Measurements of Precision in Platinum Thermometry. 561 



have to be made — -as in thermometric work — it will be a 

 great help to have a small variable resistance in circuit 

 with one o£ the leads so as to adjust L 2 and L 3 to near 

 equality. 



Having once designed a suitable adjustable resistance, 

 such as that described for R (Method 1) and X (Method 2), 

 it is possible to develop very many bridge methods suitable 

 for the measurement of a resistance with current and 

 potential leads. In addition to the two low resistance 

 bridges which we have described we have developed two 

 other low resistance bridges, but they are not described as 

 they are not so convenient in practice. In addition the 

 method described by Dr. Searle * might be used for precise 

 work, but in such a case the ratio coils should be exactly 

 equal and the galvanometer connexions should be reversed 

 for the second balance in order to keep the current through 

 the galvanometer, due to any thermal E.M.F. in P, constant 

 in direction. Even then there is some disturbance owing to 

 the changes in the resistances of the bridge arms, but tho 

 method is of considerable value. 



The chief points in connexion with the two bridges we 

 have described are :==* 



(1) No slide wire and therefore no thermal E.M.F. at 



contact, and no variable contact resistance in galvano- 

 meter branch. 



(2) The variable resistance may be rapidly adjusted to 



obtain a balance. 



(3) As the galvanometer circuit is always closed, no 



thermal key is required. 



(4) The resistance of the thermometer leads is eliminated 



as a source of error. 



(5) Secular changes in the resistance of the thermometer 



■' bulb" or " spiral" may be detected with certainty. 



(6) With Method 2 the current through the thermometer 



is the same at all temperatures without adjustment of 

 any external resistance. 



Third Method. 



Of the methods so far described we believe that Method 2 

 is the most convenient, although very slightly less sensitive 

 than Method 1. One feature of the bridges to which ob- 

 jection may be taken is the use of mercury contacts for 

 many of the coils. This objection may be disregarded when 



* Searle, 'The Electrician,' March 31 to April 21, 1911, Section 26, 



