Rays of Positive Electricity and Their Application to Chemical Analyses/Retrograde and Anode Rays

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The rays we have hitherto been considering consist of positively charged particles travelling in the direction in which such particles would be moved by the electric field in the discharge tube. In addition to these there is another system of rays travelling in the opposite direction. By far the larger portion of these rays are cathode rays, i.e. negatively charged corpuscles moving with great velocity, but as the author showed long ago1 these are mixed with rays which are evidently of a different character, for unlike the cathode rays they are not appreciably deflected when a permanent magnet is brought near them. It was afterwards shown by Villard2 and the author3 that some of these new rays were deflected by strong electric and magnetic fields and that the direction of the deflection indicated that the particles forming the rays were charged with positive electricity. The fact that these rays travel with high velocities away from the cathode and thus in the opposite direction to the electric forces acting upon them makes their investigation a matter of very considerable interest The apparatus I have used for this purpose is represented in Fig. 40.

Thomson RoPE 1913 Fig40.jpg

A is a perforated electrode through which the rays pass on their way to the willemite screen or photographic plate S. On their journey to S the rays traverse the usual electric and magnetic fields. B is a plane rectangular electrode at the other end of the discharge tube: it is carried by a stopper working in a ground glass joint and thus can be rotated about a vertical axis. C is a wire fused in the side of the tube for use as an auxiliary electrode. D is a side tube in which a closed glass vessel containing a piece of iron can slide up or down: this vessel carries a piece of fine metal rod which, by moving the iron by means of a magnet, can be inserted in or withdrawn from the line of fire of particles projected from B. When the stopper carrying the electrode B is turned so that the normal of the plane of the electrode either coincides with the axis of the hole through A, or makes but a small angle with it, then if B Is made cathode and a discharge sent through the tube, the cathode rays pass down through the tube in A and produce vivid phosphorescence on the screen. In addition to these rays there are others which produce a phosphorescence different In colour from that due to the cathode rays and are deflected in the opposite direction by the and the magnetic fields: the amount of electrostatic deflection is about the same as that for the cathode rays but the magnetic deflection is very much less. It can easily be shown that these are not ordinary positive rays due to A becoming cathode through accidental reversals of the coil. For in the first place they disappear when the electrode B Is twisted round so that a normal to its plane no longer nearly passes down the tube through A: and secondly the rays persist when A is disconnected from the induction coil and the auxiliary electrode C used as a cathode. Again when the rod attached to D Is put in the line of fire a shadow is thrown on the phosphorescence on the screen, due to these rays. These rays are strongest when the electrode B is placed so as to be at right angles to the axis of the tube through A. If the electrode Is rotated they diminish rapidly In intensity but can be detected until the normal to B make an angle of about 159 with the axis of the tube through A; they appear in fact to follow much the same path as the cathode rays from B, for much the same rotation was required to prevent the cathode rays getting through the tube In A and producing phosphorescence on the screen.

These rays get exceedingly feeble when the pressure of the gas in the discharge tube Is very low and they are no longer observable at pressures when the ordinary positive rays give quite vigorous effects; even when most fully developed they are feeble In comparison with the ordinary positive rays, so that It Is necessary for the tube through A to have a much wider bore than Is required for experiments with positive rays. As these rays travel In the opposite direction to the positive rays they are called retro-grade rays.

Using a tube through A about .5 mm. in diameter I obtained a photograph of the retrograde rays which gave the following results:—

There are in the retrograde rays positively electrified atoms and molecules of hydrogen and positively electrified atoms of oxygen: there are also negatively electrified atoms of hydrogen and oxygen, and with these rays the intensity of the lines corresponding to the negatively electrified particles is greater than that of the positively electrified ones; with the ordinary positive rays the positive lines are much stronger than the negative. In. the retrograde as well as in the positive rays are large numbers of uncharged particles. The photograph taken with the retrograde rays shows that the maximum velocity of the negatively electrified atom is about the same as that of the corresponding positively electrified one and differs but little from the velocity of these atoms in the ordinary positive rays. This result is suggestive because the electric field in the tube would accelerate the negatively electrified retrograde rays and retard the positively electrified one. It points, I think, to the conclusion that the origin of the retrograde rays is analogous to that of the negatively electrified particles which accompany the positive rays, the difference between them being that the retrograde rays acquire their negative charge before passing through the cathode, while the negative constituent of the positive rays do so after passing through the cathode. We may suppose that the process by which the retrograde rays are produced is somewhat as follows : neutral atoms or molecules acquire a negative charge when they are just in front of the cathode, they are then repelled from the cathode and driven through the dark space, acquiring under the electric field in the discharge tube a velocity of the same order as that acquired by the positively electrified particles of the positive rays during their approach to the cathode. Some of these rapidly moving negatively electrified particles will in their course through the gas come Into collision with the corpuscles and molecules In the discharge tubes; the first collision will detach a corpuscle leaving the particle in the neutral condition; another collision will detach another corpuscle and leave the particle positively charged The particles which have made two collisions form the positively electrified portion of the retrograde rays, those which have made one collision the portion which is without charge, and which have not made a collision the negatively electrified portion of these rays.

Thomson RoPE 1913 Fig41.jpg

These retrograde rays are very well developed when a double cathode of the kind introduced by Goldstein (see p. 5) is used instead of a flat cathode. If a cathode consisting of two parallel triangular plates. Fig. 41, is substituted for the flat cathode B in the apparatus, shown in Fig. 40, a plentiful supply of retrograde rays come from the cathode when it is turned into a suitable position. By twisting the triangle round by means of the glass stopper the emission of the rays, both cathodic and retrograde, could be determined. In this way it was shown that the maximum emission of cathodic rays is along the line starting from the middle points of the sides. At the higher pressures this is practically the only direction in which cathode rays can be detected; at very low pressures, however, cathode rays can be detected coming from the corners of the triangle as well as from the middle points of the sides. Few, if any, however, are given out in any intermediate direction. The positively electrified particles stream off at all pressures from both the corners and middle points of the sides, but not from the intermediate positions. The most abundant stream comes, as for the cathode rays, from the middle points of the sides but the disproportion between the streams from the corners and from the middle points of the sides is nothing like so large as for the cathode rays, so that the ratio of positive to cathode rays is much the greatest at the corners of the triangle.

A simple method of demonstrating the existence of retrograde rays and also of the places at which the positive rays originate Is founded on the difference between the phosphorescence of lithium chloride under cathode and positive rays. When lithium chloride is struck by cathode rays, the phosphorescence is a steely blue giving a continuous spectrum. When struck by rapidly moving positively electrified particles the phosphorescence is a rich deep red, and the red lithium line is very bright in the spectrum. To explore the tube for positive rays a thin rectangular strip of mica or metal Is covered with fused lithium chloride, the strip Is attached to a piece of iron4 which rests on the bottom of the discharge tube. By moving the iron by means of a magnet the strip can be moved towards the cathode or away from it. If we start with the mica strip close to the cathode we find that there Is no red light to be seen on the side of the lithium chloride next the cathode. The anode side of the chloride Is a brilliant red, showing that the strip Is being struck by the positive rays before they reach the cathode but not by the retrograde ones. If the mica strip is pulled farther away from the cathode until the distance between them is about half the thickness of the dark space, red light appears upon both sides of the strip showing that now it is struck by the retrograde as well as by the positive rays. This state of things continues until the mica reaches the limit of the dark space and approaches the negative glow; In this position the cathode side of the strip Is red but the other side Isdark showing that now it is struck only by the retrograde rays. Another way of making this experiment is to keep the strip fixed at a distance of between one or two centimetres from the cathode. Beginning with a fairly high pressure so that the strip is outside the dark space, we find that the cathode side of the strip is reds while the other side is dark; in this position the strip is struck only by the retrograde rays. If the pressure is gradually reduced so that the dark space increases until it reaches just past the mlcas both sides of the strips will now show the red light, showing that now positive as well as retrograde rays strike the strip. When the pressure is further reduced until the dark space is three or four centimetres long, the red light disappears from the cathode side but is very bright on the other.

These experiments show that many of the positive rays start from close to the junction of the dark space and the negative glow. It is surprising to find how short is the distance which the screen has to travel from the negative glow before the redness of the side remote from the cathode shows that positive rays are striking against it. As at this end of the dark space the electric force is very feeble, the charged particle can only have fallen through a small fraction of the potential difference between the anode and cathode; yet as we have seen it has the power of exciting the lithium red light. The reason that in the preceding experiment the retrograde rays are not observed when the screen is close to the cathode is due I think to the shadow cast by the mica on the cathode. The mica stops the positive rays on their way to the cathode so that the parts in shadow are not struck by these rays and so cannot be the origin of retrograde rays, if these are produced in the way we have described.

This view is confirmed by the following experiments. If the cathode is placed near the middle of a large bulb and the mica screen is put a little on one side of the cathode, the red lithium light can be observed on the side of the screen turned towards the cathode even when the screen is quite close to the cathode and the dark space 5 or 6 cm. long.

Again if the cathode stretches across a tube of uniform bore, and the screen is moved towards the cathode, the shadow thrown on the cathode becomes much more marked and suddenly increases in size at the place where the red light fades away from the cathode side of the mica strip. The increase in size is due I think to the screen getting positively electrified when in the region close to the cathode. We know by the distribution of electric force In the dark space that there is a dense accumulation of positive electricity just in front of the cathode, which naturally would charge up an insulator placed within it. The positively electrified screen repels the positively electrified particles which pass it on their way to the cathode and deflects them from their course, so that they strike the cathode beyond the projection on it of the screen. In this way a considerably Increased area is screened from the impact of the positively electrified particles. The portion so screened no longer emits cathode rays. Thus the region in front of it is traversed by little if any current and there is consequently no bombardment of the screen by retrograde rays.

Somewhat similar effects are obtained if the mica screen Is replaced by a very fine platinum wire. If this wire is slowly moved, towards the cathode, starting from a place inside the negative glow, the following effects are observed: almost immediately after entering the dark space the wire becomes red hot and remains so until it reaches the velvety glow immediately in front of the cathode (known as Goldstein's first layer). Here It becomes cold and the shadow which before could hardly be detected now becomes well marked, and much thicker than the wire. The change takes place very abruptly. In some cases just before entering this layer the shadow is reversed, i.e. the projection of the wire on the cathode is now brighter than the rest of the cathode, indicating I think that the wire when In this position gets negatively electrified and attracts the positively electrlied particles Instead of repelling them.

The retrograde rays are well developed with cathodes made of wire gauze.

1 J. J. Thomson, " Proc. Camb. Phil. Soc.," IX, p. 243.

2 "Comptes Rendus," CXLIII, p. 673, 1906.

3 J. J. Thomson, " Phil. Mag,," XIV, p. 359, 1907.

4 It is better to put the iron in a closed tube and attach the mica strip to the tube, otherwise so much gas is given out by the iron that it is difficult to reduce the pressure sufficiently.