MEASUREMENT OF HIGH tofcl^ANgE^ ,»- -^/l09 





3.— ON THE MEASUREMENT OF HlO^EJlSISTANCES 

 AND ON GALVANOMETERS SUITABLE FOR THE 

 PURPOSE. 



By Richard Trelfall, M.A., Professor of Physics, University 



of Sydney. 



[Abstract.^ 



A galvanometric method was adopted partly with a view to 

 finding how far the sensitiveness of a high i-esistance galvano- 

 meter could be conveniently carried. The substances examined 

 were samples of Australian gums. 



In order to obtain an accurate estimate of the specific resistance 

 of a substance, it is in general possible to proceed in one or two 

 ways ; either by making a comparison of the resistance of the 

 substance in a known form with the resistance of some standard, 

 or by observing the electromotive force and current. The metliod 

 adopted by the author may be considered as a sort of compromise, 

 for in it the resistance of the gum under one electromotive force 

 was compared with that of a standard megohm under another. 

 The electromotive force was compared by an extension of Lord 

 Rayleigh's method of balancing and estimating differences instead 

 of ratios, and the experiments were arranged so that alternate 

 equal readings were obtained by means of a galvanometer : — 



(a) When an electromotive force nE acted through the gum, 

 (b) when an electromotive force E acted through a megohm. 



The experimental problems are the following : — 



1. The production of a slab of gum of accurately known 



dimensions. 



2. The construction of the most sensitive foi'm of galvano- 



meter. 



3. The investigation of the sources of electromotive force, 



and the comparison of the same. 



L The first condition was achieved by the use of accurately 

 flattened brass rectangular plates, covered with an electric deposit 

 of reguline platinum. The measurement of the thickness of the 

 slab of gum was attained by the use of three micrometer screws. 



2. A great deal of labour had been wasted in attempting to 

 obtain good results from a galvanometer of the Gray pattern, the 

 fatal objection seemed to be that the time of swing was so great 

 as to prevent a due discrimination of the real deflexions from 

 those produced by unavoidable air currents. The form finally 

 arrived at was exhibited and explained ; and a good deal was 

 said about the practical arrangements necessary to secure a fair 



