240 



Mr. F. E. Smith. On the Construction of [Mar. 10, 



formula. If such an axis were projected and examined, the curve 

 could not be distinguished by the eye from a straight line. 



The correction for conical form was obtained by a method very 

 similar to those previously employed by other observers. 



(b) Determination of the Mechanical Constants. 



The lengths of the mercury columns, and of the tubes, were 

 measured in a water bath comparator by Mr. B. F. E. Keeling, the 

 temperature of the mercury and of the glass being accurately known 

 to o, 02 C. The mass of the mercury was determined with a probable 

 error of about 0*0005 per cent. These determinations enabled the 

 mean cross-sections of different parts of the tube to be evaluated. From 

 the results, the mean cross-section of the standard portion was deduced, 

 and, finally, that length of a uniform mercury column of similar cross- 

 section that would have a resistance of 1 international ohm. The 

 consistency of the results obtained from different fillings of the tubes, 

 each thread of mercury occupying a slightly different position, may 

 best be judged from the results of actual observations. The following 

 values are for the first three standards constructed. The horizontal 

 lines in the table represent separate fillings of the three tubes : — 



Standard. M. 



P. 



T. 



Length of the mercury column pos- j 59 "0336 cm. 

 sessiug a uniform cross-section ' 59 '0326 ,, 

 equal to the mean of that of the j 59 -03i'8 

 standard, and having a resistance 1 59 "0337 

 of one international ohm. 



I 



63*5493 cm. 

 63 -5485 „ 

 63 5494 „ 

 63-5484 „ 



57 -7801 cm. 

 57-7808 „ 

 57-7805 „ 



The results with the remaining eight tubes are equally good. 



(c) Measurement of Resistance, and Deductions. 



The standards were erected in three fashions. In the first of these, 

 two other glass tubes were connected to the ends of each standard 

 by means of a special connector. The ends of the tubes thus brought 

 into contact were of the same cross-section, and the shapes of the 

 sections were also identical. An adjustment was provided for, which 

 insured the absence of internal irregularities at the junctions. Thin 

 pieces of platinum foil, interposed at the points of union, acted as 

 potential leads, and enabled the resistance to be measured. The 

 value thus obtained was for a column of mercury completely filling 

 the tube, and terminated by planes at the ends of the tube. This 

 mode of erection answers capitally with care, but many precautions 

 have to be taken. 



The second and third modes of fitting up the standards introduced 



