66 MU. F. E. SMITH OX THE CONSTRUCTION OF 



Variations in cross-section as great as 10 per cent, were common, and it was a matter 

 of some difficulty to select a dozen tubes all of which should have a small sectional 

 variation. 



The preliminary choice made, a long mercury column was introduced and its length 

 and mass determined. A first approximation to the length for one international ohm 

 was thus obtained. It was desirable that the shortest tube should be at least 

 60 centims. long, or the difficulty of accurately determining the mass of the column 

 would be considerable ; on the other hand, lengths greater than a metre would prove 

 inconvenient, as they could not be readily measured. 



Calibration. 



As a preparation for calibration and other operations the tubes were cleaned by 

 passing through them hot water, soda solution water, nitric acid, water, distilled 

 alcohol, and twice distilled ether in the order named. In many cases a thin cotton 

 covered wire, having attached to it a small plug of cotton wool, was passed through 

 the tul>es in a damp condition. This served to remove from the walls any adhering 

 particles of solid matter. A rapid flow of the cleansing liquids through the tubes was 

 produced with the aid of a filter pump, the latter also serving for the aspiration of air 

 in the final drying. In every case the air used for drying was purified by its passage 

 first through caustic soda solution, then through strong sulphuric acid, passing 

 afterwards over dry phosphorus pentoxide and through a glass bulb heated to 

 a temperature a little over 100 C. The air was thus ensured being warm and dry, 

 while at the same time the tube itself was heated to a temperature approximating to 

 80 C. On cooling, the mercury was introduced by slight exhaustion. 



The calibration was effected by mercury threads 5 centims. long. The reason for 

 this choice is in part practical and in part the result of the theoretical reasoning 

 already given. For the linear measurements a simple comparator by Messrs. PYE, of 

 Cambridge, was employed, and the arrangements were not such as to facilitate the 

 measurement of shorter lengths. For the tubes employed, however, 5-centim. lengths 

 were sufficiently short for the necessary degree of accuracy to be obtained. As an 

 indicating mark, a short line at right angles to the axis was etched near one end of 

 the tube. 



The 5-centim. thread being introduced, the ends of the tube terminated in two 

 calcium chloride bulbs with small orifices. These latter dried the entering air and so 

 prevented the mercury from becoming moist. The lengths of the menisci, however, 

 were not so constant as when the tube was open to the atmosphere, owing to the 

 slight variations in pressure produced by the motion of the thread. The order- of 

 measurement was as follows : - 



The thread length for different mean positions along the axis of the tube was first 

 determined. These first mean positions were at 5-centim. intervals, and in the 



