ON STANDARDS OF ELKCTKICAL RESISTANCE. 363 



To calibrate a tube it was taken and carefully cleaned with pure nitric acid, 

 and then with a solution of caustic potash. It was then weU rinsed with 

 distilled water, and dried by passing a current of hot air through a chloride- 

 of-calcium bulb, and then through the tube. A small column of mercury 

 was put in the tube, and the length of column measui-ed by the beam-com- 

 pass. Tho column was shifted along the tube by sucking up or blowing- 

 through an india-rubber tube with a chloride-of-calcium tube inserted be- 

 tween it and the tube to be calibrated. By this arrangement the column 

 could be adjiisted with the greatest nicety to the place in the tube required. 

 The lengths of the column were taken at equal intervals from one end of tube 

 to the other. The formula for correction used is given below. 



Let C be conducting-power of a tube of uniform bore and of length, ca- 

 pacity equal to that of tube considered ; C observed conducting-power. 

 Then 



,1 



^■X2f' 



where n is the number of measurements made, \ the length of the column of 

 mercury in any position. The summation extending to all the readings 

 taken 



The ends of the tubes were ground by putting some emery powder and 

 naphtha on a slate table, holding the tube vertically iipright with the left hand 

 and with the right hand rubbing the end of the tube in contact with the 

 table round the circumference of a small circle. Thus the end of the tube 

 was made slightly convex, the opening being at the apex of the convexity. 

 To measure the tubes they were placed under the beam-compass, and a stout 

 pin inserted partially into each end. 



Fi'om the shape of the ends of the tube, the point where the pin emerged 

 from the tube coidd be exactly seen and the measurement made with cer- 

 tainty. Many measurements were made turning the tube round its axis through 

 a small angle before each measurement, and the mean of the lengths found 

 taken for the true length. To find the weight of the tube full of mercury it 

 was carefully cleaned, filled with mercury, and placed in a long narrow 

 trough fuU of pure mercury. The tube was held down by iron weights, a 

 thermometer inserted in the trough, and the apparatus allowed to stand until 

 the temperature was constant. After the true temperature had been obtained 

 the tube was taken out of the trough and the contents weighed. 



This was managed in the following manner. One operator took hold of 

 the tube by pressing a finger against each end and lifting from the ti'ough ; 

 the httle globules adhering to the outside of the tube were then rapidly re- 

 moved by two assistants with brushes. ■ ■ 



The mercury was then allowed to flow slowly out into a small porcelain 

 crucible and weighed. In this way pretty consistent results were obtaiued 

 if the tubes were cleaned before each filliag. 



To determine the resistances of the tubes they were placed in the water 

 trough, with bent pieces of tubing fastened on to the ends with india-rubber 

 tubing and reaching above the surface of the water. 



The terminals were of copper, well amalgamated. They dipped into the 

 bent tubes and came flat against the ends of the tubes, the resistance of which 

 was to be determined. In the calculation of the weight of mercury at 0° in 

 the tube from the observed weight, Regnault's value for the expansion was 

 used. 



2b2 



