128 THE ROYAL SOCIETY OF CANADA 



The only experiments which have been made hitherto on the 

 mobiUties of ions in air at high pressures appear to be the ones made 

 by Dempster^ and those made by Kovarik^. 



In his work Dempster used pressures to as high as 100 atmos- 

 pheres and he found that over the range from one atmosphere to this 

 limit the mobility of the positive ion made in air by alpha rays varied 

 inversely as the pressure. He found, however, that the mobility of 

 the negative ion at the higher part of the range did not appear to vary 

 inversely as the pressure; but it decreased less rapidly with the pres- 

 sure than it should have done if the inverse pressure law had been 

 valid. Kovarik in his experiments, on the other hand, worked with 

 pressures from 13-3 to 74-6 atmospheres and over the whole of this 

 range he found that the mobilities of both positive and negative ions 

 made in air by alpha rays followed the inverse pressure law. 



In the present investigation the mobilities of the two kinds of 

 ions were measured in air over a range of pressures commencing at 

 66-86 atmospheres and extending to 181-5 atmospheres. At the 

 lower pressures of this range the mobilities obtained agreed with the 

 results of Kovarik; but at the higher pressures it was found that the 

 mobilities of the two kinds of ions began to approach each other in 

 value and both decreased less rapidly with increases in pressure than 

 they should according to the inverse pressure law. 



II. Apparatus. 



In making the measurements the apparatus shown in Fig. 1. was 

 used. AB was a thick circular plate of brass about 8 cms. in diameter, 

 into which a polonium-coated copper plate CD was inserted. GH 

 was a circular plate of brass 2 cm. in diameter and EFKL was a 

 circular guard plate surrounding GH. The plate GH was held 

 firmly in position with ebonite supports, with its lower face flush with 

 that of the guard plate EL. The upper face of CD, which was the one 

 coated with polonium, was also flush with the upper face of the plate 

 AB, into which it was inserted. The plate CD was square and has an 

 area of 16 sq. cm. The plates GH and EL were kept at a distance of 

 1 cm. from the upper face of AB by means of ebonite supports. The 

 clearance between GH and the guard plate EL was less than one-half 

 a millimiCtre. 



When this ionisation chamber was in use, it was placed in a 

 strong steel cylinder which had a capacity of about 1 • 5 litres. The 

 guard plate was electrically connected to the steel chamber, which was 

 itself kept joined to earth. One terminal of a battery of small cells 



iDempster. Phys. Review, Vol. XXIV. No. 1. Jan. 1912, p. 53. 

 ^Kovarik. Proc. Roy. Soc. A. Vol. 86, 1912. p. 154. 



