542 Prof. H. L. Callendar on Electrical 



The actual value o£ the current through the galvanometer 

 in case (A) is greater than in case (B) in the proportion of 

 3 to 2. Maxwell's rule is so far justified. But the current 

 through the resistance to be measured is 50 times greater in 

 (A) than in (B), and the heating effect of the current is 

 2500 times greater. With an ordinary platinum thermo- 

 meter arrangement (A) might give an elevation of tem- 

 perature of about 5° C.j which would be fatal for any purpose 

 of measurement. Whereas arrangement (B), w T ith nearly 

 the same sensitiveness, would give an elevation of "002° C. 

 only, which is sufficiently small for the most accurate work. 



The essential point is to observe that the ratio c/C of the 

 galvanometer current to that through the thermometer, 

 which is the true measure of sensitiveness in this case, is 

 34 times greater in (B) than in (A). (B) is therefore by 

 far the better arrangement in the case where the heating 

 effect of the current on the resistance to be measured is the 

 primary consideration. 



The rule to replace Maxwell's in this case, if there is any 

 choice as to the arrangement of the battery and galvanometer 

 connexions, is : " Connect the battery so as to make the 

 resistance in series with the thermometer greater than the 

 resistance in parallel/' This will make n greater than m in 

 fig. 1, and it is evident that the ratio c/C will be diminished 

 or the sensitiveness reduced, if n and m are interchanged. 



In the practical use of platinum thermometers, we have 

 the further restriction that n must be equal to unity, because 

 it is necessary to compensate the changes of resistance of 

 the leads by the equal changes of resistance of a pair of 

 compensating leads on the opposite side of the galvanometer 

 contact on the bridge- wire. The loss of sensitiveness, as 

 compared with the case of n very large, does not amount to 

 more than 20 or 30 per cent., and is of no consequence as 

 compared with the trouble and uncertainty involved in 

 measuring the resistance of the leads separately at each 

 observation. Moreover, if n is made large, the heating effect 

 of the current C on the resistance nB/ may become serious. 

 As a matter of fact this is a common and very insidious 

 source of error in the use of a P.O. box with ?* = 100, 

 especially if the coils are of german silver or platinoid. 

 With maganin resistances the effect is much less marked. 



It is still possible to gain some advantage in point of 

 sensitiveness by making m smaller than unity, provided that 

 it is not made so small that the heating effect of the current 

 C/m on the resistance mW becomes appreciable. For the 

 most accurate work I have generally employed thermometers 



