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current, and consequently that of the resistance, is much greater when the sounds are immersed in the obscure space near the negative electrode than in the luminous part of the stream near the positive one. Thus, under the pressure of 2mm, it is impossible in hydrogen to perceive the least derived current in the black space, while this derived current is at the same time 35° in the luminous space; at a pressure of 15mm it was 90° in the neighborhood of the two electrodes alike, but there was as yet no formation of the obscure space, and consequently the state of density of the gas was the same at the two extremities of the tube. The resistance of the obscure space is also very feeble in nitrogen under a pressure of 2mm, since the derived current is only 3°, while it is 18° in the luminous space; but the difference between the two derived currents is less than in hydrogen. This difference results from the fact that hydrogen, having a conductibility much superior to that of nitrogen on the one hand, the absolute intensity of the current is greater,, which explains why we have 35° instead of 18° in the luminous space; on the other hand, the derived portion must then be less where rarefaction still more augments the conducti. bility of the gas, which accounts for our having 0° in place of 30 in the obscure space.

Let us here remark, that all the results which show the unequal resistance presented by different parts of the same gaseous column to the propagation of electricity are readily comparable with one another, since it is the same electric stream which successively traverses these different and unequally conducting

parts.

If we place the sounds in a portion of the stream which is of the distance from one of the electrodes, and consequently from the other, we have for the intensity of the derived current, under a pressure of 2mm in air or nitrogen, 89 when the negative electrode, 12° when the positive, is nearest to the sounds. In hydrogen we have 200 and 36o. Thus, the conductibility of the gaseous column goes on diminishing gradually from the obscure space, where it is at its maximum, to the space near the positive electrode, where it is at its minimum.

By placing the sounds always in the same portion of the stream, we can find in the intensity of the derived current a sufficiently exact expression of the degree of resistance of different gases at different degrees of pressure, provided we take care, by means of a rheostat, to give to the principal current in each case the same degree of absolute force. This is an investigation with which I am at present occupied, and which is not yet finished.

We see, then, that the obscure space near the negative electrode offers much less resistance to the passage of the current than does the luminous part near the positive electrode. It thence results that, for the same reason that the less conductive portion of the gaseous column is more luminous than that with greater conducting capacity, which remains nearly dark, the temperature of the first should be higher than that of the second-an inference which experiment has fully confirmed.

Two thermometers of mercury, with cylindrical reservoirs, were placed in the interior of the tube, which is 16 centimetres in length and 4 in diameter, at the respective distance of one centimetre from each of the electrodes-a distance sufficient, as was ascertained, to annul the cooling or heating influence of those electrodes. That the influence would rather have been refrigerant, was found susceptible of verification by bringing them nearer the reservoir of the thermometers, which is not surprising, in view of their dimensions, (full metallic balls one centimetre in diameter.)

By causing the electric stream to traverse the rarefied nitrogen or hydrogen, a great difference was at once perceptible between the temperature acquired by the thermometer placed in the dark space near the negative electrode and that acquired by the thermometer placed in the luminous space near the positive electrode. These differences observe nearly the same ratio between the pressures of 1 to 10mm, even when the absolute temperatures, with which they must not be confounded, vary with the pressure and with the nature of the gases. Thus, even when there is no longer any sensible obscure space at the negative electrode, the thermometer is less elevated there than in the neighborhood of the positive, wbich proves that the gas is there more dilated and of more conducting capacity. The difference of temperature, then, should be a still more sensible criterion than the difference of brightness, of the greater or less electric resistance of different parts of the gaseous column. The absolute temperature is in general less in hydrogen at all degrees of rarefaction than in nitrogen and atmospheric air, which offer more resistance to the passage of electricity. The difference between the two thermometers was, moreover, never so great in hydrogen as in nitrogen, or atmospheric air. Thus it was at the maximum of 430* in hydrogen, under the pressure of 9 to 12mm, the thermometer having risen, in two minutes, from 21° to 261° at the negative electrode, and 21° to 31° at the positive. In nitrogen the maximum difference was 5° under the pressure of 5mm, (20° to 24° at the negative thermometer, 20° to 29° at the positive.) In atmospheric air the maximum difference was, at a pressure of 6mm, 6°, (from 18° to 260 at the negative thermometer, and from 18° to 32° at the positive.) At a pressure of 20mn the difference was not more in hydrogen than 21°, (from 21° to 28.1°, and from 21° to 26°;) in nitrogen but a half degree, (from 20° to to 25°, and from 25° to 25.1°;) and in atmospheric air it was null, (from 19o to 28° at the two thermometers alike.) When there is no longer a difference between the indicatious of the two thermometers, or that difference is very slight, it will be observed that the appearance of the luminous stream is perfectly uniform through its whole extent. Here we give a table of some experiments :

ATMOSPHERIC AIR, (duration of the experiment, two minutes.) Pressure. Positive thermometer. Negative thermometer. Difference. 2mm........ 16° to 25°......... 16° to 21° ........ 4° 18° to 31°.

18° to 25.1o.
6mm.... 18° to 32°

180 to 260
gmm...
184 to 31

18° to 277
10mm... 18° to 31°

18° to 285 15mm.... 181° to 31o.

181° to 290 20mm..

197 to 28°...... 19° to 280
NITROGEN, (duration of the experiment, two minutes.)
2mm.
19° to 24°

19° to 22°......
4mm..
201° to 28°

201° to 25°.....
5mm.
200 to 29°

200 to 240
6mm
20° to 31

20° to 27°
gum...
20° to 31°

20° to 27°
15mm.... 210 to 30°

21° to 27°. 20mm... 20° to 251°........ 20° to 25°.

HYDROGEN, (duration of the experiment, two minutes.)
2mm.

21° to 27° ...... 21° to 25°
5mm.
20° to 28.

20° to 2519
21° to 293 .. 20° to 2570.
9mm
21° to 31°

20° to 261°.
15mm
21° to 30°.

21° to 26 ....
20min
21° to 28.1°...

21° to 26°
21° to 25 .

21° to 231°...... * The thermometric indications are in degrees of Reaumur,

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The following is the result of an experiment in which the duration of the passage of the current was prolonged beyond 2 minutes, through atmospheric air at a pressure of 5mm: Duration of passage. Positive thermometer. Negative thermometer. Difference. 2'.......... 18° to 31°...

..... 18° to 26° to 37°.....

to 3010 - to 40°...

- to 327 . - to 42°.....

: - to 33° ..... 10'.......... - to 43o.......... - to 34o ......... 9° In proportion as the duration of the experiment increases and the absolute temperature rises, the differences between the temperatures indicated by each of the two thermometers become proportionally less; the indications of the two thermometers end by approximating, and even becoming the same after the lapse of a certain time. Hydrogen and nitrogen give the same results.

The numbers which express the temperatures in the preceding tables cannot be given as being of perfect exactness; they vary, in effect, in their absolute values with the intensity of the electric stream; but they are sufficiently constant and exact to demonstrate : 1st, that there is a sensible elevation of temperature, which accompanies the propagation of the electric current in rarefied gases; 2d, that this elevation is sensibly less in the neighborhood of the negative electrode than near the positive, when once the gases are sufficiently rarefied for the discharge to pass easily and the electric light to be stratified ; 3d, that the absolute elevations of temperature at the two electrodes, and their differences, vary with the density and the nature of the gas.

A fact which shows well all the calorific and luminous power of electricity, is, that hydrogen reduced to 1 mm of pressure can become luminous and be heated in a very sensible degree* by the passage of electricity, although at that pressure it has a density so inconsiderable that a cubic centimetre of the gas does not weigh more than barely soho of a milligramme.

When we see matter so subtle as hydrogen reduced to one or two millimetres of pressure, becoming luminous under the influence of electricity, the temptation can hardly be resisted of surmising an analogy with the matter at once so subtle and so luminous which constitutes the cometary bodies. This analogy becomes still more striking when we examine closely the appearance presented, in the tube which contains the rarefied hydrogen traversed by the electric current, by those species of luminous mists which manifest themselves at the moment when we introduce a little gas into the tube, and which we also see in the obscure space when a certain degree of rarefaction has been attained. Undoubtedly the gaseous matter is there still more rarefied than it is in the rest of the mass where it is already extremely so, and it offers at the same time a remarkable resemblance with the luminous matter which constitutes the comets.

DIV.INFLUENCE OF MAGNETISM ON ELECTRIC CURRENTS PROPAGATED

IN HIGHLY RAREFIED GASEOUS MEDIUMS. This influence, whose existence I have shown under the form of a rotation communicated by the pole of a magnet to the electric currents which radiate from it, is, as might be expected, and, as M. Plucker has evinced by several remarkable experiments, general. The luminous filaments which display themselves in rarefied gases, traversed by the discharges of the Ruhmkorff apparatus,

* The heating of the gas must in fact be very considerable to be capable, in two minutes, of raising by nearly 30 the temperature of a thermometer whose reservoir is a cylinder of mer. cury 21 millimetres in diameter by 3 centimetres in length. Besides, the single fact that the gas is luminous well evinces its high temperature; for the light is evidently but the effect of its incandescence.

are attracted or repelled by magnets as electric currents circulating in metallic wires would be. In a word, this action is subject to all the laws of electrodynamics, with this difference, that all the parts of the mobile conductor being independent of one another, instead of being united with one another, as they are in a rigid wire, they completely obey the forces which act upon them, and take the positions of equilibrium which result therefrom. Hence it is that the luminous filament takes the form of a magnetic curve; a necessary condition, in order that the equilibrium should take place, since the action of the magnet on the element of . current is then nothing, the direction of the action being perpendicular to that element when it is a tangent to the magnetic curve. I have verified in sundry cases the law just recited, and have even succeeded in showing that, conformably to the law of Ampère, two electric streams having the same direction in a rarefied gas attract each other as two voltaic currents transmitted across movable metallic wires would do. I have not realized the repulsion of two electric streams passing in contrary directions, by reason of the practical difficulty which I have hitherto encountered in constructing an apparatus for the purpose. I do not, however, renounce the hope of being able to do so. I shall return to this subject in an article in which I propose to consider the mutual action of electric currents on one another. I restrict myself, for the present, to an investigation of the effects of magnetic action on those currents. My researches on this subject comprise two series of experiments: first, those in which the electro-magnet from which the electric action emanates is placed externally to the rarefied gas through which the electric stream is propagated: secondly, those in which the magnetized iron is situated in the gas itself. One of the most simple cases is that in which one of the tubes of which I have spoken in preceding experiments is placed either axially or equatorially in relation to the poles of a strong electro-magnet. The following is what is observed when care has been taken to rarefy well the gas which transmits the electric current. The portion of this current submitted to magnetic action is condensed towards the walls of the tube in the part nearest, or that most remote from, the magnetic poles, according to the direction of the current and that of the magnetization; the striae become much more compressed and more brilliant. If the portion of the tube placed in the neighborhood of the electro-magnet is that where the negative electrode happens to be, the obscure space is immediately seen to become luminous, and to present close and brilliant striae as would be the case with the constantly luminous portion of the current which seems to advance. At the same time, the bluish photosphere which surrounds the negative ball contracts to at least half its size, becoming more brilliant, and the sort of bluish sheath which surrounded the metallic rod, at the extremity of which is the negative electrode, completely disappears. All that bluish atmosphere is concentrated on the ball. It seems that all the gaseous filaments, which may be considered as so many conductors of the discharge, instead of radiating from all points of the negative ball and rod, and being disseminated through the entire gaseous mass as far as the positive electrode, radiate only, when the magnetic action is exerted on them, from the negative ball, becoming condensed towards the walls of the tube, on one side or the other, as far as that portion of their course at which, the action being no longer sensible, they resume their normal position. This condensation explains why the part of the current which was obscure because the gas was there too much dilated, becomes luminous, and why that part which was already luminous becomes more slender and brilliant, with stratifications more closely compressed. The action of the magnet produces the same effect, which would be produced by a local augmentation of density in the rarefied gaseous matter. Further, it is not necessary that the action of the magnet should take place exactly on the obscure part in order to its becoming luminous; it equally becomes so, even when the magnetism acts on another portion of the current, provided it be not too remote from the negative electrode. The consequence of the explanation just given, and which it is easy to verify by experiment, is, that the portion of the gas which transmits the discharge must, when it is subjected to the action of the magnet, become a more imperfect conductor, and that consequently the electric current must, on the whole, encounter a greater resistance in its passage along the interior of the tube when one part of the tube is approached by the electro-magnet than it encountered previously. Thus the tube of one metre being filled with rarefied hydrogen, and the apparatus of derivation placed in the circuit," we obtain the following results:

Pressure. Intensity of the derived current. Without magnetization. Magnetization at the Magnetization at the positive electrode. negative electrode. 4*- - - - - - - - - - - - - - 33°-------------- 30°-------------- 20° 8*- - - - - - - - - - - - - - 30°-------------- 30°-------------. 109

With the tube 50 centimetres in length, filled with nitrogen, we have:

Pressure. Intensity of the derived current. Without magnetization. Magnetization at the Magnetization at the positive electrode. negative electrode. 2*-- - - - - - - - - - - - - 57°.------------- 52°-------------- 429 4*- - - - - - - - - - - - - - 37°. . . . . . . . . . . . . . 27°-------------- 17o 6*- - - - - - - - - - - - - - 25°. . . . . . . . . . . . . . 20°-------------- 120

The effects are more marked when the tubes are placed equatorially between two soft-iron armatures of the electro-magnet, which are immediately in contact with the walls of the tube, than when they are placed axially on the poles themselves. We see that there is a much greater increase of resistance when the magnetism acts on the portion of the current near the negative electrode than when it acts on the portion near the positive electrode. The reason of this difference is, that the first portion which, as we have seen in the preceding paragraph, is endued with a much greater share of conductibility, must naturally experience a more considerable diminution of that property by the condensation of the gaseous matter produced by the action of the magnet, than is experienced by the second portion, where the gas is less rarefied. The direction of the magnetization has no influence on the results: it has no other effect than to elevate or depress the current, which, when the magnet does not act, is simply horizontal.

Among the experiments which I have made regarding the influence exerted by the exterior action of magnetism on rarefied gases enclosed in tubes, I will further cite those in which the tube is convoluted into a flat spiral terminated by two prolongations perpendicular to the plane of the spiral which serve to introduce and rarefy the gas, as well as to give a passage to the discharges; the tube of the spiral and its prolongations is a little less than one centimetre in diameter, and its total development nearly eighty centimetres. It is necessary that the gas should be rarefied at least as much as 2" for the discharges to pass when nitrogen or atmospheric air is employed. With hydrogen, a pressure of 5 or 6mm suffices for their transmission. But whatever the gas or its degree of rarefaction, it is only after the lapse of some minutes from its being placed in the circuit that the discharge begins to pass. . It is evidently necessary that it should be sometimes charged with static electricity for the resistance to the establishment of the continuous stream to be surmounted. But that

* It should not be forgotten that here the derived current is proportional to the principal current, so that its intensity may be regarded as being quite approximately the measure of that of the discharge which traverses the tube.

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