ON PKACTICAL STANDARDS KOK ELECTRICAL MEASUREMENTS. 51 



cause of variation, it cannot be modified to explain all the gradual 

 increases in resistance, owing to the negligible effect of humidity on very 

 low-resistance standards. Cause No. 1 appears, therefore, to have been 

 the chief agent in the cases we have considered. 



It is necessary, however, that we should say something about other 

 manganin coils. In 1903 the resistances were measured of some manganin 

 coils (1 to 5,000 ohms) in a box by R. W. Paul, London. The coils 

 could not readily be immersed in oil, and the measurements were 

 therefore uncertain to about 1 part in 100,000. The resistances were 

 again measured in 1904, 1906, and December 1907. The maximum 

 change in the resistance of any coil is 5 parts in 100,000, while the 

 mean increase in resistance during 1903-7 is 4 parts in 100,000. 



In 1902, and again in 1907, the resistances were measured of some 

 manganin coils (1 to 10,000 ohms) in box No. 1723 by O. Wolff, Berlin. 

 The maximum change in resistance during the period 1902-8 is about 

 6 parts in 100,000, and a few of the coils have kept practically constant. 

 Many manganin coils in other boxes are known, however, to have 

 changed very considerably. 



It will be seen that of the manganin standards we have examinea 

 some have kept remarkably constant, while others are practically useless 

 as standards. It must not be concluded, however, that all manganin 

 resistances are subject to such changes. Drs. Jaeger and Lindeck have 

 shown that the manganin standards of the Reichsanstalt keep very 

 constant, and the manganin coils at the Bureau of Standards also appear 

 to be of a fairly constant type, though subject to considerable cyclic 

 changes owing to variable atmospheric humidity. The manganin 

 standards reported on in this Appendix comprise every standard resistance 

 of manganin in use in the Standards Department of the National 

 Physical Laboratory. 



APPENDIX II. 



Specifications for the Practical Realisation of the Definitions of the Inter- 

 national Ohm and International Ampere, and Instructions for the 

 Freparation of the Weston Cadmium Cell. 



(From the National Physical Laboratory.) 



The following specifications have been prepared after consultation 

 with various authorities, and will form a basis for discussion at the 

 forthcoming Congress on Electric Units in London. They have not 

 been authoritatively adopted, and are subject to amendment. 



In the last Report specifications for the realisation of the inter- 

 national ampere and for the construction of the cadmium cell were 

 given, the processes of preparation, Ac, being described with considerable 

 detail. These specifications appeal to a much wider circle than the 

 present ones, for the latter are intended mainly to serve as a guide to the 

 standardising institutions of the various countries in order to obtain, as 

 far as possible, complete agreement in the units of electric measurements. 

 Certain instructions, such as the purification of mercury, have therefore 

 been omitted, but all which is thought to be essential for jm exact repro- 

 duction of conditions is still included. Instructions for the erection of 

 mercury standaids have not previously been issued. 



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