484 Wellisch and Bronson — Distribution of the Active 



2. Experimental Procedure. 



The radium emanation used in our experiments was obtained 

 from two sources, for the use of both of which we are indebted 

 to the kindness of Prof. Boltwood. One source consisted 

 of a solution of radium salt contained in a glass vessel so con- 

 structed as to permit vigorous bubbling when a current of gas 

 was passed through it ; the other source was a quantity of 

 carnotite contained in a glass jar, which was adjusted to per- 

 mit of rapid connexion to the testing vessel. 



The emanation was in most cases passed through glass-wool 

 and phosphorus pentoxide before entering the test vessel; no 

 effect on the distribution was observed, however, by omitting 

 these precautions. 



Different forms of test vessels were employed ; the vessel 

 which was used practically throughout and to which all our 

 experiments apply, unless mention is made to the contrary, 

 consisted of a brass cylinder with an aluminium bottom and a 

 brass central electrode sprung into a stout brass holder so as to 

 permit of rapid detachment. The vessel was provided with a 

 guard tube which was connected to earth ; this guard tube was 

 slightly tapered so as to make a good fit with the ebonite plug 

 which supported the central electrode. Stop-cock grease was 

 employed to ensure complete tightness. 



The dimensions of this vessel were as follows : 



Height of containing portion 140 mm 



Inner diameter 58 " 



Exposed length of central electrode 101 " 



Diameter of central electrode 1 *83 " 



The measurements of the activity of the central electrode were 

 made in a vessel identical to the above except that, as men- 

 tioned below, no ebonite and in consequence no guard tube 

 was employed. 



The diagram of connexions and the disposition of apparatus 

 are given in fig. 1. The test-vessel A was supported by brass 

 clips mounted on ebonite ; these clips were connected to the 

 battery through carbon resistances B, R/, the changes of poten- 

 tial being effected by means of the adjustable contacts s, $', 

 and the key L. B represents a parallel plate vessel which was 

 employed in order to communicate definite induced charges to 

 the electrometer system whenever occasion arose. C is a 

 capacity consisting of two thin sheets of tinfoil separated by a 

 thin sheet of mica ; this capacity, together with that of B, 

 could be added to the system by means of the key K, and the 

 total capacity of the system was then increased 21£ times. 

 The electrometer was of the Dolezalek pattern with a platinum 



