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resistance once surmounted, we may interrupt the passage of the discharge without incurring the necessity of waiting more than an instant for the transmission to recommence, when we close the circuit anew, provided the interruption does not exceed an hour or two. The luminous current presents, with hydrogen under a pressure of 5 or 6", very neat and distinct striae of a rose color; at a pressure of 2", they become much larger and less distinct; the color is also paler. The same occurs with air and with nitrogen, but the effects are more striking with hydrogen. A remarkable appearance presented by the current in the interior of the spiral is, that it seems to undergo a very distinct movement of rotation, in a direction which appears to vary with the direction of the discharge; but this last result is not very constant, which has led me to think that the rotation is only apparent, and that it is the effect of the discontinuity of the discharges which constitute the current, a discontinuity which produces the illusion of a displacement. This point, however, deserves to be studied anew. In order to observe the action of magnetism on the spiral current, I place the spiral of glass between the two poles of the electro-magnet in such a way that its plane shall be the same with that of the two polar surfaces, the two prolongations being thus rendered vertical, the one above, the other below, that plane. The magnetization, according to its direction, either condenses the current towards the interior walls of the spiral tube, or, on the contrary, repels it towards the exterior walls, rendering it very diffuse. In the first case, it becomes highly brilliant, and the stratifications are very distinct; in the second case, they are but slightly visible, and the current itself is much larger and quite dim. It appears to undergo, in even a more sensible manner, the movement of rotation, of which we have spoken. A quite curious fact is, that in the vertical branch of the tube which is below the spiral, and consequently between the two branches of the electro-magnet, the current divides itself, under the influence of the magnetism, into two streams or filaments, which tend, respectively, to one and the other side of the tube. Of these two filaments, one is very small, and of little brilliancy, in comparison with the other. The cause of this separation consists, very probably, in the fact that the inductive current of the Ruhmkorff apparatus is really composed, as we have already taken occasion to say, of two successive and opposite inductive currents, one having much more tension, and passing almost exclusively through the gas, while the other is transmitted with much difficulty, but yet passes, in very small proportion, it is true, since the action of the magnet separates it from the principal current, which is the only one in general that it is requisite to consider in this kind of phenomena, because it is by much the strongest. I have sought to determine in the case of the spiral tube, as I had done with the large rectilinear tube, the influence of magnetization on the resistance of the gas to the transmission of the discharge, and I have obtained a rather curious result. The two points of platina of the apparatus of derivation being at a distance of ten millimetres from one another in the distilled water, I obtained a derivative current of 20°, the spiral tube being filled with hydrogen under the pressure of 2mm. The spiral was placed vertically between the two horizontal armatures of the electro-magnet which were exactly in contact with its two faces. As soon as the magnetization took place, the derivative current was reduced to 15°, when the discharge was repelled, and driven towards the exterior walls of the spiral with an apparent movement of rotation, and it was raised, on the contrary, to 25°, when the discharge was condensed towards the interior walls of the spiral. Does this influence of the direction of the current or of the magnetization depend on the particular form given to the stream, or to the small diameter of the tube, in comparison with its development in length It is a point for future elucidation. I pass now to the case where the magnetic pole is in the midst of the which transmits the discharge. I have first operated with a spherical o

about fifteen centimetres in diameter, furnished with four tubulures situated at the respective extremities of two diameters of the globe, which intersect one another at right angles. Two cylindrical rods of soft iron are fixed by means of two of these tubulures in the interior of the globe, in the direction of the same diameter, so that their interior extremities may be at a distance of about eight or ten centimetres from one another, while their exterior extremities project from the tubulure nearly two centimetres. It is these exterior extremities which are to be placed in contact with the poles of a strong electro-magnet, in order that the interior extremities may thus become two magnetic poles. The two other tubulures serve to introduce into the interior of the globe two isolated metallic rods, terminated by balls which are at a distance of about ten centimetres from one another, and which serve as electrodes to the electric stream whose direction is thus equatorial, that is to say, perpendicular to the right line which joins the two magnetic poles. As long as the rods of soft iron are not magnetized, the electric stream remains perfectly rectilinear; but so soon as magnetization takes place, the stream, which we will suppose to have a horizontal direction, takes the form of a half circumference of a circle situated either above or below the line which joins the magnetic poles, according to the direction of the magnetization or that of the discharge. The form of the luminous arc is that of a half ring much flattened, as well as widened. The striae are strongly marked in it, more than they were in the rectilinear current, and its exterior part is much serrated, especially when the gas contains a little vapor of alcohol or ether. If the electric current, instead of being equatorial, is axial, that is to say, directed from one of the magnetic poles to the other, these two poles serving it as electrodes, it experiences no sensible modification under the influence of magnetization. If, however, the discharge is made to pass between a ball of brass and one of iron, placed at the extremity of an iron rod so as to be capable of being magnetized, there is observed, at the moment of magnetization, a movement of depression, or of elevation in the luminous atmosphere which surrounds the ball of iron. This movement pertains evidently to the change of direction undergone by the electric filaments which radiate from the ball. But the best mode of studying the action of magnetism in the cases where the magnetized bar is in the interior of the gas, is to make use of a bell or cylindrical jar sixteen centimetres in diameter by twenty centimetres in height, in the axis of which is placed a rod of soft iron having a diameter of about three centimetres, whose rounded end is situated at the middle of the axis of the cylinder. This rod is planted in a circular disc, which serves to close the jar. A metallic ring, about twelve centimetres in diameter, formed of wire from 3 to 4” in diameter, and having for its centre the top of the iron rod, is situated in a plane perpendicular to the axis of the jar. This ring communicates, by means of a rod covered with an isolating coat which is soldered to it, with one of the poles of the Ruhmkorff apparatus, while the other pole is placed in communication, outside the jar, with the extremity of the rod of soft iron, which, in the interior of the jar, is also covered with an isolating coat, except at its summit. It is between this summit and the ring of which it is the centre that the discharge takes place. In order to magnetize the rod of soft iron, it now suffices to place it in contact, by its exterior extremity, with the pole of an electro-magnet, taking care to place between the two a thin strip of caoutchouc to serve as an isolating layer, so that the whole apparatus shall '. well isolated. The cylindrical jar is also closed at that one of its two extremities where the rod of soft iron is absent, and it is there furnished with two cocks, of which one serves to form a vacuum, and to introduce a gas which is more or less rarefied; and the other, constructed in Gay Lussac's manner, permits the introduction into the ball of a greater or less quantity of vapor of whatever nature.

I have made many experiments with this jar by filling it successively with atmospheric air, with nitrogen, and with hydrogen, at different degrees of rarefaction, these gases being at times perfectly dry; at others, containing a greater or less proportion of vapor, either of water or of alcohol. Atmospheric air and nitrogen, when both dry, give nearly identical results. with this difference, that the light is more vivid and clearer with nitrogen. If the soft iron be taken for the positive electrode, and the ring for the negative one, the luminous current is seen to form, at a certain degree of rarefaction, a sort of peach-red envelope around the top of the soft iron, and a sheath of a pale violet color along an arc of a greater or less number of degrees around the ring. At a very weak pressure this sheath encompasses the whole ring, while the top of the soft iron is completely enveloped with a rose-colored aureole, from which issues a very short stream of the same shade, and presenting the form of a large virgule, or comma. This virgule, when the iron is magnetized, is distinctly seen to turn in one or the other direction, with the aureole from which it emanates, according to the direction of the magnetization. The violetcolored sheath which surrounds the ring is also seen to turn in the same direction with the rose-colored aureole, although they are separated by a space completely obscure. By changing the direction of the discharges, there will be seen at the negative electrode a violet-colored envelope, which only covers the whole surface of the top of the iron rod when the gas is very much rarefied, and at the positive electrode, brilliant points, separated from one another by a roseate glimmer which surrounds the entire ring, and whence emanate regular stratifications, internally concentric to the ring. When the gas is not greatly rarefied, there is seen to issue from the ring a luminous jet which tends to the summit of the central rod of soft iron, being only separated from it by a small, black space, and which undergoes a movement of rotation in one direction or the other, like the hand of a watch, according to the direction of the magnetization. In this case there is but a portion of the top of the iron rod which is covered with the violet envelope, and this luminous segment turns with the brilliant jet. I have made a great number of experiments, under the conditions just indicated, with atmospheric air, with nitrogen, and with hydrogen, whether dry or more or less charged with vapors. I shall proceed to give a description of them in a summary manner, first remarking, however, that, whatever be the gas and its degree of elasticity, whether it be dry or impregnated with vapor, the rapidity of rotation is always much greater when the ring serves as the positive than when it serves as negative electrode, and that this rotation, which increases in rapidity in proportion as the tension diminishes, ceases to be appreciable at a much less tension in the second case than in the first. In my earlier experiments I had made use of a large globe, twenty-five centimetres in diameter, in which the ring was twenty centimetres in diameter, and the central iron rod three. This globe was furnished with two tubulures, one serving to introduce the iron rod, whose top reached the centre of the globe, and its lower extremity issuing from the tubulure, so as to be capable of resting on the polar surface of an electro-magnet. The other tubulure was closed by a cock, which served to introduce the gas and vapor, and from it there issued an isolated conductor, which supported the ring and admitted of its being placed in the circuit. The discharge thus passed between the summit of the rod of soft iron and the metallic ring. This globe was filled with air rarefied to 4 mm. The discharge took place under the form of a stream which turned with a rapidity of sixty revolutions per minute when the ring was positive, and twenty when it was negative. At a pressure of 6" the velocity was only forty revolutions per second in the former case and twenty in the latter. With vapor of alcohol, at a pressure of 5", the velocity was respectively twenty-two and eleven revolutions perminute. After these first experiments, which served as my introduction to this sort of researches, I resumed the study by availing myself of the jar of twenty by sixteen centimetres, described above. The following are the results obtained with dry atmospheric air:

Pressure. Number of revolutions in a minute. - Ring, positive. Ring, negative. 16”------------------------------ 55 --------------------- 36 12"------------------------------ 83 --------------------- 55 9mm. . . . . . . - - - - - - - - - - - - - - - - - - - - - - - 99 --------------------- 63 6"------------------------------ xx - - - - - - - - - - - - - - - - - - - - - 100 3*------------------------------ -- - - - - - - - - - - - - - - - - - - - - - 128 2"------------------------------ ** - - - - - - - - - - - - - - - - - - - - - ->

At 9mm, with the ring serving as a positive electrode, there is no longer a stream, but a dilatation of the discharge, forming a sector of from 30° to 45°; and this sector obeys the movement of rotation as the stream before obeyed it. But it enlarges in proportion as the pressure diminishes, and at 6" forms a complete circular sheet, and it is then that the rotation, which, up to this point, had increased in rapidity, becomes no longer sensible. When the ring serves as a negative electrode it is covered with a violet sheath, whose size likewise increases in proportion as the pressure diminishes, but which occupies only half the circumference of the ring under a pressure of 4*. It is seen to turn very rapidly, but at a pressure of 2mm it occupies the whole circumference of the ring, and there is no longer any sensible rotation. At the summit of the magnetized iron rod there is a roseate aureole, from which, as has been said, emanates at one point a very short jet in shape of a comma, which turns with the violetcolored sheath, from which it is separated by a very considerable obscure interval.

It should be remarked that, at a pressure of 6, of 4, and sometimes of even 3”, it most often happens, when the ring serves as positive electrode, that at the first moment of the circuit being formed there issues a stream which turns too rapidly to allow its velocity of rotation to be measured, but which quickly expands so as to form, for some instants, a sector which continues to revolve, and soon after a complete circular sheet, which no longer manifests any movement.

It does not follow that the action of magnetism is annulled when the gas is too much rarefied for the continuance of a sensible rotation. That action is manifested under another form, as is shown by experiments made under a pressure of from 3 to 2mm. Thus, if the ring serves as negative electrode, the violet sheath which surrounds the soft iron is seen, at the moment when this last is magnetized, to subside sensibly, and to rise at the instant of its being demagnetized. If, on the contrary, the ring serves as positive electrode, the rose-colored sheet which fills the interval between the ring and the summit of the central rod of iron is raised, as well as the violet sheet which issues from that summit, at the moment of magnetization, and depressed at the instant of demagnetization.

The following is a more complete experiment with dry nitrogen, and shows that rotation begins to manifest itself at stronger pressures when the ring is positive than when it is negative:

Pressure. Number of rotations in a minute. Ring, positive. Ring, negative. 35* - - - - - - - - - - - - - - - - - - - - 12 --------------- - - - - - - - -> 29* - - - - - - - - - - - - - - - - - - - - 27 ---------------------- x 21* - - - - - - - - - - - - - - - - - - - - 45 - - - - - - - - - - - - - - - - - - - - - - 36 16** - - - - - - - - - - - - - - - - - - - - 67 ---------------------- 51 12" . . . . . . . . . . . - - - - - - - - - 99 . . . . . . . . . . ------------ 59 8* . . . . . . . . . . . . . . . ----- 115 . . . . . - - - - - - - - - - - - - - - - - 70 6" . . . . . . . . . . . - - - - - - - - • ?? - - - - - - - - - - - - - - - - - - - - - - 115

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- - - - - - - - - - . . . . . . . . . . . . . . . . . . --------------- 150

At 4mm the rotation is too rapid to allow its degree to be observed; at 3mm it appears completely to cease. The rose-colored aureole is very vivid when the summit of the rod of soft iron is positive. When there is no longer any rotation, there is observed, as with atmospheric air, a movement of depression and of ascension under the influence of magnetization.

The presence of vapor modifies in some important particulars the results obtained with dry gases. The following is an experiment made with ordinary air subjected to a pressure of 2", into which vapor of water has been introduced in successive quantities, so as to increase that pressure solely by the effect of the presence of the vapor:

Pressure. Number of rotations in a minute.
Ring, positive. Ring, negatire.

2* - - - - - - - - - - - - - - - • - - - - ?? - - - - - - - - - - - - - - - - - - - - - - --
4* - - - - - - - - - - - - - - - - - - - - xx - - - - - - - - - - - - - - - - - - - - - - ->
6mm . . . . . - - - - - - - - - - - - - - - xx - - - - - - - - - - - - - - - - - - - - - - 92
8* - - - - - - - - - - - - - - - - - - ... 140 - - - - - - - - - - - - - - - - - - - - - - 70

10* - - - - - - - - - - - - - - - - - - - - 120 ---------------------- 52

12" -------------------- 90 ---------------------- 50

14" - - - - - - - - - - - - - - - - - - - - 80 ---------------------- 48

We see that at an equal pressure the rapidity of rotation is greater with vapor of water than with dry air, which is attributable probably to the greater facility with which the electric discharge is transmitted. With the external air of a mean humidity, we have, with a pressure of 14”, 72 revolutions instead of 80 when the ring is positive, and 44 instead of 48 when it is negative. But the most characteristic fact produced by the presence of watery vapor is the division, under the influence of magnetism, of the single current into several small distinct and equidistant currents, which turn like the radii of a wheel. This division is only observed when the ring serves as a positive electrode. At a pressure of 6mm the single current begins with turning, then expands, whereupon the rotation is no longer perceptible; but at the pressure of 8, of 10, and of 12mm this current, from the commencement of its rotation under the action of magnetism, divides into five or six small streams which turn, as was just said, like the radii of a wheel; while, when the air is dry, the current never divides; but, under a weak pressure, it merely expands into a sector or a circle of which all the parts are continuous. When the ring is negative, and there is vapor present, it will be seen that the current which issues from the summit of the iron rod presents, where it is in contact with the iron and at the moment when this is magnetized, instead of a continuous surface, a series of small brilliant points, which seem points of emanation for as many small currents, too little distant from one another to become distinct. Here, then, this current, which does not divide into separate filaments, simply undergoes dilatation or expansion at the point where it is in contact with the iron. The vapor of alcohol produces similar effects with the vapor of water. The single current is, in this case, much more brilliant than with dry air or with the vapor of water; it presents fine stratifications, which give it an appearance not unlike that of a caterpillar. Magnetization expands and divides it into several currents, sensibly larger than those observed with the vapor of water. If, however, the diameter of the ring is too large, greater, for instance, than fifteen centimetres, the subdivision of the current is not effected without difficulty, unless the intensity of the discharge and that of the magnetization be very considerable. The following is an experiment in which, the rarefied gas being hydrogen, different portions of alcoholic vapor were successively introduced. The pressure

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