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LIV. The Measurement of High Specific Resistances. By 

 R. Threlfall, M.A.j Professor of Physics, University of 

 Sydney, N.S.W* 



[Plate XIV. fig. 3.] 



THE experiments which form the subject of this paper 

 were begun almost immediately after my arrival in New 

 South Wales, in June 1886, and have been continued at 

 intervals ever since. The original object was to measure 

 accurately the resistance of certain gums produced by trees 

 growing in the Colony. The only gum thoroughly examined 

 however, up to now, is that produced by the " grass tree " 

 (Xanthorrhoea hastilis) . This gum, in spite of many attempts 

 to improve it by various methods of purification and by 

 mixing with other substances, turns out to be useless as an 

 insulator, having in fact no higher resistance than, say, 

 ordinary samples of resin, that is about 4'1 xlO 3 megohms 

 per cubic centimetre. Besides this, the gum in question is 

 faulty in other ways. It is of the nature of shellac, but 

 cannot compete with the shellac as ordinarily supplied either 

 in price or purity. In addition it has the two fatal defects of 

 being partly soluble in water and of decomposing before it 

 melts. Long-continued gentle heating does not seem to im- 

 prove it in this latter respect ; while the texture of the material 

 becomes looser, it grows friable and very dark in colour. 

 Benzoic acid appears to be given off in large quantities 

 during the process. In spite of many attempts I have 

 hitherto failed to obtain any considerable quantity of the 

 fig-tree gums which are said to be produced in large quantity 

 in the northern parts of the Colony. This paper therefore 

 will be devoted to a description of the method adopted in 

 measuring these resistances, a method which ultimately 

 reached a considerable degree of perfection ; partly on ac- 

 count of the modifications introduced in the construction of 

 high-resistance galvanometers, and partly on account of the 

 peculiar property of Clark cells. The method has since 

 been employed for other measurements, as will be shown 

 later on. 



I do not wish to claim any superiority for the galvanometer 

 over the electrometer methods, except that, given the gal- 

 vanometer and cells, it is certainly more easily applied, 

 especially when the determinations are numerous. I was 

 forced to adopt the galvanometer method in this instance 

 through not having an electrometer ; but I was by no means 



* Communicated by the Physical Society: read March 23, 1889. 



