wenner: wire resistance standards 



415 



of one of these standards changes appreciably it is replaced by 

 another whose resistance is known to be more constant. In 

 March, 1909, values were assigned to the standards of the refer- 

 ence set, and since that time it has been assumed that their mean 

 value has remained constant. The values assigned to all the 

 standards belonging to the Bureau and to the standards submitted 

 for test all involve this supposition. In May, 1910, certain stand- 

 ards from the national laboratories of England, Germany, France 

 and the United States were compared, and considering all the 

 data available the most probable value in International Ohms 

 was assigned to the various standards. This resulted in an 

 addition of 7 parts per million to the mean value of the reference 

 set. Values obtained from measurements made before May, 

 1910, have had this correction applied so that all are on the same 

 basis. That is, all values are based on the supposition that the 

 mean value of the ten 1-ohm standards constituting the refer- 

 ence set has remained constant since March, 1909. 



Values on different dates and at a temperature of 25° C. of 

 a number of 1-ohm standards of the sealed type are given in 

 table I. Here the identification number of the standard is given 



table I 



1-Ohm Standards — Sealed Type 



Excess of resistance above one ohm, on different dates at 25°C, and change in 2\ years 



in millionths of an ohm 



This table shows that during 2\ years the resistance of only one of these stand- 

 ards changed by more than 0.002 per cent and that the average change was less than 

 0.001 per cent. 



