362 Prof. J. A. McClelland on the vompar ison 
the radium atom breaks up. The radium atom certainly 
gives off positively charged particles—the a rays. The 
emanation particles cannot “be what remains of the atom after 
the emission of one or more a rays, because, in that case, it 
would be negatively charged. The atom must have parted 
with an equal negative charge, either by the emission of 
negative particles or in some other way. 


XLII. The Comparison of Capacities in Electrical Work; 
an Application of Radioactive Substances. By J. A. 
McCurtiann, MW.A., Professor of Experimental Physics, 
University College, Dublin *. 
hee 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 determination 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 
1908, and the other by Professor Stroud and Mr. Gates 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 considerable 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 
ionization current that can be obtained by the use of a radio- 
active substance like uranium is evtremely 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 to a given potential ; 
and then a second condenser is char ged to the same potential, 
and the time taken in the two cases obser ved, so that we get 
* From an advance proof of the Proceedings of the Royal Dublin 
Society, vol. x. part il. p. 167, communicated by the Author. 
