Power with the Velocity of Cathode Hays. G49 



small battery of storage-cells </> whose other terminal is 

 connected through a sensitive Nalder galvanometer to earth. 



It is necessary that the rays should cross the chamber 

 in a well defined beam without undergoing appreciable 

 scattering on the way by the gas left in the apparatus. To 

 examine how far this condition is fulfilled a willemite screen 

 was waxed on to the end of the chamber instead of the 

 ebonite plug and Faraday cylinder. A clearly defined spot 

 was obtained showing no signs of scattering at any of the 

 pressures used in the experiments. Its diameter was less 

 than a millimetre ; as the opening of the Faraday cylinder 

 was about 4 millimetres in diameter there is no doubt that it 

 caught all the rays. 



The Faraday cylinder also does away with any effect due 

 to reflexion of the rays. For of the rays which are reflected 

 on hitting the top of the cylinder only those which come 

 back normally can emerge from it. From the dimensions of 

 the cylinder (which has been drawn to scale in the diagram), 

 it may easily be calculated that the solid angle subtended at 

 .the point P by the aperture is less than one hundredth of 

 the whole solid angle through which we may assume the 

 reflected rays to be distributed. Thus the number which 

 re-emerge is negligible. 



The effective length of path of the rays in the chamber is 

 the distance ac. There is no field inside the cylinder and 

 any ions produced by the rays after passing c, whether 

 before or after reflexion at P, will simply recombine and 

 not affect the current through the chamber. 



It is evident that the current through the chamber 

 consists of two parts : — 



(1) the current carried by the rays themselves ; 



(2) the ionization current. 



The direction of the former is independent of the sign of 

 the potential applied to the Faraday cylinder while the 

 latter changes in direction when the field is reversed. By 

 such a reversal of the potential of the Faraday cylinder we 

 can thus obtain both the sum and difference of the effects 

 (1) and (2). 



Let n be the number of cathode rays crossing the chamber 

 per second, and a the number of pairs of ions made by one 

 cathode ray in going 1 cm. in air at a pressure of 1 mm. of 

 mercury, and let I be the length of path of the rays in the 

 chamber. 



Then the current carried by the rays =??<?. 



The saturation current carried by the ions —ncdpe. 



