542 Mr. F. Jti. Smith on Bridge Methods for Resistance 



plug-contact resistance is very great, and because o£ this, 

 plugs have, in many cases, been abandoned in favour of: 

 mercury contacts. This is so in many of the Callendar- 

 Griffiths bridges made by the Cambridge Scientific Instru- 

 ment Co., and mercury contacts could be used in the bridge 

 employed by Professor Callendar in 1902. It is satisfactory 

 to know that the mercury contacts fulfil the requirement 

 with respect to constancy. 



Of the other improvements suggested by Dr. Chree, three 

 related to the resistance bridge and one to the current used 

 in the measurements, These are as follows :- — 



1. It is desirable that some simple and more perfect 



method should be devised for eliminating the effects 

 of changes in the relative resistance of the leads and 

 the proportional arms. 



2. The bridge-wire scale is too much contracted for very 



exact work with thermometers of such low resistance 

 as the ordinary Kew ones; it is desirable, if accuracy 

 to a 001 C. is aimed at, that 1 cm. of bridge wire 

 should not answer to more than 0°'l C. 



3. If possible the causes of wanderings in the bridge 



centre and of thermoelectric currents — not eliminated 

 by the Griffiths key — should be removed. 



4. Unless the heating effect of the current can be exactly 



ascertained and allowed for, the sensitiveness of the 

 galvanometer should be largely increased, so as to 

 render unnecessary, currents whose heating effects 

 will sensibly influence the readings. 



We are not aware of any such improvements in the 

 resistance-measuring apparatus having been made, nor do 

 we know of any experiments on the effects of changes in the 

 compensating leads, other than those described in Dr. Chree's 

 paper. 



It was the changes in the relative resistance of the leads 

 that was the principal cause of the apparent changes in the 

 zeros of the platinum thermometers during Dr. Chree's 

 investigation. In one instance, the change in the leads in 

 less than one month was equivalent to o, 05 C, and at the 

 end of that time there was a sudden alteration equivalent to 

 0°'12 C. As these changes have beon fully discussed by 

 Dr. Chree it is unnecessary to go into details here. How- 

 ever, we thought it possible that the leads now in use might 

 be much superior to those employed at Kew 12 years ago, 

 and so we made further experiments which are now 

 described. 



