Standard Resistance- coils with Mercury- Standards. 353 



The values given in the last column are the mean of those 

 derived from the two tubes ; the greatest difference between 

 the mean and any one observed value was "000005. Strecker's 

 values would be above Benoit's. 



Lord Rayleigh found for the average change between 0° 

 and 12°, -000861. 



Of course the value of this temperature-coefficient depends 

 somewhat on the glass of the tube; but the differences between 

 the coefficients of expansion of various kinds of glass are too 

 small to account for the whole of the differences shown 

 above. 



The paper by Herr Strecker on the same subject has been 

 already referred to, and it will be noticed that the number he 

 arrives at as expressing M. in B.A. units differs by '00078 

 from that given by Lord Rayleigh ; or, if we take the num- 

 bers expressing the B.A.U. in terms of M., the difference is 

 •00086. Now Strecker had compared his mercury tubes 

 with a B.A.U. tested carefully by myself against the stan- 

 dards, and found to have the value of 



•99937 B.A.U. at 17*7, 



This coil is marked M, No. 54. 



On the conclusion of his experiments, Herr Strecker re- 

 turned this coil to me, and kindly sent with it a German- 

 silver copy of his tubes marked No. 13, and said to have, 

 according to his determinations, a resistance of 



1-00189 M. at 10° C, 

 w r ith a temperature- coefficient of 



•000247 per degree. 



It was thus in my power to repeat my comparison of Hg, , No. 

 54, with the standards, and to compare the values of M. as found 

 by Herr Strecker with the B.A.U. The value found for .^ 

 5*4 agreed very closely with that given by Herr Strecker ; the 

 results of my comparison of Herr Strecker's coil with the 

 B.A.U. are given below. The comparisons were made as 

 usual, by Carey-Foster's method; but since the difference be- 

 tween the two resistances was such as to require the use 

 of a long piece of the bridge-wire, a resistance of 20 B.A. units 

 was introduced in multiple arc with the standard 1 unit. 

 This resistance was composed of the two standard ten-unit 

 coils of the Association. 



Phil. Mag. S. 5. Vol. 20. No. 125, Oct. 1885. 2 C 



