WOLFF, SHOEMAKER, BRIGGS: RESISTANCE STANDARDS 459 



Because of the impossibility of exact realization of the above 

 conditions, principally because of the impracticability of securing 

 glass tubing of strictly uniform bore, certain specifications were es- 

 sential. Those adopted specified that the tubes used must be made 

 of a glass the dimensions of which change little with time; that 

 they be well annealed and straight ; that the bore be as nearly as 

 possible uniform and circular ; that the area of cr©ss-section of the 

 bore be approximately one square millimeter; and that the mer- 

 cury have a resistance of approximately one ohm. It was also 

 specified that each tube be accurately calibrated, and that no 

 tube have a caliber correction greater than 5 parts in 10,000. 

 The length of the tube, the mass of mercury the tube contains, 

 and the electrical resistance of the mercury must all be determined 

 at a temperature as near to 0°C. as possible, all measurements 

 being corrected to 0°C. 



The four tubes used at the Bureau were selected from a large 

 number of tubes of Jena 59 III glass, specially drawn by Schott 

 and Gehossen, and were straightened, annealed, and graduated by 

 M. Baudin in Paris. 



At the Bureau of Standards, they were very carefully cali- 

 brated, their caliber factors, ranging from 1.000047 to 1.000096, 

 being determined to an accuracy of about one part in a million. 

 The tubes were cut at points giving a resistance, including the 

 end correction, of approximately one ohm, the cuts being located 

 at points for which the cross-section could be most accurately 

 calculated. The ends were ground and plane polished. The 

 tubes were tapered at the ends to fit into the end bulbs, used in 

 making electrical measurements, and into the glass cleaning and 

 drying fittings. 



The Reichsanstalt method was employed for the determination 

 of Mo, the mass of mercury required to just fill a tube at 0°C. 

 A tube was exhausted, filled with mercury and placed vertically 

 within a double walled ice bath, the lower end of the tube being 

 sealed by a plane polished plate, and the upper end, carrying a 

 slight excess of mercury, being protected by a ground glass cap. 



The excess of mercury was removed by stroking off with 

 a plane polished glass plate in a gimbal mounting, the conden- 



