542 Mr. G. H. Henderson on the Range and Ionization 



from source to chamber was less than "02 cm., involving an 

 error in range of less than "05 per cent. 



As will be seen shortly, the ionization curve near the end 

 of the range is extremely steep, and a small change in pressure 

 causes a comparatively large change in ionization current. 

 It will thus be seen that great accuracy was not required in 

 the measurement o£ this current, the probable error in which 

 was less than 1 5 per cent. It was difficult to measure 

 ionization currents directly to much greater accuracy without 

 unduly prolonging the time of observation. It should be 

 pointed out that at the extreme end of the range, when 

 measuring the last traces of ionization current due to the « 

 particles, the number of these particles entering the chamber 

 was small. Probability variations in this number increased 

 the error in the measured current rapidly as the current 

 decreased towards zero. This error could not be avoided by 

 any increase in accuracy of the measuring instrument. 



The ionization current (corrected for natural leak) was 

 corrected for the decay of the source by means of the tables 

 published by Lawson and Hess * for radium C. The decay 

 of thorium C was allowed for by assuming a period of 60 6 

 minutes, the mean of the most recent determinations |. 



This corrected ionization current I 2 was then reduced to an 

 equivalent current I at 0° 0. and 760 mm. pressure by the 

 formula 



760 T 

 l ~~> 273 lj ' 



where T and p were the absolute temperature and pressure 

 (reduced to mercury at 0° C.) at which I x was measured. 



I was the current which would be measured in air at 0° O, 

 and 760 mm. in the vessel, if the ionization chamber were 

 placed at a distance x from the source of the rays, where 

 x was given by 



lT_ 760-^r •^ i -' 



38*12 was the fixed distance from the source to the ionization 

 chamber. 



The values of I and x so found were plotted on squared 

 paper. It was possible to cover only a portion of the whole 

 ionization curve in one series of observations. A fresh 

 source was then prepared and another series of observations 



* Lawson and Hess, Wien. Ber. II a. cxxvii. p. 599 (1918). 

 t Meyer u. Schweidler, Badioahtivitat, p. 412 (1916). 



