422 Prof. McLennan on Induced Radioactivity 



the wire to be tested was wound. The brass tube C, which 

 was earthed throughout the measurements, served as a guard- 

 ring and prevented any leak from the vessel A to the rod D 

 across the ebonite plugs. 



The saturation current due to spontaneous ionization of 

 the air in the vessel having been first determined by con- 

 necting the supporting rod D to the electrometer in the usual 

 way and applying a potential of 100 volts to A, the exposed 

 wire was then wound on the reel, inserted in the vessel, and 

 the saturation current again ascertained. Any increase 

 observed in the ionization current was taken as a measure of 

 the radioactivity induced in the exposed wire. 



As it was impossible, in setting up the apparatus in two 

 different localities, to be certain that it was of exactly the 

 same sensibility, a standard of ionization was deemed necessary 

 for purposes of comparison. A radioactive substance which 

 the writer had in his laboratory at the time of making the 

 experiments was chosen, and a small quantity in a glass phial 

 being found to give a constant ionization current when placed 

 in a given position in the chamber A, this current was adopted 

 as a standard and the ionization currents measured in the 

 various tests were expressed in terms of it. 



3 . — Experiments. 



Both before proceeding to Niagara, and after returning, a 

 series of exposures was made in the quadrangle of the Uni- 

 versity at Toronto. Copper wires, approximately 30 metres 

 long, were exposed for periods of two hours at a potential of 

 8000 to 10,000 volts on a number of days. After a wire had 

 been exposed it was placed in the ionization chamber, the 

 saturation current measured, and its radioactivity, thus ascer- 

 tained, was expressed in terms of the standard radioactive 

 substance. 



The resulting values, taking the standard saturation current 

 as unity, showed considerable variation in the amount of 

 radioactivity excited, the highest value observed being 1*75 

 and the lowest 0*6. The intermediate values ranged between 

 these limits, but were quite irregular and seemed to depend 

 more upon the existence of wind and its velocity than other 

 changes in the weather. The occurrence of showers did not 

 exert a noticeable effect on the amount of radioactivity ex- 

 cited. It was frequently observed that exposures made in 

 the morning gave greater induced radioactivity than those 

 in the afternoon, but that exposures made after sunset gave 

 values generally equal to, and sometimes greater, than those 

 of the morning. 



