64 E LEGTBIG.I T Y DISRUPTIVE Convec- tive dis charge. Intimately connected with the glow is the convective discharge, if indeed they are not degrees of the same phenomenon. " The electric glow is produced by the constant passage of electricity through a small portion of air in which the tension is very high, so as to charge the surrounding particles of air which are continually swept off by the electric wind, which is an essential part of the phenomenon." 1 Now there seems little reason to doubt that at lower tensions 2 discharge of this kind may occur without the luminous phenomenon at the surface of the conductor. If this be so, then the convective dis charge is only a different degree of the glow discharge. Discharge by convection plays a very important part in all elec- trostatical experimenting. The air in the neighbourhood of an electrified conductor gets charged, forming anelectrical atmosphere, which surrounds the conductor, being more extensive in the neighbourhood of salient angles than elsewhere. Such electrical atmospheres are often a source, of great inconvenience in the labora tory and lecture-room when delicate electrical experiments are in progress. A curious little instrument, called the electrical tourniquet or windmill, depends for its action on the electrical wind which accompanies convective discharge. A small rectangular cross, with equal arms, is made of light wire ; the extremities of the arms are bent through a right angle in the plane of the cross, so as to point all one way. The little cross thus made is poised, like a compass needle, on a vertical wire connected with an electrified conductor. Convective discharge takes place at the points, giving rise to an electrical wind, the reaction of which causes the little machine to revolve with great rapidity. If the experiment be con ducted in the dark, a glow usually appears on the revolving points. The experiment also succeeds when the cross is immersed in a non-conducting liquid. Dark We have already alluded to the dark spaces that some- mterval. times appear in the spark in gas at the atmospheric pres sure. Faraday observed that a phenomenon of this kind was very common in coal gas. When the discharge takes place in highly rarefied gas, a dark space of this kind almost always separates the positive from the negative light, its situation having a certain degree of fixity with respect to the negative, but not to the positive electrode. It is very difficult to form an idea of the exact nature of the discharge which takes place in this space. Discharge there undoubtedly is of some kind; and pending further investigation, Faraday called it the dark discharge. The fact that its real nature is still undiscovered amply justifies the separate name. Faraday found that it occurred in dis charges that pass almost instantaneously, and concluded that it could hardly be due to convection of the ordinary kind, which requires time. De la Rive and Hittorf have made out many peculiarities connected with its appearance in vacuum tubes, the phenomena in which we now attempt briefly to describe, Pheno- A variety of forms may be given to the vessel in which the rarc- iiiena in fied gas to be experimented on is inclosed. rarefied One of the most common used to be the electric egg, which is gases. simply an oval glass vessel furnished with two small metal spheres for electrodes ; the stems which carry these electrodes pass air-tight through tubes cemented to the ends of the vessel ; the stem which supports the whole is perforated and fitted with a stop-cock, so that the apparatus can be exhausted to any required extent and then temporarily closed. The commonest of all instruments of this kind now-a-days is the Geissler tube This is simply a glass tube, into which are fused two electrodes of platinum or other metal ; a capillary tube allows the apparatus to be connected with an air- pump, and exhausted ; when this is done, the capillary tube is sealed up by means of a spirit-lamp. A very common form of such tube is the spectrum tube (see art. LIGHT), consisting of two wider parts, connected by a capillary part, in which the light of the discharge is much more intense than elsewhere. Complicated tubes of all kinds have also been constructed as electric toys. The reader must not forget that the form of the tube exercises a great influence on the phenomena, whether at the positive or negative electrode. In the summary description that follows the 1 Maxwell, Electricity and Magnetism, i. 55. 9 The reader will not forget the exact seaise in which we use the word tension. Of course, low tension does not mean low potential. electric egg is referred to, unless it is otherwise stated. We fur ther assume that the electromotor used gives currents in one direc tion only. A Holtz machine would satisfy this condition, within certain limits at least. When the gas is rarefied to a considerable extent, the spark loses its sharp outline, becomes interspersed with nebukms portions, and by-and-by loses its characteristic form altogether. As the rarefaction goes on, the discharge ceases to reach from the positive to the negative electrode. The latter now displays a patch of lavender-blue light, separated from the positive light by a dark interval, the length of which depends on the distance between the elec trodes. In certain cases the positive light terminates in a cup-shaped depression, whose concavity is turned towards the negative electrode. As the rarefaction is still further increased, the positive light tends more and more to fill the tube, although in general it recedes from the nega tive electrode, over which, on the other hand, the beauti ful lavender glow spreads more and more, exhibiting at the same time a growing tendency to fill a limited space surrounding the electrode. At a still higher degree of rarefaction, the positive light, which now occupies a con siderable space, and takes a shape more or less correspond ing to that of the inclosing vessel, is divided transversely into a number of cup-shaped striae, separated from each other by darker intervals. These striae vary in form and appearance considerably, according to circumstances. In the neighbourhood of the positive electrode, their con cavity is turned towards the positive electrode ; but towards the other end of the positive light, the concavity may be turned the other way, especially in the electric egg. The positive light, in vacuum tubes, shows there fore the same remarkable variability, and the negative light the same measure of stability that Faraday remarked in gas at ordinary pressures. The colour of the positive light varies very much in different gases ; in nitrogen and air its rosy-red colour contrasts very sharply with the blue of the negative light. The negative light is remarkable for its power of producing fluorescence. It is very depen dent as to its extent on the form and size of the uncovered surface of the electrode ; anything placed on the electrode cuts it off sharply, as if the light were projected from the electrode and stopped by the obstacle. Disintegration of the negative electrode also goes on very rapidly, so that, after a vacuum tube has been used for some time the glass all round the negative electrode is blackened, browned, &c., as the case may be, with a deposit of finely divided metal. The quantity as well as the quality of this deposit depends very much on the nature of the metal ; it is smallest with aluminium, which is on that account much used for electrode terminals. The negative light occasionally shows one, two, or even three stratifica tions; but in this respect it never equals the positive light. When the rarefaction is carried to the utmost, both positive and negative lights fall off greatly in splen dour. The negative light contracts more and more in upon the electrode, and confines itself even there to a small patch near the end, showing, however, a tendency to pass along the axis of the tube towards the positive elec trode. The positive light, on the other hand, gradually draws inwards, till at last it is only a star on the end of the electrode, which now disintegrates, owing to the great tension. The temperature at the two electrodes is, in general, very Tern different. The true explanation of this difference has not iatu been made out, although it is doubtless connected with the equally unexplained differences in the light phenomena. A general rule has been laid down, that the temperature of the negative electrode is always higher when the dis
charge takes place through the gas alone, and the tempera-