ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 123 



APPENDIX II. 



On a Determination of the International Ohm in Absolute Measure. By 

 Professor J. V. Jones, F.R.S., Principal of the University College of 

 South Wales and Monmouthshire, Cardiff. 



The apparatus for tlie absolute measurement of electrical resistance 

 in my laboratory at Cardiff" -was completed in 1890, and I first used it 

 for the determination of tlie specific resistance of mercury in absolute 

 measure.^ This determination -was made by direct measurement on a 

 mercury column contained in a trough of paraffin wax. The I'esults of 

 five complete sets of observations were as follows : — 



94103 

 94074 

 94093 

 94045 

 94021 



The mean of these is 94067 ; and the extreme variation from the 

 mean is 46, or about four pai-ts in 10,000. 



I suspected that much of the variation was due to the paraffin trough, 

 the temperature of which varied slightly (about half a degree) during 

 the observations, and was not accurately measurable owing to the low 

 conductivity of tlie material. With variation of temperature there was 

 variation of breadth, and the breadth of the trough entered as a primary 

 factor into the calculation of the specific resistance. 



When I proceeded to use the apparatus for the measurement of low- 

 resistance standards of solid metal this was conclusively shown to be the 

 case. I brought a set of measurements made on such a standard under 

 the attention of the Section last year at Nottingham, in which the extreme 

 variation from the mean was only about one part in 12,000. 



This may be taken to be the normal performance of the apparatus ; and 

 seeing that it is an instrument of such precision, it seemed to me of 

 interest to determine by the use of solid metal standards the relation 

 between its indications and the results obtained by other observers for the 

 value of the ohm. 



With this end in view I obtained four coils from Messrs. Nalder 

 Brothers — two platinum-silver ten-ohm coils and two manganine tenth- 

 ohm coils. Mr. Glazebrook has measured them in terms of the inter- 

 national ohm ; and I am much indebted to him for the pains he has been 

 kind enough to take in making the determination. The following table 

 gives their resistances and temperature coefficients : — 



These coils were arranged in manner similar to that adopted by Lord 



' Phil. Trans., 1891, A. 



