244 Sir J. J. Thomson : Further 



relative amounts, I have supplemented it by another method 

 which enables us to measure the number of charged particles 

 of any particular kind. The arrangement used for this 

 purpose is represented in fig. 22. The particles, after 

 passing through the electric and magnetic fields, fall upon 

 the end of a closed cylindrical box, B. In the end of this 

 box nearest the discharge-tube a parabolic slit 1 mm. in 

 width is cut, the vertex of the parabola being the point 

 where the undeflected rays would strike the end of the box, 

 and the tangent to the vertex the line along which the 

 particles would be deflected by the magnetic force alone. 

 This slit is the only entry into the box. Thus all the particles 

 which enter the box have the same value of <?/m, and by 

 altering the value of the magnetic field, one set of particles 

 after another may be driven on to the slit and thus admitted 

 into the box. Inside the box, and immediately behind the 

 slit, there is an insulated long and narrow metal vessel placed 

 so that every particle passing through the slit has to fall into 

 this vessel. This vessel is connected with a Wilson tilted 

 electroscope, so that the charge received by it can be 

 measured. 



At first considerable trouble was caused by the corpuscles 

 which the metal around the slit emitted when it was struck 

 by the positive rays ; to prevent these entering the Faraday 

 vessel a permanent magnet was placed so as to produce a 

 strong field between the slit and the Faraday vessel. This 

 deflected the corpuscles away from the vessel without having 

 any appreciable influence on the positive rays. A part of 

 the front face of the vessel in which the Faraday cylinder 

 was enclosed was cut away and a willemite screen inserted ; 

 by deflecting the positive rays on to this screen and observing 

 the brightness of the parabolas, one could ascertain whether 

 the conditions of the discharge-tube were such as to ensure 

 a plentiful supply of positive rays. The face of the vessel, 

 with the parabolic slit and phosphorescent screen, is repre 

 sented in fig. 22. 



The pressure in the observation tube was made as small as 

 possible by the use of charcoal and liquid air ; the pressure 

 in the part of the tube through which the discharge passed 

 was generally about 3/1000 of a millimetre. 



The Wilson electroscope gave a deflexion of about 100 

 divisions for a volt. The method of observation was as 

 follows : — the positive rays were deflected by a constant 

 electric field, and then the magnetic field was increased 

 by small increments, and the deflexion of the Wilson 

 electroscope in 10 seconds observed. Unless a parabola 



