Imágenes de páginas
PDF
EPUB

on the part of negative electricity, a great facility in dispersing itself in the ambient medium, when once that medium is rarefied. The agitation of the striae in the luminous part of the current becomes very considerable under the slight pressure of two millimetres. It manifests itself at first very sensibly in the neighborhood of the positive electrode, from which the luminous stream issues under the form of an outspreading cone, which, in proportion as the pressure diminishes, becomes more and more cylindrical, until it assumes altogether the form of a cylinder of whose circular base the electrode is the centre, the agitation of the striae being, at the same time, general throughout the whole extent of the current. When the discharge is effected in a cylindrical jar, between a ball serving as a negative electrode and a metallic ring of which that ball is the centre, and which serves as a positive electrode, the bluish atmosphere which surrounds the ball enlarges by several centimetres at a pressure of 2", and its exterior outline is covered with small filaments, presenting a tuftlike appearance. These filaments are probably formed by the series of molecules which transmit the discharge. They are much more distinct with hydrogen (a good conductor) than with other gases. If the ball serves as a positive electrode, it is surrounded with a lively rose-colored halo of about a centimetre in diameter, presenting well-marked stratifications; then comes a dark annular space, which terminates at the ring, which is itself completely invested with an envelope or sheath of clear violet, with opaline tints. Nitrogen presents the same phenomena as hydrogen, though the stratification of the electric light does not begin, except under a much feebler pressure. In the long tube (one metre in length) the agitation of the striae, under a pressure of 2mm, is even more considerable than with hydrogen. These striae seem to form an animated helix, with a movement of rotation around its axis. The light is also more vivid, the tint being of a peach-blossom rather than pale rose, color. The phenomenon is of a most brilliant description. Further, there is the same obscure space in the vicinity of the negative electrode, the same glimmer of palish rose color at a weak pressure of from 1 to 2" in this obscure space, the same appearance in this ...; mist of well-defined and motionless rings more luminous than the space which surrounds them. Atmospheric air corresponds in its phenomena with nitrogen. I have ob. served only that here the agitation of the striae is less striking, and the light of a rose color less deep than in nitrogen. The appearances which I have just described are, therefore, within some mere j precisely the same in hydrogen, nitrogen, and atmospheric air; they are equally the same, whether these gases are dry, or contain the vapor of water or of alcohol in more or less quantity; there are no differences, except that the pressures at which the various phenomena, and the tints of light which accompany them, are observed, vary with the nature of the rarefied elastic fluid. We cannot, then, attribute the effects just considered to an electro-chemical decomposition which cannot take place in a simple and well-desiccated gas, nor to any action appertaining to the chemical nature of the elastic fluid. They are evidently the result of a mechanical action which accompanies the transmission of electricity—an idea first advanced by M. Riess, who showed that an analogous phenomenon presents itself, under a little different form, it is true, in liquids and in solids. The phenomenon in rarefied elastic fluids would consist in the alternate contractions and dilatations of the gaseous medium produced by the series of discharges, always more or less intermittent, of which the electric stream is formed. In fact, whether it be by Ruhmkorff's apparatus, or an ordinary electric machine, or by a hydro-electric machine of Armstrong, that the stratifications are produced, there is never a continuous discharge, but, in reality, a series of dis. charges which may succeed one another so rapidly that the intermission shall not be betrayed, even by a galvanometer. But it does not the less exist, as M. Gassiot has shown in operating with a pile of Grove at high tension, which, with the same electrodes, and in the same medium, will give rise, first to stratifications, and afterwards, when the current has become continuous, to a voltaic arch. The mechanical action of the series of discharges on the rarefied elastic fluid may, indeed, be directly verified by the very marked oscillations of the column of mercury of the manometer placed in communication with the elastic fluid, which accompany the propagation of electricity in that fluid. These oscillations rise to two or three tenths of a millimetre in hydrogen, under a pressure of 16”. They begin to be sensible when once the stream passes, that is to say, at 36” of pressure; attain their maximum of three-tenths of a millimetre between 20 and 12” of pressure; and diminish rapidly in descending from 12 to 5mm, at which last pressure they no longer take place. With nitrogen and atmospheric air, and employing the same tube 16 centimetres in length and 5 in diameter, the oscillations begin, at the moment when the stream passes, under the pressure of about 20mm; attain their maximum of from four to five tenths of a millimetre between 12 and 8mm of pressure; and then continue to diminish until 2 or 5", at which pressure they cease to be sensible. With the tube one metre in length, and even with that of 50 centimetres, I have not succeeded in observing any appearance of oscillation accompanying the transmission of the electric current, whatever might be the gas enclosed in these tubes, and whatever the pressure to which it was subjected. On the other hand, I have obtained very distinct ones, of one and two tenths of a millimetre, under pressures varying from 30 to 15", in a jar 20 centimetres in height by 16 in diameter, filled with rarefied hydrogen, and in which the electric stream assed from a central ball to a ring 12 centimetres in diameter concentric to that ball. This last result shows that the absence of oscillations in the long tubes has less connexion with the volume of the gaseous medium, which is less than in the vessel of the last experiment than with the influence of the sides of the tubes which embarrass the movement of the gas. It is also a proof that the oscillations proceed from a mechanical action, and not from an elevation of temperature. As regards their intensity, the oscillations evidently depend on the greater or less resistance which the gaseous medium opposes to the transmission of the electric current, since the oscillations are more considerable with nitrogen than with hydrogen, and diminish as the pressure does, reckoning from a certain point of the pressure, which is that at which the discharge can take place in a complete manner, and at which the intensity of the oscillations attains its maximum. The stratification of electric light would appear then to be a phenomenon analogous to the production of undulations of sound, that is to say, a mechanical phenomenon proceeding from a succession of isochronous impulses communicated to the rarefied gaseous column by the series of electric discharges rapidly succeeding each other. We find a new proof favorable to this view of the phemomenon in the perturbation which a displacement of the gaseous matter occasions in the stratifications, and, consequently, in the disposition of the elastic fluid which permits those stratifications to appear. To produce this perturbation, it suffices to introduce into the tube in which there , a rarefied elastic fluid, and while the electricity is in process of propagation, an additional quantity of the same gas already enclosed therein, so as to increase the pressure by one-fourth or one-half of a millimetre at most. Let us see what then occurs with hydrogen, remarking that the effects are the same with the three tubes, respectively, 15, 50, and 100 centimetres in length. e begin by rarefying the gas to the extent of 2", so as to have the phemomenon of the stratifications as distinct as possible. We then introduce a small quantity of hydrogen; if the introduction takes place on the side of the negative electrode, striae of a fine rose color are immediately seen to form in the obscure space, their diameter being that of the stratified column—that is, of the tube, while they are at the same time very narrow and well defined. They are gradually propagated in the tube, confounding themselves with the original striae, which are much larger and less distinctly limited; then, as soon as the entrance of the gas is arrested, the luminous column is seen to recede slowly from the negative electrode, and resume gradually its primitive appearance. When the introduction of the gas takes place on the side of the positive electrode, in place of the striae occupying the whole cavity of the tube, we see a brilliant stream of very small diameter (2 to 3") distinctly striated, and quite similar to a minute spiral spring (ressort a boudin,) advance along the axis of the tube in the relatively obscure interior of the luminous column, which itself, as soon as the gas begins to enter by the positive as well as by the negative electrode, immediately advances so as to occupy almost entirely the obscure space up to the negative electrode, from which it is only separated by the interposed stratum, 2" in thickness, which it cannot surmount. Then, the introduction of the gas once stopped, everything returns quickly to the normal state. By whichever of the two extremities of the tube the gas is made to penetrate, we see, on the entrance of the gas, a very subtle mist of a roseate white color make its appearance, and diffuse itself in the tube; but this, as soon as the introduction of the additional quantity of gas has ceased, passes over from the negative to the positive electrode, leaving the obscure space to form itself anew, and momentarily hiding in its passage, by enveloping them as it were with a light cloud, the successive stratifications of different parts of the column; then this mist disappears, and the luminous column resumes its primitive appearance. which it maintains so long as nothing is changed either in the electric current or the state of the gas traversed by it. The appearance of this mist, which perfectly resembles that I have mentioned as existing in the dark space of the column in a state of repose, well denotes the agitation into which the introduction of a small additional quantity of gas throws the whole column—an agitation so conspicuously manifested by the progression of the striae and their encroachment on one another. The phenomenon presents this further feature: that the definiteness and brightness of the striae in the gaseous portion introduced, which make them so plainly distinguishable from the gas which was already in the tube, enable us to follow the progressive movement of that portion from one end of the tube to the other. The experiment may be repeated several times in succession by successive introductions of additional quantities of gas, provided that each time the pressure be not increased more than 4 of a millimetre, and that the total pressure do not in all exceed 5 or 6". With nitrogen and atmospheric air the incidents are the same, only we cannot push the experiment so far, the pressure at which the phenomenon ceases to take place with these gases being much less than it is with hydrogen. The narrow striae which display themselves at the moment of the entrance of the gas on that side where the entrance takes place are also less distinct and less bril. liant, but there is equally a momentary disappearance of the obscure space, the production of a roseate mist, and progression of this mist, on the cessation of the introduction of gas, from the negative electrode to the positive. With the three gases alike, we see, when the introduction is effected on the side of the negative electrode, the mist advance at first like the slender striated thread which follows the axis of the tube from the positive electrode to the negative; then, having arrived at this extremity of the tube, it turns back, passing over, as has been said, from the negative to the positive electrode. This mist evidently j. from a portion of the gas which, in entering the tube, is excessively dilated, and becomes visible by the electricity which traverses it. From the slowness with which the mist is propagated we may judge of the feeble degree of elastic force in the gas. It is to the same cause ...; that we should ascribe the slowness with which the mixture of the gas which is entering the tube with that already present is effected—a slowness which is manifested by the circumstance of the definite and narrow striae appearing in the new portion of gas, while in the old the striae are much larger, and by no means so distinctly defined—a phenomenon which can only proceed from the former not being, at the moment when it enters the tube, so much dilated as the gas which was already there. . In fine, the fact that the gaseous column with narrow striae is much larger when the gas which produces it enters on the side of the negative electrode than when it enters on that of the positive, is a proof that, before the new introduction of gas, the gaseous column already in the tube was much more dilated in the neighborhood of the negative electrode than on the side of the positive. So, then, the passage of the electric discharges very rapidly succeeding one another across a rarefied gaseous column would produce therein, when the rarefaction had reached a certain degree variable with the nature, and consequently with the conductibility of the gas, first, a considerable dilatation of the gaseous matter around the negative electrode, and next, beginning in this i. portion of the column, a succession of alternate contractions and dilatations as far as the positive electrode. It is highly probable that the same effect takes place when the gas is not sufficiently rarefied for producing stratification of the electric light. But in that case, the greater elastic force of the gas, joined with the necessarily less rapid succession of the discharges, allows the immediate return of the contracted and dilated strata to their state of normal density, and thus prevents that double state from manifesting itself; while when the gas is less elastic, and the discharges succeed one another more rapidly, the state of dilatation and contraction of successive strata roduced by a first discharge still subsists when a second arrives, the result É. that it becomes sensible. The transmission of electricity, then, through a gaseous column occasions a movement in the particles of gas, and that movement seems to be an impulse emanating from the negative electrode. Might not this effect be attributed to the static electricity with which the molecules are charged, and which would augment their constitutive repulsion ? We know, and it is seen by the luminous aureoles which surround the negative ball and rod, that, at an equal tension, the negative electricity issues more readily than the positive from its metallic electrodes in order to penetrate into the rarefied ambient medium. Hence, the portion of that medium nearest to the negative electrode must be more charged with static (negative) electricity than is (with positive) the portion of the rarefied gas near the positive electrode; it is not, then, surprising that the repulsion of the gaseous molecules, and consequently the rarefaction of the gas, should be greater in the first of these two portions than in the second." . Now, why does negative electricity diffuse itself more easily than positive under the same conditions of intensity, magnitude, and position of electrodes, nature and rarefaction of the ambient medium ? Here is the mystery, or at least a point of most interesting consideration as regards the theory of electricity.

$ III.—PARTICULAR PHENOMENA PRESENTED BY DIFFERENT PARTs of THE STRATIFIED ELECTRIC CURRENT.

The gaseous column traversed by the electric current is composed, as we have said, when it has been brought to a certain degree of rarefaction, of strata alternately dilated and contracted, with an obscure space greatly dilated in the neighborhood of the negative electrode. The more dilated parts of the column offering less resistance to the transmission of electricity must remain obscure. while the more contracted, with less capacity of conduction, grow warm, and become luminous, even when it is the same discharge which traverses them. We should here expect a phenomenon exactly analogous to that which is produced when we place in the circuit of a voltaic pile a chain formed of alternate wires of platina and silver, having the same length and diameter; although they both transmit the same current, the wires of platina, offering most resistance, grow hot, and become even incandescent, while those of silver, being better conductors, remain cold and opaque.

* Tole fact that the electricity of tension is more easily propagated around the negative than around the positive electrode may be readily verified by experiment, as well as the permanent state of electric tension of the F. column during the passage of the electric current, whatever may be the rarefaction of the gas.

To demonstrate that in fact the space remaining opaque offers less resistance to the transmission of electricity in the stratified column than the luminous part of that column, I have arranged two small disks of platina, 7” in diameter, each attached by a point in its circumference to the end of a wire of platina, enclosed in a tube of glass, in such a way as to be kept parallel to one another at a distance of three centimetres. The two disks are connected in a solid manner, though very carefully isolated, and without any possible electric communication except by means of the wires of platina soldered to their circumference, and enclosed in a tube of glass. The free extremities of the two wires of platina can be respectively placed in communication with those of the wire of a galvanometer. The apparatus is arranged in such manner that the two disks of platina may be introduced into the stratified electric stream so as to cut it transversely, and to have their centres situated in the very axis of the stream. They thus serve as sounds destined for the derivation of a part of the current. and the intensity of that derived portion, which is so much less as the conductibility of the interval of derivation is greater, is measured by the deviation of the needle of the galvanometer put in communication with the free extremities of the platina wires which support the disks; these wires, as has been said, are themselves enclosed in tubes of glass where they traverse the recipient which contains the rarefied gas, with a view to their remaining well isolated, and that the disks alone may be in contact with the gaseous substance which transmits the discharges. Now, it suffices to change the direction of these discharges in order that the sounds, without being displaced, shall be immersed either in the obscure space near the negative electrode, or in the luminous space hear the positive one. The apparatus is, moreover, so contrived that the sounds may be !. in other portions of the current. It is proper to add, that the electrodes

etween which the electric stream passes are two disks of platina, each five cen

timetres in diameter, placed parallel to one another at a distance which may vary from forty to thirty centimetres, and consequently, like the little disks serving as sounds, perpendicular to the axis of the stream.

The following are some experiments made successively with nitrogen and bydrogen:

Nithogen, or ATMospheric Air.

Pressure of the gas. Intensity of the derived current. Sounds near the positive electrode. Sounds near the negatire electrode. 6”---------------------. 70°---------------------- 18° 4"---------------------- 40°---------------------- 8° 2*----------------. ---- 18°---------------------- 39 HYDROGEN. 15*-----...... - - - - - - - - - - - 90°---------------------. 90° 6”------------.......... 82°---------------------. 65° 4”-------------......... 92°. --------------------- 29 2*- - - - - - - - - - - - - - - - - - - - - - 35°---------------------. 0°

We see from these tables that the intensity of the derived current diminishes with the pressure, although the transmitted current be much stronger, which shows with what rapidity the resistance of the gas diminishes in proportion as its rarefaction increases. But at the same time the diminution of the derived

« AnteriorContinuar »