Experiments on Positive Rays. 211 



rays do not penetrate right through the film, the most 

 suitable plates for these experiments might be expected to be 

 those which had very thin films containing as much of the 

 silver salts as possible. Guided by this consideration, I tried 

 the old Daguerrotype silver plates treated with iodine, but 

 did not obtain such good results as with the Sovereign 

 plates. 



For some experiments it is important that the magnetic 

 and electric fields used to defiVct the rays should be as co- 

 terminous as possible, and so arranged that if necessary 

 the length of these fields can be reduced to a few millimetres 

 and the intensity increased sufficiently to give measurable 

 deflexions. The arrangement represented in fig. 1, which 

 was designed by Mr. Aston, has been found to give good 

 results. The soft iron pole-pieces, which have flat plane 

 ends A and B, are in an ebonite box which forms part of the 

 observation^tube, the ends of: the electromagnet CD fitting 

 into recesses cut in the outside of the box. The flat ends 

 of the pole-pieces, which are insulated, are used as the 

 plates to produce the electric field. Care must be taken 

 that the positive rays do not strike either ebonite or glass, 

 as if they do they charge up the surface and produce an 

 electric field which deflects the rays. Earth-connected pieces 

 of tinfoil were placed on these insulators to prevent any 

 charge of electricity accumulating upon them. A water- 

 jacket J is placed round the observation-tube to prevent the 

 wax getting hot and giving off vapour. 



We shall now proceed to disdiss some results obtained 

 with this apparatus. 



Secondary Lines on the Photographs of the Positive Pays. — 

 These photographs show lines of two types : one type con-- 

 sists of a series of arcs of parabolas which, when the deflexion 

 due to the electrostatic field is horizontal, start in most cases 

 from points lying on a straight vertical line. Since the 

 horizontal electrostatic deflexion is inversely proportional to 

 the kinetic energy of the charged particles, this shows that 

 the maximum kinetic energy possessed by the particles 

 producing these lines is the same for each kind of particle. 



These parabolic arcs are often of considerable length, 

 indicating that there is a considerable range of velocities 

 among the particles of the same kind. The ratios of the 

 latera recta of the different parabolas are equal to the ratios 

 of the values of e/m for the particles producing the parabolas, 

 and are independent of the strength and distribution of the 

 electric or magnetic fields. We attribute the formation 



P 2 



