Reading Resistance- Thermometers. 719 



Hence 00-2) 



m 



Therefore . . )2 



Hence from (7) rB »_ am _ /8= o, 

 Therefore , / ■ ,- 



-H iC^v 1 -*-?) (9) 



A1S ° «w , 



»=^g +2 (10) 



anc * aw -4-/3 



& = + .& (11) 



urn + 2/3 



Since a and £ are both positive (for metals like platinum), 

 real values for m, n, and a; can thus always be found by 

 equations (9), (10), and (11). As no terms in t d and higher 

 powers have been neglected, this arrangement gives an exact 

 equivalent of the parabolic formula. In other words, when 

 m. n, and k have the values found from a and j3 by these 

 equations, the alteration in x necessary to produce a balance 

 lor a given change of temperature will be proportional to 

 that change of temperature. 



From equation (9) it will be seen that two values of m 

 satisfy the required conditions. The value got by taking the 

 I ositive sign in (9) is the convenient one to use, "as the other 

 value gives very small change of reading for a given change 

 of temperature. 



In order to make the increase of x by O'l ohm correspond 



Fig. 5. 

 D 



rise of temperature of 1° C, as in Method (A) the 

 resistances (fig. 5) in the loop are raised in suitable pro- 



3 B t 



