1904] Construction of Mercury Standards of Resistance, etc. 239 



" On the Construction of some Mercury Standards of Eesistance, 

 with a Determination of the Temperature Coefficient of 

 Eesistance of Mercury." By F. E. Smith, A.E.C.Sc., Assistant 

 at the National Physical Laboratory. Communicated by 

 E. T. G-lazebrook, F.B.S. Eeceived March 10,— Head 

 March 17, 1904. 



• (From the National Physical Laboratory.) 



(Abstract.) 



This paper contains an account of the construction and measurement 

 of eleven mercury standards of resistance at the National Physical 

 Laboratory, Teddington. 



It is divided into three parts : (a) theory of construction ; (b) deter- 

 mination of the mechanical constants : (c) measurement of resistance, 

 and deductions. 



(a) Theory of Construction. 



The definition of the international ohm necessitates the determina- 

 tion of a length and a mass only, the resistance of a uniform column 

 of mercury of length, L. cm., and of mass, W. gr., being 



W ' ^L06^3 2 ^ mterna ^ ona l °hms. 



The difficulties of accurately evaluating L 2 /W are discussed in the 

 paper. The method finally adopted for the determination of this ratio 

 differs from those which have been previously employed. A quantity 

 proportional to L 2 /W was first of all determined for different, but 

 practically similar portions of a selected tube. The tube was then 

 broken into three parts, the positions of fracture being carefully chosen, 

 so that a mercury column occupying the central portion should have 

 a nominal resistance of 1 ohm. The ratio L 2 /W for this central portion 

 was calculable from the previous observations. It is shown that this 

 method admits of measurements of a high degree of accuracy, while 

 all the operations involved are of a simple nature. 



The effect of liquid and gaseous films in contact with the glass is 

 briefly discussed. If the same cycle of operations is always adhered 

 to, it is shown that such films produce no disturbing effects. 



The fact that the axis of a tube may be undulatory in character, is 

 proved to be a very serious obstacle to an accurate computation of the 

 resistance of a mercury column. Thus, it is quite possible for the 

 radius of curvature at any part of the axis to be as small as 40 cm., 

 and for the maximum displacement of the curve from a straight axis 

 to be 0*015 mm. only, and yet, under such circumstances, the computed 

 resistance is 0*0036 per cent, greater than that calculated by the usual 



