ox PRACTICAL STANDARDS FOR ELECTRICAL MEASHRE^IENTS. 45 



terms of the unit of resistance employed for the purposes of Table IX. 

 are approximately 



A = l-OOOoO F = 1-0008S 



B = 100026 G = 1-00103 



C = l-OOlOl H = 0-99972 



D = 1-00020 Flat = 1-00053 



E = 1-00080 



Manganin Standards of Resistance. 



The manganin standards of the National Physical Laboratory are in 

 constant use and have proved of very great value. They not only facilitate 

 electrical measurements, but they bring them to a far higher degree of 

 accuracy than was formerly attainable. Nevertheless, the variations in 

 these resistances have ift many cases been a source of trouble, and attempts 

 have been made, and are being continued, to construct standard coils of 

 manganin -which shall remain practically constant in resistance. 



Since 1903 the manganin standards have been intercompared at least 

 four times every year, and the probable changes have been deduced from 

 occasional comparisons with mercury standards and from tables of dif- 

 ference values, due regard being also paid to the past history of the coils. 

 As an example of the comparisons we take the case of seven 1-ohm coils 

 which wei-e intercompared in January, April, July, and October 190G. 

 The observed differences are given in Table X. 



Table X. 



Maximum | 



Difference j 



- 1 



1-12 

 0-88 



0-78 5 



0-47 I 



3-83 I 



1-67 I 



Any one of these differences was not obtained from a single observa- 

 tion, but is the mean of six differences. All possible combinations of the 

 seven coils were taken — 21 in all— and the differences observed. From 

 these 21 observations six values resulted for the difference between any 

 two of the coils ; it is the mean of these six values which is recorded. 

 The temperature during the observations was very nearly 17°-0C,and 

 the differences were corrected to 17° C. before taking the mean. 



An analy.sis of the figures given in Table X. indicates that the coils 

 L-17 and L-IS probably changed most during 1906, and that the other 

 five coils changed by amounts less than 3 parts in 1,000,000 from January 

 to July 1906. From July to October the difference 1690-780 changed 

 by an appreciable amount and the differences in the values for July and 

 October, viz. : 



1690- 780 Change = 1 12 X Kr ohm. 

 1690-2351 „ = -0 57 ., 



1690-2483 „ = -0-5(; „ 



1690- 381 .,, = -0.30 „ 



indicate that 1G90 probably fell in resistance in this period by about 

 G parts in 1,000,000, and 780 rose by about 5 parts in 1,000,000. The 



