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ROYAL SOCIETY OF CANADA 



in the testing vessel. Further, by placing some form of potentio- 

 meter between the standard high resistance and the earth, the range, 

 over which we can measure currents, can be made very large. 



The great difficulty in getting a resistance, which is sufficiently 

 high and at the same time constant, is probably the reason that the 

 method has not come into more general use. The purpose of the 

 present paper is to discuss the possibility of getting such a resistance, 

 and to describe the results that have been obtained by the writer 

 in this direction. These results are still far from satisfactory, but 

 it is hoped that they may be a step in the right direction. 

 f"^ It is well known that resistances from one to ten megohms can 

 be made from fine carbon lines or films, or from certain liquids, such 

 as ■■ xylol, or amyl alcohol. One of the most satisfactory of these 



Key 1 Key 2 



9 9 



Earth 



Testing Vessel 



Earth 



Potentiometer 



l|l| Ih 



Fic. I. 



liquid resistances, known as Hittorf's solution, is a solution of amyl 

 alcohol containing 10 per cent of cadmium iodide. If the electrodes 

 are of cadmium, this resistance should show no polarization. Re- 

 cently C. W. Stewart (Physical Review, Vol. XXVI, page 302, 1908) 

 has described a new form of carbon resistance. It is obtained by 

 mixing lamp-black with a lacquer called " Zapon," and spreading 

 the mixture on an insulating surface. In this way he obtained re- 

 sistances varying from 0.3 to 3 megohms, and he speaks very highly 

 of their small temperature coefficient (.001 to .0015), and of their 

 permanency and constancy, there being only very slow changes in 

 the resistance with the time. The films used by Stewart were about 

 4 mm. long and 2 mm. broad, and he did not attempt to deal with 



