Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/73

Rh DISRUPTIVE DISCHARGE.] E L E T K 1 1 T Y 63 time its great brilliancy gives an impression of whiteness. In nitrogen the appearance is much as in air, only the colour tends more to bluish purple, and the spark is more sonorous. In oxygen the spark is whiter and less brilliant than in air ; in hydrogen crimson-coloured ; in carbonic acid greenish ; in hydrochloric acid white, and never broken by dark parts ; in coal gas green or red, with occasional dark parts. If the spark be carefully examined, especially when the pressure is greater than an atmosphere, it will be seen that the central bright streak is surrounded by an envelope, of somewhat nebulous form, and of a lavender- blue colour. This envelope tends to spread over the nega tive electrode, where it is moie conspicuous as compared with the central streak than elsewhere. This envelope ap pears to be due to the glowing metal particles torn from the electrodes. It has, unlike the central streak, a sensible duration, on account of which it happens in many cases that a much greater quantity of electricity passes through it than through the infinitely more brilliant but less endur ing part of the discharge. The envelope can be actually separated from the streak by a current of air properly directed, or by the action of a magnet (vide infra, p. 74). When the discharge in air at the atmospheric pressure takes place between a salient but not pointed part of one conductor and another conductor of considerable surface (e.g. between one sphere 2 cm. diameter and another 13 cm. diameter), the luminous appearance very often takes a characteristic form, which has been called the brush dis charge. The name is to a considerable extent descriptive of the phenomenon; if the word broora had been applied it would have been even more appropriate, and a rough idea of the variety of forms the brush may assume will be obtained by thinking of the various forms of the domestic article in question. At the surface of the smaller conductor appears a short, straight, luminous stem differing in appear ance very little except in brightness from a spark. From this radiate a series of twig-like branches of much inferior brilliancy, having a purplish-violet colour. These sub divide in many cases into still smaller ramifications, and are ultimately lost in the medium. &quot;When the large conductor is either altogether absent or very distant, the general ten dency of the branches is to spread outwards more and more in all directions ; but when the large conductor is brought nearer, the branches have a tendency to bend down towards it, so that the whole assumes an ovoid shape. The brush is generally accompanied by a crackling or hissing sound, or even a musical note. On approaching the hand or a con ductor of extended surface, the pitch of this sound rises con siderably. This at once suggests that the brush is an inter mittent phenomenon. That this really is so was clearly proved by Wheatstone in one of the earlier applications of his rotating mirror. 1 Wheatstone saw in his mirror not one image of the brush, but several arranged in succession at regular intervals. Each of these images corresponds to a single discharge, and each appears less complicated than the brush as viewed by the unaided eye, which is, in reality, a superposition of a considerable number of brushes, the number depending on the time taken by a light impression to fade on the retina. At the same time each individual image is a little drawn out in the direction of motion of the mirror, which shows that the brush has a sensible duration. Faraday speculates very acutely concerning the nature of the brush discharge (see jcp. Res., 1425 sqq.). He finds that, although it is generally accompanied by a current of air, yet it is not always or necessarily so. He also care fully illustrates the difference between the positive and negative brush. If we have a small ball on the end of a wire projecting freely into the air, the positive brushes 2 ob tained from it are much larger and finer than the negative brushes so obtained. Again, if we charge a large metal ball positively, and bring an uninsulated metal point up to it, a star appears on the point, which gets brighter and brighter as the point approaches the sphere, but the form does not change until the distance is very small. If the sphere be charged negatively, the star appears as before when the distance is considerable, but at a mode rate distance (1 to 2 inches) a brush forms, and when the distance is still farther reduced a spark passes. It seems, therefore, that the negative discharge keeps its form un changed under considerable variety of influencing circum stances, whereas the form of the positive discharge is more readily affected. The explanation of these differences he finds in the fact, which he established by experiments already alluded to, that the limiting tension is smaller at positive than at negative surfaces ; so that, cceteris paribus, the negative discharge occurs of tener than the positive discharge ; but, on the other hand, when the latter does occur, more electricity passes. This, no doubt, accounts for the lower pitch of the sound of the negative brush, and the greater extent and brilliancy of the positive one. Faraday found great differences in the character of the brush in different gases ; in none apparently does it reach the brilliancy attained in air or nitrogen. He also observed that rarefac tion up to a certain point favoured the production of brushes. When discharge takes place from the rounded end of a Glow, wire projecting freely into the air, the brush is very often replaced by a quiet phosphorescent glow, which covers a greater or less extent of the end of the wire. The noise which accompanies the brush is entirely absent in this form of the discharge, and the means by which the brush can be analysed into a series of successive discharges give no cor responding result for the glow. In the rotating mirror it simply stretches out into a uniform band of light. The glow is therefore either a continuous discharge or an intermittent discharge of incomparably shorter period than the brush. Diminishing the discharging surfaces favours the produc tion of glow. 3 Increase of power in the electric machine which is furnishing the electricity has a similar effect. Rarefaction of the air has also a great effect in facilitating the production of glow, especially in the case of negative, glow, which is extremely hard to produce in air at common pressures. In Faraday s opinion, the star which is ob tained with a positive sharp point is a positive glow ; but he thinks it not improbable that the negative star is not a negative glow, but a small negative brush. The glow is invariably associated with a current of air to or from (generally both) the glowing conductor. Everything that favours this air-current increases the glow; e.g., a brush may sometimes be converted into a glow by properly directing an air-current near it. Again, everything that prevents or retards the formation of an air-current has a similar eS ect on the glow : a glow can be converted into a brush in this way. Lastly, everything which tends to prevent abrupt variation of the tension favours the glow, and everything having an opposite tendency is destructive of it. Faraday concludes, therefore, that the glow is due to a gradual discharge by convection, in which the agents are the particles of the gas. The order of the appearance of spark, brush, and glow at positive and negative surfaces is, in general, the same ; but the gradation is different. Positive spark does not pass intp brush so soon as negative spark does ; but, on the other hand, positive brush turns to glow long before negative brush. positively charged surface. * EJ-J&amp;gt;. Res., 1527.
 * Phil. Trans., 1834, &c.
 * By positive brush, of course, Is meant brush emanating from a