for Measuring Ionization. 715 



By this arrangement any desired amount of ionization 

 could be obtained between the plates e and / by simply 

 moving in and out the cover, which was thick enough to 

 absorb all the rays except the 7. 



In the present experiments the movement of the electro- 

 meter-needle was observed by means of a spot of light on 

 a ground-glass scale placed about one metre from the mirror. 

 The earth-contact key K was operated by a cord running 

 over small pulleys. 



In making a test, the unknown radioactiA^e substance which 

 it is desired to measure is placed on the lower plate of the 

 testing-vessel X, and the contact m made with, the middle of 

 the battery. Both the testing-vesseJ and the standard have 

 now the same difference of potential between their plates, 

 the plate b being charged positively above and / negatively 

 below earth. 



If now the contact-key K is separated from earth, the 

 electrometer quadrants c d begin to charge up with positive 

 electricity, and the needle swings towards the left, its rate of 

 movement being proportional to the intensity of ionization 

 in the vessel X. The cover of the standard / is then opened 

 until the needle "just begins to swing towards the right, and 

 then closed a slight amount until it begins to swing back 

 again towards the left. The mean of these last two positions 

 of the standard is taken as the position of equilibrium, at 

 which the supply of positive electricity from the testing- 

 vessel X is just balanced by the supply of negative electricity 

 from the standard S. Under these conditions the electro- 

 meter merely acts as an indicator and not as a measuring 

 instrument. The unknown activity is thus determined in 

 terms of that of the standard. 



Since in general we are only concerned with the relative 

 ionizations, the standard need only be calibrated in arbitrary 

 units. For the preliminary set of observations the standard 

 was calibrated in the following manner. The contact m 

 was moved to L thus earthing the positive side of the 

 battery. The cover g was then opened a distance x, the 

 quadrants separated from earth and allowed to charge up 

 with negative electricity from the standard. The steady 

 rate of movement of the needle was then observed with a 

 stop-watch as in the usual " rate of leak " method. This 

 reading in arbitrary units (scale-divisions per second) would 

 be proportional to the ionization in the standard due to an 

 opening x. 



In the same manner readings were made for various other 

 openings, and the results plotted in a calibration curve for 



