212 Sir J. J. Thomson : Further 



of these parabolic arcs to particles which were charged 

 before they entered the electric and magnetic fields, and 

 which have retained their charges unaltered during their 

 passage from one end of the field to the other. The value of 

 e/m for the particle producing a line of this type can be 

 determined at once by comparing the latus rectum of its 

 parabola with that of a parabola due to a known particle, 

 say the atom of hydrogen, or the mercury atom ; if the 

 intensities of the electric and magnetic fields are known, we 

 can determine the absolute value of e/m for any parabola 

 without using any comparison lines. We shall call the lines 

 of this type the primary lines. 



When, however, the pressure of the gas traversed by the 

 rays after passing through the cathode is not too low, there 

 are other lines to be seen on the plate which have quite dif- 

 ferent properties from the primary lines (PL IX. fig. 2) . They 

 form curves of fairly uniform intensity which do not stop short 

 at a finite distance from the vertical axis but are continued 

 until they meet this axis : in some cases they pass through 

 the origin itself, in others they start from points vertically 

 above or below it. They are to be found on the side of the 

 plate corresponding to negative charges as well as on that 

 corresponding to positive. Their shape and also their position 

 with respect to the primary lines depends upon the dis- 

 position as well as on the intensity of the electric and 

 magnetic fields. If care is taken to make the electric and 

 magnetic fields uniform and coterminous, the curves we are 

 discussing become straight lines passing either through the 

 origin or very near to it; examples of these are shown in figs. 3 

 and 11 (PL IX.). We see from the figure that though all 

 the lines of this type pass nearly through the origin, one set 

 actually passes through it, while another starts from a point on 

 the vertical axis a little above it, the curves having the shape 

 shown diagrammatically in fig. 4 a. Fig. 4 b (PL IX.) is a 



photograph. A careful examination of the plate shows that 

 the lower curve turns up abruptly towards its termination A 



