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

From Wikisource
Jump to navigation Jump to search
Thomson RoPE 1913 Fig42.jpg

The positively charged particles, which we have hitherto considered, originate in the neighbourhood of the cathode. Gehrcke and Reichenheim1 have discovered rays of positively charged particles which start from the anode. Their attention was called to these rays by noticing that a pencil of yellow light streamed from a point on the anode of a tube with which they were working. It was found that there had been a speck of sodium chloride at the points on the anode from which the pencil started. They got these rays developed to a much greater extent when they used for the anode a piece of platinum foil with a little pocket in which various salts could be placed. The foil was in circuit with a battery insulated from the one used to send the current through the discharge tube; this battery was for the purpose of raising the anode to a red heat, as these rays are not developed unless this electrode is at a high temperature. The current through the tube was produced by a battery giving a potential difference of about 300 volts which, as a Wehnelt cathode was used, was sufficient to send a very considerable current through the tube: the pressure in the tube was very low. The rays were well developed in this tube when NaCl, LiCl, KCl and the chlorides of Cu, Sr, Ba, In, were placed in the pocket. The colour of the rays corresponded with the colour given to flames by the salt They did not get any effects when the oxides of calcium or barium were put in the pocket; these oxides are known when hot to give out large streams of negatively electrified corpuscles and for this reason are used for Wehnelt cathodes. These rays are apparently only given out by the salts of the metals and not by the metals themselves ; they are called Anode rays.

Gehrcke and Reichenheim arranged a Faraday cylinder so that the rays could fall into it; they found that when the rays entered the cylinder It acquired a strong positive charge.

Gehrcke and Reichenheim subsequently used another form of apparatus which gave better results than the one just described. The anode was a rod of salt placed inside a glass tube so that only the front of it was exposed to the tube; the cathode was an aluminium ring encircling the anode, the pressure was reduced to a very small value by the use of carbon cooled by liquid air. With the discharge from a powerful induction coil, or still better from a large electrostatic induction machine, the anode got hot without the aid of an auxiliary heating current, and a bright stream of rays came from the end of the salt anode; the appearance of this beam is represented In Fig. 43. It was found that a mixture of two or more salts with powdered graphite gave brighter rays than a simple salt, the best mixture seemed to be LiBr, Lil, Nal and graphite. The rays come off at right angles to the surface of the salt; thus if the surface is cut off, as in Fig. 44, the rays come off In the direction ab.

Thomson RoPE 1913 Fig43.jpg
Thomson RoPE 1913 Fig44.jpg

Gehrcke and Reichenheim found that there was a very considerable difference of potential between the surface of the anode and a point a centimetre or two away: in some of their experiments it was as much as 2300 volts. By assuming that the energy acquired by the rays was due to the fall through this potential V, and measuring the radius of the circle into which the rays were bent by a strong magnetic field H, the values of v and m/e can be determined, for we have

and if r is the radius of the circle Into which the rays are bent by a magnetic force H at right angles to their path



In this way the following values were obtained :—

Salt v. cm/sec e/m ratio of mass of particles to that of an atom of hydrogen
LiCl. 2.40–2.71x107 1.11–1.15x103 8.6–8.3
LiCl. 1.89–2.26x107 .69–.875x103 14–11
NaCl. 1.87–1.76x107 .46–.41x103 21–23
SrCl2. 1.08x107 .21x103 90 (if the atom is doubly charged)

The results for LiCI given in the first line relate to the brightest part of the rays, those In the second to the least deflected rays. It would appear from this that the charged particles are the atoms of the metal in the salt and that in the case of strontium they carry a double charge, A very Interesting case of these anode rays is that of a discharge tube with a constriction In the middle. When two bulbs A and B, about 10 cm. in diameter, with the anode in A and the cathode in B, are connected by a narrow tube: then when the pressure in the tube Is very low and a small quantity of iodine vapour is introduced into it, anode rays start from the constriction c at the cathode end of the narrow tube and cathode rays from d, the anode end of this tube. If the connecting tube were quite straight these anode rays might be the positive rays corresponding to the cathode c, but as they appear when the tube is bent this cannot be their origin. It is especially to be noticed that the anode rays do not appear unless iodine, bromine, or chlorine is in the tube. This is perhaps due to the fact that the atoms of these substances are excellent traps for negatively electrified corpuscles as they hold these corpuscles imprisoned. Any positively electrified particles in the tube will thus have a much better chance to escape being neutralized by these corpuscles when these gases are present than when they are absent: and thus the number of anode rays will be increased.

The most natural explanation of these rays is that the hot salts from which they originate act like fused electrolytes, and that the current through them into the discharge tube is carried by the ions into which the salts dissociate, the positive ion, which is a charged atom of the metallic constituent of the salt, following the current will come to the surface of the anode, will get detached from it, and under the influence of the strong electric field which exists in the gas close to the anode will acquire the high velocity characteristic of the anode rays.

1 "Verh. D. Phys. Gesell.," 8, p. 559; 9, pp. 76, 200, 376; 10, p. 217.