II. On the Construction of Some Mercury Standards of Ramvftnlo^ with a 

 Deteriniiiftlioii of the Tem/terature Coefficient of Resistance of Mercury. 



By F. E. SMITH, A.U.C.Sc., Assistant at the National Physical laboratory. 



Communicated by R. T. GLAZEHROOK, F.R.S. 

 (From the National Physical Laboratory.) 



Keceivcd March 10, Read March 17, 1904. 

 [PLATES 1-3.] 



INTRODUCTION. 



ACCORDING to the Order in Council of August 23, 1894, by which certain fundamental 

 electrical units were made legal 



"The ohm which has the value 10 in terms of the centimetre, and the second of 

 time, and is represented hy the resistance offered to an unvarying electric current l>y 

 a column of mercury at the temperature of melting ice, I4'452l grammes in mass, of 

 a constant cross-sectional urea, and of a length IOG'3 centims.," is one of the units of 

 electrical measurement on which " denomination of standards required for use in 

 trade " is to be t>ased. 



The establishment of the National Physical Lalx>ratory rendered it jnissible to 

 realize this unit in England. With this object, a numl>er of selected tubes of 

 " Verre dur" were obtained from M. BAUDIN, with the kind assistance of the officials 

 of the Bureau International, while others of Jena 16'" glass were procured from 

 Messrs. SCHOTT and Co., Jena. The work, however, has necessarily occupied a long 

 time. With the increasing accuracy of electrical measurements it appeared desirable 

 to determine the constants of the tul)es so that the probable error should not exceed 

 I part in 100,000. Preliminary oljservatioiw of various kinds were essential to secure 

 success, while the work of organising the various departments of the Lulx>ratory also 

 tended to delay matters ; hence it was not until the autumn of 1902 that the real 

 start was made. 



VOL, CCIV, A 373. I 15.10.04 



