274 Prof. A. P. Chattock on the Forces at the 



There is, however, an electrical effect of the negative 

 discharge which requires consideration. 



The bounding surface of the discharge cone separates two 

 fields — an outer one composed of lines which pass unbroken 

 from the point into the gas, and an inner one composed 

 partly of unbroken and partly of broken lines. 



Suppose that in fig. 2 we pass from A outwards along the 

 discharge cone. The number of lines of force contained bv 

 the cone decreases to a minimum near the centre of the 

 ionizing region, and then increases until this region is passed. 

 It follows that if we draw side by side with the discharge 

 cone a second similar cone in the outer field, this second cone 

 must contain, on the average, a number of lines which lies 

 between the maximum and minimum numbers in the dis- 

 charge cone if the outer and inner fields are to be in 

 equilibrium with one another. 



Owing to the narrowing of the discharge area, and the 

 increased thickness, t, of the ionizing region, the field in the 

 latter, unlike that for a positive point, is chiefly composed of 

 broken lines. This would result in P being far too small 

 to give a correct value of/ if the discharge area covered the 

 end of the point ; but as it is, P is almost wholly due to the 

 lines of the outer field. If therefore we write 



/ = 1-085 </$Plr 0} 



we shall obtain a number wdfich is less, but perhaps not much 

 less than the value of the field at the bottom of the discharge 

 cone. Except when the current is small, the measurement 

 of / for negative discharge is thus somewhat indefinite. 



Discharge between two Points. 



Suppose that to a positive point P (fig. 3) negative ions are 

 sent from a second point !N in its neighbourhood, and that 

 the average field in the ionizing layer 

 remains unaltered. The momentum effect &' ° # 

 of the N ions on P may be conveniently , | 



discussed under two heads. - ^sf^ 



1. That of N ions which will ultimately ? J* 



reach the hemispherical end of P by the . r^\^ 



arrow-marked paths. * j \ 



These will behave like C ions reversed, 

 with this difference — the momentum they impart to the gas 

 keeps to the cones of discharge down which tbey pass to A 

 much more than with C ions because the sides of the point 



