ON THE SPECIFIC RESISTANCE OF MERCURY. 



in various poMtiuns close up to the end. The same process was repeated 

 at the other end : let the mean of the lengths thus found be ^. Then, if we suppose 

 that the curvature of the meniscus did not alter (and experiment showed that this 

 assumption is nearly true), we have very approximately the equality 



Thus, in tube VI., which is slightly smaller at both ends than it is in its central 

 part, the value of 7 is 3'63 mm., while the length of the thread was the same at each 

 end, and was equal to 378 mm. For this tube the mean value of (L J)/L was 

 003G7, and the average value of the correction '000150. It is larger in the case of 

 this tube than with any other used, for ija most the effect of one end was opposite to 

 that of the other. 



Table V. gives the values of the lengths of the threads at the two ends respectively, 

 the mean of these two or l lt and of / for the various tubes. 



TABLE V. 



The mercury was weighed, using a balance by OERTLINO and the weights employed 

 by Lord RAYLEIGH ; these were compared with each other and with a set of weights 

 which Mr. SHAW had previously compared with the standard 500-gramme weight of 

 the Laboratory. A small correction of about '1 milligramme on 10 grammes was 

 found, and has been introduced, but it is too small to affect our results. 



The mercury was weighed in both pans of the balance, and the weighings repeated 

 on two or more different days. 



In the electrical measurements the tubes were compared directly by CAREY 

 FOSTER'S method with the B.A. standards, using the bridge designed by Dr. FLEMING 

 which was employed by Lord RAYLEIGH. The tubes had been so adjusted that the 

 difference between them amounted only to a few centimetres, at most 70 divisions of 

 the bridge wire. We were thus independent of variations in the resistance of the 

 wire due to temperature changes, and the value of the bridge wire division was taken 

 as -0000498 B.A. unit. 



The ends of the mercury tube were connected, in a manner to be described shortly, 



3 A 2 



