86 ROYAL SOCIETY OF CANADA 



diatiou from the insulated system be a strong one the movable system of 

 the electrometer will take up the equilibrium position practically at once 

 for any particular pressure and so the pressure may be lowered by stages 

 and the corresponding scale readings observed. But if the radiation be 

 a weak one it may take the movable system a long time to acquire the 

 equilibrium potential corresponding to any particular pressure, and under 

 these circumstances it is best, if the object of the investigation is to as- 

 certain the character of the excess emitted radiation, to reduce the pres- 

 sure as rapidly and as low as possible, and then, while maintaining the 

 low pressure, to observe whether a movement occurs in the movable 

 system of the electrometer. 



If such a motion occurs, the direction of the motion will indicate 

 the sign of the charge acquired and the rate of movement will give la,. 

 measure of the magnitude of the intensity of the charge producing ra- 

 diation. 



II. Charging Action" of the Eadiation from Polonium. 



Some preliminary attempts were made to obtain a charging effect 

 with some potassium salts in a high vacuum, but it was soon found that 

 such charging action was exceedingly small, and it was thought best to 

 carry out a few parallel experiments with some of the better known, 

 radioactive substances, in order to gain some information regarding the 

 pressures at which a charging action would be exhibited by various types 

 of rays from active substances placed in a number of differently shaped 

 vessels. 



The first experiment was made with polonium deposited on a strip 

 of copper. The area of this deposit was about 6 sq. cms. The copper 

 strip which carried it was supported by a piece of amber insulation at 

 the centre of an air-tight brass cylinder 20 cms. long, and about 5 centi- 

 metres in diameter. This cylinder was connected to a McLeod Gauge 

 and also to a Gœde mercury exhausting air pump. The insulated copper 

 strip bearing the polonium was also connected to the free quadrants of a 

 Dolazaleck electrometer, which gave about 200 mm. divisions deflection 

 per volt. With this arrangement it was found repeatedly when the earth 

 connection to the free quadrants was broken, that the needle moved 

 slightly in the positive direction and came to rest about three centimetres 

 from the zero reading. This deflection was taken as a measure of the 

 volta effect. As the pressure was lowered no further change occurred in 

 the electrometer reading until a pressure of approdmately .4 mms. was 

 reached. At this pressure the quadrants always commenced to gain a 

 positive charge, and as the pressure was still further lowered the needle 

 at once moved out and took up a definite position corresponding to each 



