756 



Sir J. J. Thomson on 



Fisr. 4. 



field. With this arrangement a particle, after leaving the 

 space between the plates, enters a region where it is exposed 

 to magnetic but not to electric forces, i. e. when it is deflected 

 vertically but not horizontally. In this case the appearance 

 presented by the phosphorescence patch at the pressure, when 

 under normal circumstances it would be as represented in 

 fig. 1, is shown in fig. 4. 



There is now a vertical portion OA due to 

 rays which have been deflected vertically but 

 not horizontally, i. e. which have been acted ; 

 upon by magnetic but not by electric forces, 

 and which must therefore have been produced 

 between the ends of the parallel plates and 

 the screen. Connected with the vertical piece 

 OA there is a curved part AB due to rays 

 which have been deflected by the electrostatic 

 as well as the magnetic forces. The rays 

 lulling on the portion of AB near to A have 

 been produced inside the parallel plates close 

 to the end next the screen, and have only 

 been exposed to the electric force for a small 

 portion of their path. As we approach B 

 the corresponding rays have been produced 

 nearer the cathode, while the rays at the 

 very end were already produced before the 

 the plates was entered, for we find that the 

 phosphorescent patch is in the same position as where the 

 fields of action of the magnetic and electric forces were 

 coincident. 



If we reverse both the electric and magnetic forces so as to 

 bring the negatively charged particles on to the part of the 

 screen previously occupied by the positive ones, we find that 

 the phosphorescent band due to the negative rays is an exact 

 reproduction in shape, size, and position of that due to the 

 positively charged particles. 



If this experiment is repeated when the pressure has been 

 reduced to the stage when the phosphorescence splits up into 

 two bands as in fig 3, the contrast between the behaviour of 

 the two bands is very instructive. The lower band (7. e. the 

 one where the magnetic deflexions are the greatest) is bent 

 in the way we have just described, and is below the position 

 it occupied when the magnetic and electric fields were coin- 

 cident. The upper band on the other hand is bent in the 

 opposite way and is above the position it occupied when 

 the fields coincided. The appearance of the phosphorescence 

 is represented in fig. 5, where the dotted lines show the 



space between 

 end B of the 



