CAPACITY KOI; IIKAT (!' MKTAF.S AT I 'IFFKKKNT TKM I'KKAT! I;KS. 



127 



tliink it necessary to encumber the paper with a full table of the results. By the 

 introduction of the new coil, the mean bridge unit (approximately T i of an ohm) 

 suffered alteration. Hence the values of R, and R,, for AB and CD differ somewhat 

 from those previously published. The alteration, however, is in the unit employed, 

 rather than in the thermometers themselves, and the value of R|/R and $ may be 

 regarded as unchanged. 



One addition to the bridge, however, is worthy of mention, as it may be found 

 useful in other cases. To obtain good contact by means of plugs, considerable pressure 

 lias to be exerted. As the insulating surface holding the 

 brasses (in this case marble) is always somewhat yielding, the 

 tendency of the brasses is to gape and thus contact is only 

 made round a small horizontal section of the plug. 



We therefore affixed to each pair of brasses an additional 

 contact maker of the kind shown in fig. 2. The spring of 

 the strip AB caused the wedge fastened on its lower surface 

 to spring clear of its twin wedge on the block C when the 

 screw-head E was sufficiently raised. The upright holding 

 the screw passed through a slot in the strip without making 

 contact therewith. The plug being firmly inserted in its hole, 

 the head E was tightly screwed down ; thus the wedge 

 surfaces were firmly pressed together, and that with a sliding 

 movement. The strain on the insulating surface was thus 

 relieved, excellent contact was made by the wedge surfaces, 

 also by the screw E connecting the strip with C and also by 

 the plug itself. By this arrangement consistent results were obtained and one of the 

 troubles of exact resistance measurements eliminated. 



The constants of AB and CD were found to be as follows : 



Fit 



The difference at 100 C. resulting from the separate standardisations is therefore 

 AB-CD= +0-157. 



The thermometers were then coupled up differentially and together immersed in the 

 hypsometer. The differential reading was then found to be +0'158. 



' The barometer used when observing R! was one whose scale coefficients, &c., had been determined by 

 companion with a French Standard of the Bureau International in 1896. Owing to an accident we had 

 to refill it prior to this work : this was done with all the usual precautions. 



