172 Scientific Proceedings, Royal Dublin Society. 



rapid when only the capacity c is in use. The following are the 

 numbers in this determination : — 



c + -ooi 



50 divisions in 75-8' 

 50 divisions in 75*7 



100 divisions in 21'9" 



100 divisions in 21 -8" 



100 divisions in 22'2" 



Mean, 7575" Mean, 21-97" 



„, , C + -001 15150 



Therefore, ___=___. 



These equations give 



c = '000169 microfarad ; 

 C = -001817 microfarad. 



To give somewhat of a test of the reliability of the method, the 

 same capacity was determined on another occasion, taking no 

 care to use the same quantity of uranium, and, in fact, having 

 very different ionisation currents from those used in the first case. 

 The following numbers were obtained, only one observation 

 being taken in each case : — 



G+c -001 + c 



50 divisions in 48-2" 50 divisions in 28-6" 



C + -001 



50 divisions in 56*5" 100 divisions in 16*5" 



Calculating as before, we get 



c = -000170 microfarad ; 

 C = -001802 microfarad. 



The agreement with the preceding numbers is very good, 

 especially when we consider that only one observation was taken 

 in each case. 



(c) To show that this method is suitable for much larger 

 capacities than those used in the preceding examples, we shall 

 give an example in which a capacity known to be about -5 micro- 

 farad was determined by comparing it with a standard capacity 

 of -1 microfarad. For this purpose, an electrometer of the 



