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XIII. On the Distribution of Electric Force in the Discharge 

 at Low Pressures. By T. Harris, B.A., B.Sc, A.B. C.S* 



ff^HE object of the work to be described below was to 

 A determine the distribution of the electric forces in a 

 discharge through air at low pressures, in a cylindrical 

 discharge-tube. 



Similar measurements at higher pressures have been made 

 by many workers on this subject who have mostly used a 

 probe method, in which a wire is inserted into the gas and 

 assumed to take up the potential of the gas at that point. 

 Objections have been raised against this method f, and at 

 low pressures it is specially unreliable, so that the method 

 was adopted which has been used by Thomson J and Aston §, 

 in which a fine beam of cathode rays is projected through 

 the field of force, the magnitude of which is estimated from 

 the deflexion of the beam. 



Thus if v is the velocity of the cathode rays passing 

 through d cm. of a uniform field, X, at right angles to the 

 direction of the field, thence proceeding to the screen upon 

 which they excite a phosphorescent spot situated at a dis- 

 tance I cm. from the edge of the field, then the deflexion of 

 the spot on the screen from its position when X=0 is given 



by 



where e/m is the ratio of the charge to the mass of one 

 electron. This holds if the assumption is made that the 

 deflexion of the beam is small enough for the path of the 

 rays to be considered always as perpendicular to the field. 

 The velocity v is given by 



±mv 2 =Ye, 



wliere V is the generating potential of the rays. 



Substituting this value of 



o 



V 



8= 2V' i G + 0" 



* Communicated by the Author. 



t For experimental work see "Wehnelt, Deutsch. Phys. Gesell. Verh. 

 xiii. xiv. pp. 505-510, July 1911. 



I J. J. Thomson, Phil. Mag. xviii. pp. 441-451. Oct. 1909. 

 § Aston, Proc. Roy. Soc. A. 1910-1911, p. 526. 



