20 Messrs. Hutchinson and Wilkes's Comparison of 



in comparison with the diameter of the tube, Lord Kayleigh 

 has calculated that a correction of *82 diameter, additive to 

 the true length of the tube, is necessary in order to allow for 

 the resistance of the terminal connexions. Mascart, Nerville 

 and Benoit, and also Grlazebrook and Fitzpatrick have verified 

 this result experimentally. This quantity is SL in the above 

 formulae. 



In commencing our experiments in the spring of 1888, our 

 first object was to determine the best methods that had been 

 used for the different determinations involved. About two 

 months were spent in standardizing resistances that were to 

 be used (the comparisons being made in a constant tempera- 

 ture vault, using a Fleming bridge) and in testing different 

 methods of measurement. 



An attempt to measure the lengths at 0° C. was made, only 

 the portion of the tube to be observed by the microscopes 

 being scraped clear of ice. It was found that this method 

 presented great difficulties ; and as an error of 10 per cent, in 

 the assumed expansion of glass, in reducing the length of the 

 tube from 20° C. to 0° 0., would only cause an error of 

 TFooVoo ^ n ^ ne l en gth, the plan was abandoned. 



The measurement of a column of mercury a little less than 

 the length of the tube, which was covered with ice except at 

 points over the ends of the column, was tried repeatedly, both 

 by observing the meniscus and by flattening the ends of the 

 mercury with hard rubber plugs (as suggested by Lord 

 Rayleigh) without success. The lenses of the microscopes 

 would naturally coat rapidly with moisture, and the unoccupied 

 parts of the bore of the tube become so wet that minute glo- 

 bules of mercury would be left behind when the column was 

 run out to be weighed. We endeavoured to obviate this by 

 plugging the ends with soft wooden plugs ; but still the 

 moisture got in, making the meniscus uncertain, and inter- 

 fering with the removal of the column. 



Plate-glass end-pieces, held in place by elastic bands, were 

 tried, hoping thus to obtain full tubes at 0° ; but, owing to 

 the grinding of the plates against the mouths of the tubes and 

 the old trouble with wet mercury, these were given up. 



The tubes were at first secured to straight, narrow, well- 

 seasoned boards, and (the end-cups being in place) were put 

 in watertight rectangular boxes (lined with waxed duck) 

 about 5 inches wide and 5 inches deep. Crushed ice was 

 then packed in over them. Though observations taken ten 

 or fifteen minutes apart would apparently agree, it was found 

 that, owing to the proximity of the board, the mercury would 

 sometimes not have reached its minimum resistance in four 



