ON PRACTICAL STANDARDS FOR ELECTRICAL MEASUREMENTS. 35 



The question of the permanence of mauganin standards has been 

 discussed recently by Messrs. Rosa and Babcock ^ and by Drs. Jaeger 

 and Lindeck,- and it seemed desirable to bring together all the informa- 

 tion available regarding the changes which have taken place in the coils 

 of the Association, and of others which have from time to time been 

 compared with them. 



At the National Physical Laboratory the primary standards of resistance 

 are of mercury, and the secondary standards are of platinum, platinum- 

 iridium, gold-silver, platinum-silver, and of manganin. It will be shown 

 that the mercury standards have kept constant, that the platinum coils 

 have probably kept constant, that the platinum-iridium, gold-silver, and 

 a few of the platinum-silver coils have changed considerably, while other 

 platinum-silver coils have kept very nearly constant. Of the manganin 

 coils a few have kept very nearly constant, bur. most of them have 

 increased in resistance. 



The platinum, platinum-iridium, and some of the gold- silver and pla- 

 tinum-silver coils are the property of the Association, and many of them 

 were first compared by Matthiessen and Hockin in 1865-7. Most of tlie 

 manganin standards were constructed by O. Wolff, Berlin, but four were 

 built by Mr. Melsom at the National Physical Laboratory. These 

 standards vary in nominal value from one-thousandth of an ohm to 

 10,000 ohms. 



The method of comparing resistances has been dealt with in a previous 

 Report,^ and for the purposes of this communication it will be sufficient tc 

 state that, on all occasions when mercury standards were erected, the 

 resistance coils were measured in terms of the mean unit represented by the 

 mercury columns, with a probable error of about 5 parts in 1,000,000. 

 In the intervals between the comparisons with mercury standards the 

 values of the coils in international ohms were at times uncertain within 

 1 to 2 parts in 100,000, but the relative values of the unit coils of man- 

 ganin with potential leads could at all times be determined with an error 

 not greater than about 2 parts in 10,000,000, and the one-thousandth 

 ohm and 10,000 ohms manganin standards could in general be measured 

 in terms of the unit coils within about 5 parts in 1,000,000. In the 

 intervals between the erections of the mercury tubes a very careful survey 

 of the history of the coils was often necessary to determine the most prob. 

 able changes in the coils, and a slight readjustment of the values allotted 

 to the coils was sometimes made when the mercury standards were next 

 employed. The probable error of the resistance values assigned to the 

 manganin standards on any date is almost certainly less than 1 part in 

 100,000. ^ 



Mercury Standards of Resistance. 



The mercury standards of resistance are 10 in number, and were con- 

 structed in 1902-3. The mean international ohm as realised by the ten 

 standards is taken as the unit, and each tube is measured in terms of it. 



' The Electrician, June 14, 1907, and November 15, I!)07. 



" Ibid., August 2, 1907. 



' B.A. Report, Section A, 1906. 



D 2 



