486 Prof. McLennan and Mr. Keys on the 



II. Apparatus. 



In making the measurements the apparatus shown in 

 fig. 1 was used. AB was a thick circular plate of brass 

 about 8 cm. in diameter into which a polonium-coated 



Fiff. 1. 



F G R 



TO ELLCTR.OAETER 



H K 



mmmmMMM MMmmAmmMMm ^ 



LARTH 



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 had 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 millimetre. 



When this ionization 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. 

 Gne terminal of a battery of small storage-cells was joined 

 to earth, and the other terminal was joined by a wire, which 

 passed through an insulating plug of ebonite in the walls of 

 the steel cylinder, to the plate AB. An insulated wire PR 

 also passed out through the walls of the steel cylinder and 

 was joined up to a pair of quadrants of a Dolezalek electro- 

 meter. With this arrangement any desired uniform electric 

 field, positive or negative, could be established and main- 

 tained between the polonium-coated copper plate CD and 



