PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 161 



The values of these coefficients obtained over so short a range are not 

 of much importance. Still, in view of Mr. Solomon's determination, they 

 may be given. They are: For .3873, -000283; and for 3874, -000277. 

 These values are relative to the standard coils of the Association. 



APPENDIX II.. 



On the Determination of the Temperature Coefficients of Tivo 10-Ohm 

 Standard Resistance Coils {Nos. 3873 and 3874) used in the 1897 

 Determination of the Ohm. By M. Solomon. 



In the determination of the ohm made by Professor W. E. Ayrtou 

 and Professor J. Viriamu Jones in 1897 (Report, 1897, p. 212), four 

 standard resistance coils were used, two of which had a resistance of 10 

 ohms each, and two of 0-1 ohm each. Values for the temperature co- 

 efficients of these coils had been calculated from four accurate determina- 

 tions of their resistance made, two by Mr. Glazebrook in 1894 and 1897, 

 and two by the Board of Trade in 1896 and 1897 (' The Electrician,' 

 vol. xl., p. 39). The values thus obtained neither agreed with one another 

 nor with the coefficients given by the makers, Messrs. Nalder Bros. &, Co, 

 It therefore became necessary to make as accurate a determination as 

 possible to endeavour to find the correct values for the coefficients. The 

 following Paper gives the results of the tests made on the two 10-ohm 

 coils (Nos. 3873 and 3874), the tests on the other two coils being not 

 yet completed. These two coils are of the B.A. pattern, and are made of 

 platinum silver wire. A preliminary series of tests made on one of the 

 coils showed that to attain the required accuracy special precautions 

 would have to be taken to keep the coils at steady temperatures. Each 

 coil was therefore placed in an oil bath, the temperature of which was 

 automatically regulated. In making the determination of the tempera- 

 ture coefficient of one coil, the other was used as a standard, and was 

 kept at a constant temperature throughout the whole series of tests. 

 The coil under test was maintained at a steady temperature for some 

 time, and a measurement of the difference of resistance between it and 

 the standard was then made by means of a Carey Foster bridge. The 

 temperature of the coil being tested was then altered and a fresh measure- 

 ment taken, this being repeated for several temperatures. 



The apparatus used in the measurements was ari-anged in the following 

 manner. The standard coil was placed in an oil bath with two vessels, in 

 the inner of which the coil itself and a carefully standardised thermometer 

 were immersed. In the outer bath was the bulb of an alcohol thermo- 

 meter, the mercury index of which, when the temperature rose too high, 

 completed the circuit of an electromagnet and battery, and caused 

 the gas which heated the bath to be put out. On the bath then cooling 

 the circuit of the electromagnet was broken, and the gas turned on and re- 

 lighted by a bypass. This thermostat was very sensitive, the temperature 

 of the inner bath rarely varying so much as 0-0-5° C. in a day, and in a 

 run of ten days undergoing a maximum variation of 0"3° C. The ther- 

 mostat in which the coil under test was placed was not so sensitive, but 

 was designed to work over a greater range of temperature. The coil and 

 thermometer were placed in an inner bath, and in the outer bath was a 

 large brass bulb filled with alcohol. The expanding alcohol either passed 

 into a small reservoir, or, when the passage to this was closed by 



