THE BRITISH ASSOCIATION 

 STANDARDS OF RESISTANCE, 



1865-1932 



BY 



SIR RICHARD T. GLAZEBROOK, F.R.S., AND 

 DR. L. HARTSHORN. 



(From the National Physical Laboratory.) 



Ordered by the General Committee to be printed in extenso. 



The original Electrical Standards Committee of the British Association was 

 appointed at the Manchester meeting in 1861. In their first Report 

 (Cambridge, 1862) they point out that they had first to determine ' what 

 would be the nnost convenient unit of resistance, and second, what would 

 be the best form and material for the standard representing that unit.' 



The C.G.S. system of measurement was the outcome of their delibera- 

 tions on the first question, and they determined to adopt as a practical 

 standard of resistance the ohm equal to 10' C.G.S. units of resistance. 

 Experiments were made at King's College by Maxwell and Fleeming Jenkin 

 to obtain the ohm in a material form, and Reports giving the result of these 

 were issued in 1863 and 1864. Experiments were carried out by Matthiessen 

 and Hockin to determine the best form and material for a series of standard 

 coils. The Reports which followed give an account of the process of these 

 experiments, and in Appendix A to the 1865 Report we have their final con- 

 clusions and a description of the form of standard coil they recommended. 

 As a material for the wire of which the coil was constructed an alloy con- 

 taining 66 per cent, silver and 33 per cent, platinum was chosen, for 

 reasons given in the Report. It was agreed that copies of the standard 

 should be made and preserved at Kew Observatory, and the Report for 

 1867 contains a table of the values of the standards in question. A copy of 

 this is given as Table I. The Committee was dissolved in 1870. 



Soon after Maxwell's appointment as Cavendish Professor at Cambridge 

 the coils, with the bridge used for their comparison, were brought to the 

 Cavendish Laboratory and were used by Chrystal and Saunder in their 

 work on Ohm's Law in 1876. 



Lord Rayleigh became Cavendish Professor in 1879 and was immediately 

 interested in electrical measurements. Various investigations, particularly 

 those of Rowland at Baltimore, and some deductions from Joule's work, had 

 thrown doubts on the accuracy of the absolute measurements of the British 

 Association Committee. These doubts were confirmed by measurements 

 made at Cambridge by himself and Schuster, and from 1881 onwards 

 there was great activity at the Cavendish Laboratory and elsewhere in 

 connection with the question of electric units. 



