22 Messrs. Hutchinson and Wilkes's Comparison of 



about fifteen were selected on account of uniformity of bore, 

 being tested by moving a small mercury column along in 

 them and measuring its length with a scale. These were 

 then more carefully tested and the best seven selected, which 

 were cut as nearly as possible to represent exact multiples of 

 a B.A. unit in resistance — one tube was cut for one half B.A, 

 unit. The ends were ground convex, using a fine file and 

 camphor in turpentine. After this the tubes were carefully 

 cleaned, using distilled water, nitric acid, distilled water, 

 ammonia, distilled water, alcohol, distilled water. Before 

 using these liquids, a small piece of wet cotton-wool was 

 drawn through the tubes (always in the same direction) in 

 order to remove any solid particles that might accidentally be 

 present. This was accomplished by first drawing through a 

 silk thread by means of an air-pump, and then tying on the 

 cotton and pulling it through several times in the same direc- 

 tion. This was always done in cleaning the tubes before 

 filling. The tubes were dried by warm dry air, which had 

 passed through calcium chloride and cotton-wool, the flow 

 being kept up by a pneumatic pump. 



The values of p for the different tubes were obtained by two 

 independent determinations, using different lengths of the 

 thread of mercury. The lengths of the thread were read on 

 a dividing-engine. 





Table I 



. — Values of p. 







Tube. 



Approximate 

 resistance. 



li. 



Length of 

 thread, in 



ft. 



Length of 

 thread, in 



Mean [i. 







centimetres. 





centimetres. 





I. 



i ohm. 

 I „ 



1-00056 



5 



1-00050 



39 



1-00053 



II. 



1-00039 



3 



1-00043 



38 



100041 



III. 



2 „ 



100088 



4-8 j 



1-00080 

 1 00100 



4-01 

 3-5 j 



1-00089 



IV. 



10 „ 



1-00132 



38 



1-00133 



4-8 



1-00133 



V. 



5 „ 



100133 



4-6 



1-00122 



40 



1-00127 



VI. 



1 „ 



1-00055 



4-7 { 



1-00063 

 100062 



3-4 1 



3-8/ 



1-00060 



Determination of L. — As all the tubes except one were 

 longer than a metre we calibrated two metres of a five-metre 

 bar ; but it was found so unwieldy that better results could 

 be expected from using a metre bar and three microscopes. 

 Accordingly a comparator was placed on a long marble slab, 

 and in prolongation with it a third microscope, mounted on a 

 solid wooden block cemented to the slab, was placed. The 

 values of the micrometer-divisions were respectively *0025 

 millim., '0022 millim., -0045 millim. 



