168 Scientific Proceedings, Royal Dublin Society. 



to a given potential ; and then a second condenser is charged to 

 the same potential, and the time taken in the two cases observed, 

 so that we get the ratio of two capacities by simply observing two 

 intervals of time. 



The method will probably have occurred to anyone who has 

 been using radio-active substances; but as many workers have 

 occasion to compare capacities accurately who are not using radio- 

 active substances, I have thought it advisable to make a few 

 experiments showing the accuracy of the method, and showing 

 also how small a capacity can be detected and measured by it. 



To use the method it is not necessary to have a supply of 

 radium, as the title of the paper might suggest; uranium is 

 even better in some respects, and uranium is to be found in 

 every laboratory. 



Description of Apparatus and Method or Working. 



A and B are two insulated metal plates, one of which, B, can 

 be joined to one terminal of a battery of small storage-cells, the 



Earth 



other terminal of which is to earth. The battery may consist of 

 100 or more small test-tube cells, so that B can be kept at 200 

 volts or higher. 



A few grammes of, say, uranium nitrate are spread on a sheet 

 of paper, and placed on the plate A. The radiation from the 

 uranium ionises the air between A and B ; and so A gradually 

 rises in potential if insulated, supposing B to be positive. As is well 

 known, the ionisation current thus obtained between two plates 



