714 



Dr. S. J. Allen on a Null Instrument 



the aid of fig. 1. The testing-vessel X for the unknown 

 ionization consists essentially of two parallel plates, insulated 

 from one another and from the enclosing case, the lower 

 one b being connected to one pole I of a battery, and the 



Fig. 1. 



upper one a to one pair of quadrants c, d of a sensitive 

 Dolezalek quadrant electrometer E. The other pair of 

 quadrants is joined to earth in the usual manner. To the 

 quadrants c, d is also connected another metal plate e, which 

 together with the quadrants and plate a are connected to 

 earth through the contact-key K. Beneath the plate e, and 

 parallel to it, is placed a shallow brass box/ having a sliding 

 and close-fitting brass cover g, which can be drawn out and 

 in by means of a long wooden rod li. This box is connected 

 to the other pole n of the battery, and a movable contact m 

 is arranged so that part of the battery may be earthed as 

 desired. The inside of the box / is covered with a thin 

 uniform layer of uranium oxide; this being constant in 

 strength and giving off no emanation. 



In order to obtain as uniform a layer of radioactive 

 material as possible, and one which would stand a considerable 

 amount of rough usage, the uranium oxide was powdered 

 very fine and then sifted through a close-mesh sieve on to 

 a piece of cardboard covered with a thin coating of shellac. 

 As soon as the shellac had hardened, the card was turned 

 upside clown and all the surplus oxide tapped off, leaving 

 behind a thin and quite uniform layer of radioactive material 

 which adhered very firmly to the card. The card was then 

 fastened in the bottom of the brass box. 



The rod li moves over a scale s graduated in millimetres, 

 and serves to indicate the distance x the cover is opened. 



