ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 153 



Ohm Coils. 



Among these the coil B.A. No. 38 ^ No, 77 has a special interest ; 

 it is an original platinum silver coil -which formei-ly belonged to Professor 

 Balfour Ste-wart, and is no-w in the possession of Professor Schuster at 

 the 0-wens College. According to the label on it, it -was right at 16°'5. 

 According to the Secretary's observations, its value is one mean B.A. 

 Unit at 14'9. This coil, therefore, -would appear to have risen in value 

 since about 1867 by -0006 B.A.TJ., and this result is not in accordance -with 

 the conclusions deduced in 1888 from the observations on the other plati- 

 num silver coils then examined. 



Some further experiments have been made with satisfactory results 

 on the air-condensers of the Association. A megohm resistance box has 

 been purchased for use in comparisons of capacity. 



With a vie-w to testing the permanence of the resistance standards it 

 was thought desirable to compare them again with the mercury standards. 

 This was done in December and January by the Secretary. The coil 

 riat was compared with two mercury tubes constructed in 1884 by Mr. 

 J. R. Benoit, which had been filled at Cambridge early in the year 1885, 

 and had remained full since. An account of the comparison was read 

 before the Physical Society May 9, 1891, and appears in the ' Philoso- 

 phical Magazine,' July, 1891. 



The tubes were compared with the B.A. standards. If we take, as was 

 done in 1885, for the resistance in B.A. units of a column of mercury 

 100 cm. long 1 sq.-mm. in section, the value -95412 B.A.U., we have the 

 following results for the resistance of the tubes in Legal Ohms. 



The differences are only '00004 Legal Ohms, which is too small to feel 

 really certain about. If we accept for the resistance of mercury the value 

 •95352 B.A.U., which (B.A. Report, 1890) appears the best value, then 

 we have : 



