form and distinctness, but pale in color, with a soft and velvety appearance, and not at all equal to those in nitrogen. In the rarest state of the gas the color was a pale gray green.
“Coal gas. The brushes were rather difficult to produce.” “They were short and strong, generally of a greenish color.” “In rare coal gas the brush forms were better, but the light very poor and the
“ Carbonic acid produces a very poor brush at common pressures. “In rarefied carbonic acid the brush is better in form, but weak as to light, being of a dull greenish or purplish hue.”
“Muriatic acid gas. It is very difficult to obtain the brush in this gas at common pressures. On gradually increasing the distance of the rounded ends the sparks suddenly ceased when the interval was about an inch, and the discharge, which was still through the gas in the globe, was silent and dark. Occasionally, a very short brush could, for a few moments, be obtained, but it quickly disappeared. Even when the intermitting spark current from the machine was used a brush was obtained with difficulty, and that very short;” “in the mean time, magnificent brushes were passing off from different parts of the machine into the surrounding air. On rarefying the gas the formation of the brush was facilitated, but it was yet of a low, squat form, very poor in light, and very similar on both the positive and negative surfaces. “On rarefying the gas still more a few large ramifications were obtained, of a pale bluish color, utterly unlike those in nitrogen.”—(1456–1462.)
$84. Brush in denser media.-Electrical brushes are produced, not only in air and gases, but in far denser media. Faraday procured it in oil of turpentine, (1452,) “ from the end of a wire going through a glass tube into the fluid, contained in a metal vessel. The brush was small, and very difficult to obtain; the ramifications were simple, and stretched out from each other, diverging very much. The light was exceedingly feeble, a perfectly dark room being required for its observation. When a few solid particles, as of dust or silk, were in the liquid, the brush was produced with much greater facility.”
$ 85. Difference of the positive and negative brush discharge.—On this subject I extract the following remarks by Faraday :
" When the brush discharge is observed in air, at the positive and negative surfaces, there is a very remarkable difference. The difference in question used to be expressed in former times by saying that
a point charged positively gave brushes into the air, whilst the same point charged negatively gave a star.” This is true only of bad conductors, or of metallic conductors charged intermittingly. If metallic points project freely into the air the positive and negative light upon them differ very little in appearance.
These phenomena vary exceedingly under different circumstances, as Faraday shows :
“If a metallic wire, with a rounded termination in free air, be used to produce the brushy discharge, then the brushes obtained when the wire is charged negatively are very poor and small by comparison with those produced when the charge is positive. Or if a large metal ball, connected with the electrical machine, be charged positively, and a fine uninsulated point be gradually brought towards it, a star appears on the point when at a considerable distance, which, though it becomes brighter, does not change its form of a star until it is close up to the ball; whereas, if the ball be charged negatively, the point, at a considerable distance, has a star on it as before; but when brought nearer,
a brush formed on it, extending to the negative ball; and when still nearer, the brush ceased and bright sparks passed.
As we have already seen, $ 80, the spark discharge passes into the brush at far less distances if the surface on which the discharge begins (the small ball or the rounded end of a rod) is negative, than if it is positive; but on going further into the succession of charges we find that the positive brush passes into glow long before the negative.
“A metal rod 0.3 of an inch in diameter, with a rounded end brush. It was ascertained, both by sight and sound, that the succesisve jecting into the air, was charged negatively and gave a short noisy discharges were very rapid in their recurrence, six or seven times more numerous than when the rod was charged positively to an equal degree.
“When the rod was positive it was easy, by working the machine a little quicker, to replace the brush by a glow, but when it was negative no efforts could produce this change.”-(1468.)
“A point opposite the negative brush exhibited a star, and, as it was approximated, caused the size and sound of the brush to diminish, and at last to cease, leaving the negative end silent and dark, yet effective as to discharge.”—(1469.)
" When the round end of a smaller wire was advanced towards the negative brush, it (becoming positive by induction) exhibited the quiet glow at eight inches distance, the negative brush continuing. When nearer, the pitch of the sound of the negative brush rose, indicating quicker intermittances; still nearer the positive end threw off ramification and distinct brushes, at the same time the negative brush contracted in its lateral direction and collected together, giving a peculiar, narrow, longish brush, in shape like a hair pencil ; the two brushes existing at once, but were very different in their form and appearance, and especially in the more rapid recurrence of the negative discharges than of the positive. On using a smaller positive wire for the same experiment the glow first appeared in it and then the brush, and the two at one distance became exceedingly alike in appearance. (1470.)
“In air the superiority of the positive brush is well known. In nitrogen it is as great or even greater than in air. In hydrogen the positive brush loses a part of its superiority, not being so good as in nitrogen or air, whilst the negative brush does not seem injured. In oxygen the positive brush is compressed and poor, whilst the negative did not become less; the two were so alike that the eye frequently could not tell one from the other. In coal gas the brushes are difficult of production ;" "and the positive not much superior to the negative, either at common or low pressure. In carbonic acid this approximation of character also occurred. In muriatic acid gas the positive brush was very little better than the negative.”—(1476.)
$ 86. Glow discharge.—The glow "seems to depend upon a quick and almost continuous charging of the air close to and in contact with the conductor.” —(Faraday's Researches, 1526.) Faraday was never able to separate it into visible intermitting elementary discharges. The glow is produced by
1st. Diminution of the charging surface. At the end of a metal rod
with a blunt conical point, a phosphorescent continuous glow is obtained the more readily as the point is finer.
2d. Increase of power in the machine.-Rounded ends, which give only brushes when the machine is in weak action, give the glow readily when the machine is in good order.
3d. Rarefaction of the air.-A brass ball 24 inches in diameter being made positively inductive in an air-pump receiver, became covered with glow in part, “ when the pressure was reduced to 4.4 inches. By a little adjustment the ball could be covered all over with this light. Using a brass ball 1.25 inch in diameter, and making it inducteously positive by an inductive negative point, the phenomena were exceedingly beautiful. The glow came over the positive ball, and gradually increased in brightness until it was at least very luminous; and it also stood up, like a low flame, half an inch or more in height."-(1529.)
The negative glow is difficult to obtain in air at common pressures ; “and it is as yet questionable whether, even on fine points, what is called the negative star is not a very reduced, but still intermitting brush, or a glow.”—(1530.)
In rarefied air the negative glow can easily be obtained. If the rounded ends of two metal rods about 0.2 of an inch in diameter are about four inches apart in rarefied air, the glow can be easily obtained on both rods, covering not only the ends but an inch or two of the part behind. Balls are also covered with the negative glow in rarefied air, whether their surface is inductive or inducteous.-(1531.)
The glow occurs in all the gases examined for it by Faraday. He thought he obtained it also in oil of turpentine, though it was very dull and small.-(1534.)
“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. If the arrangements are made so that the ready and regular access of air to a part exhibiting the glow be interfered with or prevented the glow then disappears. — (1535.)
Frequently it is possible to change the brush given by the end of a rod into a glow, by simply aiding the formation of a current of air at its extremity:-(1535.)
$ 87. Dark discharge.-If to the rounded end of a metallic rod projecting from the prime conductor of a machine a similar rod be held at a little distance, it is easy to obtain the appearance of light at the ends of both rods, while the intervening space between the positive and negative light remains dark; besides this familiar phenomenon, Faraday notices a very remarkable case of dark discharge.
“ 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 while that 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 onward 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 one-sixteenth or one-twentieth 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 ends were inductive or inducteous the same effect was produced.”
Similar phenomena were obtained with balls instead of the rounded ends of rods.
§ 88. Convective discharge.— The dielectric being penetrated by the spark, the brush, and also by the glow, Faraday calls this form of discharge the disruptive discharge. With the brush, and still more with the glow, another form of discharge appears, making itself manifest by the so-called electrical wind. This is owing to the particles of the dielectric, in close contact with the charged conductor, (on the end of the electrified rod,) receiving an electrical charge, in consequence of which they are repelled ; and by a repetition of this action the condustor is discharged.
Why a point should be so exceedingly favorable to the production of currents is evident. It is at the extremity of the point that the intensity necessary to charge the air is first acquired; 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 favored by the shapes and position of the rod whose point forms the termination.”—(1573.)
Particles of dust floating in the air favor the escape of electricity.
“On using oil of turpentine as the dielectric, the action and course of small conducting, carrying particles in it, can be well observed.”
“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. 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 manner been commingled with a pint of the dielectric, the latter had lost by far the greatest portion of its insulating power;" “the fluid was slightly turbid. Upon being filtered through paper only, it resumed its first clearness, and now insulated as well as before."-(1571.)
“Conducting fluid terminations, instead of rigid points, illustrate in a very beautiful manner the formation of the currents, with their effects and influence in exalting the conditions under which they were commenced. Let the rounded end of a brass rod, 0.3 of an inch, or thereabouts, in diameter, point downwards in free air ; let it be amalgamated and have a drop of mercury suspended from it, and then let it be powerfully electrized, the mercury will present the phenomenon of glow; a current of air will rush along the rod and set off from the mercury directly downwards, and the form of the metallic drop will be slightly affected, the convexity at a small part near the middle and lower part becoming greater, whilst it diminishes all round at places a little removed from this spot.”—(1581.)