80 Mr Vegard, An experiment on ionisation with <y rays. 



sources to the centre of the cylinder were about 35 cm. The 

 a and ^ radiation was cut oif by lead plates 3 mm. thick. In all 

 other directions the radiation was stopped by piles of large lead 

 and iron blocks forming a layer about 5 inches thick around the 

 radium. 



The ionisation was measured by the saturation current between 

 the inner plate and the chamber, the latter being given a potential 

 sufficient for saturation when the inner plate had a potential 

 nearly equal to that of the earth. The procedure was as follows : 



The air in the chamber was first exposed to the radiation from 

 one source (a) alone, and the saturation current measured ; the 

 second source (6) was then placed behind the second slit, while (6) 

 was left unaltered, and the current measured ; then (a) was removed 

 and the measurements repeated. This operation was performed 

 for two different positions of the moveable source corresponding to 

 the angles, 90° and 180°, between the mean direction of the ray 

 bundles. 



The saturation current is inversely proportional to the time t 



required for the gold leaf to fall between the same two marks on the 



scale of the electroscope. This, however, is only true when there is 



no leak and the zero point corresponding to zero potential of the leaf 



remains constant. The leak observed when all radium was removed 



from the room was too large to be disregarded. The velocity of the 



gold leaf due to the leak was found for different positions of the 



leaf. From this the time T required for the leaf to move through 



the scale interval on account of the leak could be found. If the 



, . t' . 



time actually measured is t' , and if ™ is a small quantity, the time 



t, corrected for leak, will be - = t? + m- The leak might, however, 



Z V J- 



be altered by the presence of radium in the room ; by cutting off 

 the rays that went directly to the chamber the leak could be 

 estimated, and was found to be very little altered by the presence 

 of radium. It is, however, not necessary to know this part of the 

 leak, for we can assume that it is an additive property. 

 The condition for additivity can then be written : 



i 1 l_J^ = o 



ta tb -^ tab 



In general this expression may be equal to some quantity e. 

 The quantity S = tab'^ will then give a measure of the departure 

 from additivity. 



The results of the measurements are given in the following 

 table : 



