C. Barus — Standardization of the Fog Chamber. 87 



Art XL — Note on the Standardization of the Fog Cham- 

 ber by the aid of Thomson's Electron ; by Carl Barus. 



1. Advantages. — Of all the methods which I have tried to 

 evaluate the coronas in terms of the number of nuclei which 

 they represent under given conditions of exhaustion, the above 

 method is the most promising and expeditious. A single 

 experiment need take but a few minutes. Incidentally the 

 observer learns whether the negative and positive ions have 

 both been captured ; for on using the tables of coronas which 

 I developed heretofore, the value of e may be computed, and 

 the result must coincide with Thomson's value. 



2. Method. — My first experiments were made with a metal 

 plate in a fog chamber, both the coronas and the current being 

 observed successively, without changing the adjustment. But 

 this was abandoned for a method in which a cylindrical con- 

 denser is employed as follows. A closed aluminum tube, *62 cm 

 in diameter 18 cm long, containing weak radium equally dis- 

 tributed along its inside, is made the core of a cylindrical con- 

 denser, 2'l cm in external diameter, and leaded to an inch or 

 more in thickness beyond. The aluminum core in question is 

 suspended axially from a fine wire leading to a sensitive elec- 

 trometer. The voltages here to be measured must of course 

 be small, and hence all connecting wires are to be inclosed in 

 earthed metal pipes. 



The core in question is then removed from the electrical 

 condenser and put into the axis of a dust-free fog chamber 

 where the nucleation (ionization) is found on condensation 

 from the contents of the corona ; or vice versa. Here there 

 are some outstanding difficulties ; for the coronas are not the 

 same throughout the length of the fog chamber. Even 

 immediately around the radium core a single corona may be 

 green on one side and red on the other. In a fog chamber 

 45 cm long, the coronas may vary from the glass end to the 

 metal end of the chamber, in a way to correspond to from 

 100,000 to 200,000 nuclei, respectively, while the radium core 

 is fixed in the middle. Inferring secondary radiation, one 

 might naturally expect to obtain still larger coronas near the 

 metal end, if the radium core (thoroughly sealed) is placed 

 there, instead of in the middle of the chamber ; but this is not 

 the case, the coronas being markedly smaller than before, 

 decreasing uniformly in size, however, toward the glass end. 

 As. the sealed aluminum tube is within the chamber, this 



