Section III., 190S. [ 163 ] Trans. R. S. C. 



XIV, — The Construction and Calibration of Very High Resistances and 

 a Standard of High Resistance. 



By Howard L. Bronson, Ph. D, 

 Lecturer in Physics, McGill University. 

 (Communicated by Prof. H. T. Barnes, D.Sc, and Read May 28th, 1908). 



The study of the passage of electricity through gases and its ap- 

 pHcation to the phenomena of radio-activity has required us to ex- 

 tend our field of accurate electrical measurements. These ionization 

 currents, which we measure, are in general too small to even be de- 

 tected by the most sensitive galvanometers. It has, therefore, been 

 found necessary to resort to the use of the electrometer or the electro- 

 scope. The principle of the method is the same for both instruments, 

 and involves the measurement of the change of potential of some 

 system of known capacity. All who have used these instruments 

 in the measurement of currents have found the method slow and liable 

 to many sources of error. 



It is evident that we could easily measure these small currents by 

 measuring their drop in potential across a sufficiently high constant 

 resistance, whose actual numerical value was known. Such a resist- 

 ance would be found very useful as a standard of high resistance, 

 and could be used to advantage, not only in radio-active measurements, 

 but also in insulation testing, and in the measurement of capacity, 

 especially in cases where ordinary methods would be difficult to apply. 

 The writer has been applying the above principle in making radio- 

 active measurements for the past four years, and has found it very 

 satisfactory. Fig. 1 shows the arrangement of the apparatus. 



If one pair of quadrants of an electrometer is connected to earth, 

 and the other pair is not only connected to the testing vessel, but 

 also to earth, through a very high resistance, it is easily seen that any 

 current in the testing vessel will charge the quadrants, until the dis- 

 charge current through the high resistance is equal to the current in 

 the testing vessel. (This latter current is not diminished by the rise 

 of potential of the quadrants, because the voltage on the testing ves- 

 sel is always high enough to produce practical saturation.) In this 

 case, the current is proportional to the potential on the quadrants — 

 that is, to the deflection of the needle. If the sensitiveness of the 

 electrometer and the actual value of the high resistances are known, 

 then we have at once an absolute measure of the ionization current 



Sec. III., 1908. 1. 



