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III. 



A NEW FORM OF VERY HIGH RESISTANCE FOR USE 

 WITH ELECTROMETERS. 



By JOHN J. DOWLING, M.A., M.R.I.A., 

 Lecturer in Physics, University College, Dublin. 



[Read November 23, 1915. Published February 2, 1916.] 



§ 1. The minute currents met with in many modern lines of research have 

 usually been measured by observing the rate of charging of a system of 

 known capacity with an electrometer or electroscope. There are certain 

 disadvantages in this procedure, and some efforts have been made to devise a 

 " steady deflection " method, notably by Bronson 1 and by Campbell. 2 To 

 effect this it is simply necessary to use the electrometer to measure the 

 potential drop across a suitable high resistance, through which the current 

 to be measured passes to earth. 



§ 2. There is, however, one serious obstacle to be overcome, namely, that 

 the resistance required for this purpose is usually very large indeed. Thus, if 

 the electrometer gives a full-scale deflection for about -^ volt, and if a current 

 of 10" 9 ampere is to be measured, we require a resistance of the order of 100 

 megohms. 



Moreover, this resistance must not vary with the current, so that the 

 electrometer deflections may be proportional to the currents being measured. 

 It is also very desirable that the resistance should possess uo appreciable 

 temperature coefficient, and that it should be free from polarisation effects. 

 These requirements we may regard as essential ; but the utility of the 

 resistance method would be greatly enhanced were it possible to vary the 

 resistance according to the strength of the current to be measured. Thus it 

 would be very useful if the resistance could be constructed to have a definite 

 value, such as 100, 500, 1000, or 5000 megohms : but it would be still more 

 serviceable if it could be altered, while in use, from one to another of these 

 values. 



I believe the apparatus now to be described meets these requirements. 



§ 3. The method of attaining such high resistances suggested itself in con- 

 nection with Siemens' method of measuring a small capacity. In this method 



1 Bronson, Amer. Journ. Sc, 1905, 4th Ser., vol. xix., p. 185. 



2 Campbell, Phil. Mag., 1911, vol. xxii., p. 301 — and later papers. 



SCIENT. PROC. E.D.S., VOL. XV., NO. III. E 



