STANDARDS OF RESISTANCE, 1865-1932 



421 



appear all the more remarkable when it is remembered that an error of o • i 

 in the temperature means a change of resistance of o- 00031 B.A.U., while 

 a variation in the measuring current of from o • i to o • 2 ampere produces 

 an alteration of o- 00006 B.A.U., a larger amount than the whole change 

 observed. 



The inference is clear that during this period these two coils have retained 

 their values unaltered, and this is confirmed by the following statement of 

 values taken from a later table in this Report : 



Values* of Coils D and E in B.A. Units in 1888, 1908 and 1932, 

 obtained from Comparison with Mercury Tubes, assuming 

 the Resistance of i metre of Mercury to be 0-95352 B.A.U. 



* See Table VII. 



We come now to the detailed account of the recent work at the National 

 Physical Laboratory by one of us (L. H.). 



The old standard resistance coils of the British Association, made in 1865 

 by Matthiessen and Hockin, have been re-measured during 1932, and a 

 comparison of the results with the old values is of considerable interest. 



Several features in the construction of the coils make it impossible to 

 obtain the same precision in these measurements as is obtained with modern 

 coils. The coils are embedded in solid paraffin wax, so that the attainment 

 of thermal equilibrium with the bath containing them is not easy. It is, 

 therefore, difficult to obtain the temperature of the coil itself, and as in some 

 cases the temperature coefficient of the material is very large, the accuracy 

 is almost entirely limited by the thermal conditions. Also the coils have no 

 potential terminals. However, preliminary measurements having shown 

 that certain of the coils had probably remained nearly constant over a period 

 of more than sixty years, it was considered desirable to aim at an accuracy 

 of I part in 100,000 in the present determinations. This requires an 

 accuracy in temperature measurement for certain of the coils of ± 0-003° C., 

 and although it is hardly likely that this could be realised, the general con- 

 sistency of a large number of observations has shown that the temperature 

 was usually correct to ± 0-01° C. The coils were immersed in a bath of 

 water, which was surrounded on all sides by cork lagging, and kept through- 

 out the measurements in a constant-temperature vault. The measurements 

 were made by means of a Smith bridge, assembled with standard manganin 

 coils in an oil-bath, kept by means of a thermostat at 20° C. in an adjoining 

 room. The terminal rods of the B.A. coils dipped into mercury cups, and 

 from these cups double leads passed to the bridge in the adjoining room. 

 The resistance of these leads is eliminated from the results by taking two 

 readings in the manner described by Smith. Thus the observer did not 

 have to approach the coils during the resistance measurements. The 

 thermometer dipping into the water-bath was read from a distance by means 

 of a telescope ; readings were taken at intervals during each day, as well as 

 before and after the resistance measurements, and it was always ascertained 



