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XVIII. 



THE COMPARISON OF CAPACITIES IN ELECTRICAL WORK : 

 AN APPLICATION OF RADIO-ACTIVE SUBSTANCES. 



By J. A. McCLELLAND, M.A., Professor of Experimental Physics, 

 University College, Dublin. 



[Read, January 19 ; Received for Publication, January 22 ; 

 Published, February 26, 1904.] 



There are many methods by which two capacities may be 

 compared, and which are fully described in text-books of 

 Physics. 



When only approximate results are required, we have several 

 methods to choose from, any of which will give a fair result ; but 

 the problem is by no means so simple when an accurate determi- 

 nation is required, especially if we are dealing with a very small 

 capacity. That better methods of dealing with the determination 

 of capacities, especially small capacities, are still required may be 

 judged from the fact that two papers have recently appeared on 

 the subject, one by Professor Fleming and Mr. Clinton in the 

 Phil. Mag., May, 1903, and the other by Professor Stroud and 

 Mr. Oates in the Phil. Mag., December, 1903. 



Those two papers may be taken as affording examples of the 

 difficulty of obtaining accurate results in this work, both methods 

 necessitating somewhat elaborate apparatus, and involving con- 

 siderable experimental difficulties. 



My object in this paper is to describe a method at once simple 

 and accurate, and suitable for the determination of capacities of 

 any magnitude down to a few micro-microfarads, or even less. 

 The method is based on the fact that the ionisation current that 

 can be obtained by the use of a radio-active substance like 

 uranium is extremely constant, and can be made so small that the 

 time taken to charge a condenser by it can be accurately measured. 

 This small constant current is used first to charge one condenser 



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