344 Mr. K. T. Grlazebrook's Comparison of 



C, C, which fit tightly over the narrow ends of the cups, and 

 are secured by wrapping string firmly over the india-rubber. 



Fig.l. 



To make the joint tight the india-rubber and glass were var- 

 nished over with shellac varnish. The cups open at the top, 

 and can be closed by glass stoppers. 



The standards were accompanied by full directions for use. 

 I give a free translation of the greater part of the instructions. 



1. On each of the cups are engraved two marks a a, bb. 

 The lower mark a a indicates the position which the extremity 

 of the tube introduced into the cup ought to occupy. The 

 upper mark b b indicates the top of the mercury in the tube 

 when filled. These two conditions must be fulfilled approxi- 

 mately ; an error of *5 centim. in their sum changes the 

 resistance either way by about -00018 ohm. 



2. Method of Filling. — The tube may be filled in air in the 

 following manner: — Place it nearly horizontally and pour 

 some mercury into one of the cups; then incline it gradually, 

 shaking it a little, until the mercury enters the tube. The 

 mercury should fill the tube slowly, without leaving a trace of 

 air. Add mercury always by the same cup until it reaches 

 the upper mark b b in both cups. The method is not always 

 successful ; it is better to work in a vacuum. 



[Various methods for doing this are possible ; it is perhaps 

 hardly necessary to give the details of the one employed by 

 M. Benoit.] 



