230 



Sir J. J. Thomson on 



was drawn out into an exceedingly fine capillary tube, whose 

 end was left open. This capillary was fastened by an air- 

 tight joint into the receptacle containing the gas under 

 examination ; this receptacle was practically the space above 

 the mercury in a barometer tube 7 and by altering the level of 

 the mercury the pressure of the gas in the receptacle could 

 be adjusted so that when the Graede pump was kept in con- 

 tinuous action the pressure in the discharge-tube was low 

 enough for the positive rays to be well developed. The 



details of the connexion are shown in fig. 4. 



Fie;. 4. 



it 



Q 





W 



Discussion of the Photographs.— The appearance of atypical 

 photograph is shown in fig. 5 (PI. L). In all these photo- 

 graphs the vertical deflexion is due to the magnetic field, the 

 horizontal to the electrostatic. Many of the photographs 

 were taken with the magnetic field in one direction for half 

 the exposure and in the opposite direction for the other half. 

 It will be seen that the curves on the photograph are of two 

 types. 



(1) Short parabolic arcs of varying length, having their 

 heads in the same vertical line, showing that the minimum 

 electrostatic deflexion suffered by the particles which produce 

 these curves is the same whatever the nature of the particles 

 may be. The electrostatic deflexion is proportional to e/mv 2 , 

 where e is the charge, m the mass, and v the velocity of the 

 particle. Now if the energy of the particle is due to the fall 

 of its charge through a potential difference V, 



Xe — \mv 2 or e/'ntt 2 = 1/2 V. 



