DISRUPTIVE DISCHARGE.] E L E C T 11 1 C I T Y ture of the positive electrode higher when the discharges ; pass mainly through particles of disintegrated metal. The former case is commoner iu vacuum tubes, where the negative electrode may get white hot, and even melt, while the positive electrode remains quite dark. The latter case is exemplified in the voltaic arc, in which great disinte gration of the positive electrode is accompanied by a higher temperature there. Attempts have been made to investigate the temperature iu different parts of the tube, and it seems to have been made out that the temperature is lower in the dark intervals than elsewhere. When the electromotor is an induction coil, which fur- B nishes discharges alternately in opposite directions, there will ba a mixture of positive and negative light at each electrode, unless the maximum tension corresponding to
- - the inverse discharge be so small that the direct discharge
m alone can break through. If, however, the tube be examined by means of a rotating mirror, or if it be itself fastened to a rotating arm, the images of the different discharges will be separated, and it will be seen that the appearances at each electrode alternate. Again, when a Leyden jar is discharged through a 3tt vacuum tube, the appearances at the two electrodes are often very much alike, particularly when the resistance of the discharging circuit is very small. When the resist ance is increased by introducing a column of water or lengths of wetted string, the appearances are similar to those indicated iu our summary description. The reason of this is fully explained by the observations of Feddersen. He examined the spark of a Leyden jar by means of a rotating concave mirror. The machine which drove the mirror had a contact-maker, which brought on the dis charge when the mirror was at a definite position ; the image of the spark was thus thrown by the mirror on a piece of ground glass or a photographic plate, properly placed to receive it. He found that the discharge assumed three distinct characters as the resistance of the discharging circuit was gradually decreased. ar- 1. The discharge was intermittent, that is to say, consisted of a series of partial discharges all in tlie same direction, following each ta. other at more or less irregular intervals. 2. When the resistance was reduced to a certain extent, the dis charge became continuous. The image of the spark on the plate had then the form of an initial vertical strip, with two horizontal strips extending from each end, and gradually thinning off to a point. The vertical strip indicates a single initial spark, and the horizontal bands the finite duration of the light from the glowing metal par ticles, k,c., near the electrodes. 3. When the resistance is very small, the discharge is oscillatory, i.e., consists of a succession of discharges alternately in opposite directions. These oscillations are due to the self-induction of the discharging circuit ; we shall examine the matter more carefully under Electromagnetic Induction. It is obvious that when the discharge is either intermittent or continuous, the luminous phenomena will be of the normal form sketched above, but when the discharge is oscillatory there will be a mixture of positive and negative appearances at each electrode, the independent existence of which cannot be detected by the unaided eye. This is the place to remark that it is rarely that the discharge is of the simple form (2), i.e., consists of a single continuous discharge; in by far the great majority of cases it consists of a series of partial discharges. With the inductorium, both varieties (1) and (3) may occur according to the length of the air space, the resistance of the whole secondary circuit, and so on. A number of very beautiful experiments have been made to illustrate these principles, which it would take us beyond our limits to describe. Good summaries of the results of Felici, Cazin and Lucas, Donders and Nyland, Ogdeu Rood and Alf. Mayer, will be found in Mascart and Wiedemann. Recent researches of a very important character have been made by Wiilluer 1 and Spottiswoode a on the discharge in vacuum tubes. They employ the rotating mirror. It would be premature to attempt to sum up or criticise their results, suffice it to say that they show an amount of agreement which augurs well for the future of this branch of electrical science. The stride seem, according to them, . Ann., "Jubelbd.," 1374. * Proc. R, S. t 1875-6, 7. to play a more essential part in the phenomenon than was perhaps previously expected. Spottiswoode, in tact, seems to incline to the view thai all discharges having a dark interval are really stratified, although, owing to their rapid motion, the strata may not be distin guishable by the eye alone. lu connection with this subject it maybe well to mention Wheat - the early experiments of Wheatstone, 3 to determine the so- sl called velocity of electricity in conducting circuits. Six n ^ t "" balls, 1, 2, 3, 4, 5, 6, were arranged in a straight line on a Velocity board ; 2 and 5 were connected with the coatings of a of elec- charged Leyden jar ; discharge passed by spark from 2 to tricity. 1, then through a large metallic resistance to 3, thence by spark to 4, then through a large metallic resistance to 6, and thence by spark to 5. It was found, as Fedderseu observed later, that the introduction of the metallic resist ance increased the duration of the sparks at all the inter vals, so that the images in the mirror were lines of small length; but, in addition, the spark between 3 and 4 began a little later than the sparks at 1, 2 and 5, 6, which were simultaneous. From this the velocity of electricity has been calculated, by taking the interval 4 between the sparks to be the time which the electricity takes to travel through the metal wire letiveen tJie intervals. Faraday long ago pointed out that this interval depends on the capacity of the wire, and may vary very much according to circumstances. It is very great in submarine telegraph wires for instance (vide supra, p. 36). Accordingly, the values of the so-called velocity of electricity, which have been found by different observers, differ extremely. The sketch we have just given of the disruptive discharge in rarefied gases must be regarded as the merest outline. There are many points of great importance to which we have not even alluded. Hittorfs investigation on what has been called the " resistance " of different parts of a vacuum tube during the discharge has not been mentioned, although it led to results of much interest, which must come to be of great importance when the clue to an explana tion of the whole phenomena has been found. The reader who desires to study the matter will find in Wiedemann au excellent account of Hittorfs work, with references to the original sources. We have not so much as raised the deli cate and difficult questions concerning the spectroscopic characteristics of the discharge. A good part of this sub ject belongs indeed more properly to the science of Light. Miscellaneous Effects, chiefly Mechanical. Owing to the Kinners- heat suddenly developed by the electric spark, and perhaps lev . s e - to a specific mechanical effect as well, there is a sudden per dispersion in all directions of the particles of the dielectric. This commotion may be shown very well by means of Kin- nersley s older form of the thermo-electrometer ; or Gauss s instrument may be used if we replace the thin wire by a couple of spark terminals. When the spark passes, the liquid in the stem sinks suddenly through a considerable distance, even if the spark be of no great length (2 to 3 mm.). Very curious effects are obtained when an electric spark is repeated Stria; several times at a little distance above a plate strewed with finely from powdered chalk. After a time the chalk is seen to be divided by a con- network of fine lines, resembling the markings on shagreen. If a cuioa. plate of glass be covered with powdered charcoal, and the spark passed through the powder, it arranges itself in a series of strice closely resembling those seen in a vacuum tube. The power of the spark to induce chemical combination (in particular, combustion) is due no doubt mainly to its high temperature. The discharge through non-conducting liquids may take Dis place in the form of spark or brush. The brush, however, charge n. is poor compared with that obtained in air, and is very hard l 3 Phil. Trans., 1834. 4 A better statement would be "the time that elapses before sufficient electricity has reached 3 and 4 to raise the tension at their uearest points to the disruptive limit,"
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