418 wenner: wire resistance standards 



seems to become very constant within a year. Where manganin is 

 used a very similar increase in resistance takes place while the 

 standard is new. In order to make further test of the material 

 and to determine what the heat treatment should be. coils have 

 recently been constructed of hard-drawn wire and of wire annealed 

 before insulating and these have been annealed at different tem- 

 peratures after winding. 



As a further means of determining the changes in the mean 

 values of the standards of the reference set, certain standards have 

 been sent abroad and measured at the national laboratories of 

 England. Germany and France. Since May, 1910. a number of 

 measurements have been made of the resistance of seven differ- 

 ent standards. The 23 values assigned were usually each the 

 mean of the values found by a number of different measurements. 

 As compared with the values assigned to the same standards in 

 the Bureau of Standards the mean difference, taken without 

 regard to sign, is less than one part in one hundred thousand. This 

 shows that the unit used in each of the laboratories continues to 

 be very nearly the same. 



Ultimately we depend for our unit of resistance on an arbitrary 

 standard such as the mercury ohm or on an absolute measure- 

 ment of some particular resistance. 



Mercury resistance standards are sufficiently reproducible so 

 that different laboratories agree to within two or three parts in a 

 hundred thousand. Absolute measurements have not as yet 

 been made as close as this. Wire standards are now so constant 

 that a reference to mercury ohms need be made much less often 

 than formerly, and international uniformity is maintained chiefly 

 by the use of wire standards. 



