ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 



57 



published, and we have to thank him for having placed his note-book and 

 papers at our disposal. 



The question of the pennanence of wire standards has been discussed 

 recently by Professor Himstedt, ' Wied. Ann.' xxxi. p. 617, and it seemed 

 desirable to bring together all the information attainable as to the original 

 coils of the Association and others used by Messrs. Matthiessen and 

 Hockin in 1867. 



The original coils of the Association are six in number, and the tem- 

 .peratures at which each has a resistance of 1 B.A.U. are given by Mr. 

 Hockin in the Report for 1867. In addition to these six coils Messrs. 

 Chrystal and Saunder examined the coil No. 29, marked F by them, and 

 also a coil known as Flat, which are not mentioned in Mr. Hockin 's re- 

 port. The results of these two comparisons are given in the following 

 table : — 



Table I. — Table giving the results of comparisons in 186? and 1876. 



It will be noticed that the coil G of Pt Ag is the only one for which 

 the table shows any marked alteration. 



Now Matthiessen gives as the percentage increase of resistance per 

 1° C. for Pt Ir the value '032. Our own experiments show it to be lower 

 than this, and the value found for G by Dr. Fleming after a most careful 

 series of experiments is 0278. I can find no record of the temperature 

 at which Hockin actually worked. If it were below 15° and the tempera- 

 ture at which the coil was right was found by the use of the coefficient 

 *032 the temperature so found would be too low. 



If we assume Hockin's measurements to have been made at 0° C. and 

 take Fleming's value "028 for the coefficient we find the temperature at 

 which the coil was right to be 18°" 1. 



We have next to consider the very complete series of measurements 

 taken by Dr. J. A. Fleming in 1879-81 ; the results of these measure- 

 ments were tabulated on a chart which has been kept with these coils 

 since that date. For the details of the experiments we have to thank 

 Dr. Fleming, who placed at our disposal his note-books. The principle of 

 his observations was as follows. If X, Y be two coils to be compared, 

 ■one X, say, was kept at 0° C, while the temperature of Y was varied 

 from 0° to 20°. The difierences between the resistance of X at 0° and Y 

 at various temperatures were measured by Carey Foster's method in 

 terms of the wire of the Fleming bridge. The values of this difference 

 were plotted as ordinates, the temperatures being the abscissae, and thus 

 a curve was obtained giving the variation of resistance with temperature 



' In obtaining this column it was assumed that B remained unchanged between 

 1867 and 1876. 



