30 



Scientific Proceedings, Royal Dublin Society. 



a current is passed through a circuit by the alternate charging and discharging 

 of a condenser. If the capacity of the condenser is c farads, and if it be 

 charged and discharged n times per second, the charging potential being V 

 volts, it is clear that the current passing is tw V amperes. This arrangement 



is obviously equivalent to a resistance B = -— ohms. 



nc 



•001 



This equivalent resistance will usually be very large. Thus, if c 

 microfarad = 10" 9 farad, and if n = 10 per second, we obtain : — 



B = — = 10 s ohms = 100 megohms. 

 nc 



This is, therefore, of the order required, as we have seen, for the measurement 



of currents in the neighbourhood of 10 -9 ampere, by a steady deflection 



method. By varying n and c, it is possible to obtain equivalent resistances 



from 10 7 to 10 10 ohms. Indeed, wider ranges may be covered, but the 



author has not yet tried the method outside these limits. 



• To Earth 



§ 4. The apparatus is very simple. In the figure a quadrant electrometer 

 E is shown as ordinarily used with an extra capacity c u connected up to an 

 ionisation chamber / in order to measure the current therein. To this system 

 is further connected the arrangement shown in the lower part of the figure. 

 A and B are two platinum-tipped contact screws, mounted on insulating 

 pillars. Between these is mounted a steel spring, S, also supported on an insu- 

 lating pillar, and having platinum contact pieces facing those on the screws 

 A and B. The screws are adjusted so as to allow the spring to vibrate with 

 just sufficient amplitude to make alternate contacts with them. To the 

 spring S is connected one terminal of the condenser c 2 , which may con- 



