550 Mr. F. E. Smith on Bridge Methods for Resistance 



of t, the current being 0*01 ampere. The results are as 

 follows : — 



A=215 



5 — 



:0'03 



2 7" = 



: 0-0076. 



t. 



*l 





0,-6. 



e. 



1 second 



0°-016 5 





0-0089 



0-007 s 



2 seconds 







0-0048 



0-011 7 



3 „ 







0-0027 



0-013, 



+ „ 







0-0014 



0-015 1 



5 „ 







0-0007 



0-015 8 



6 „ 







0-0004 



0-016, 



7 » 







0-0002 



0-016, 



For this particular thermometer it is seen to be necessary 

 to keep the circuit closed for from 6 to 7 seconds at least, 

 when precise measurements are being made v\ith a current 

 of 0*01 ampere. For larger currents a longer interval is 

 necessary. 



Heating Effect in the Bridge Arms. 



In most resistance bridges, three of the arms are of some 

 kind of resistance alloy, and the fourth arm (containing the 

 resistance to be measured) is of some other material. If the 

 temperature of the bridge arms is varied there is in general 

 a disturbance of the balance owing to the temperature 

 coefficient of resistance of the material in three of the arms 

 being different to that of the material in the fourth arm. To 

 overcome this difficulty a compensated bridge was patenl"ed 

 by Prof. Callendar in 1887, but it has not come into general 

 use. In the case of a bridge it is obviously better to con- 

 sider the increments of resistance than the increments of 

 temperature due to a given current. 



With regard to the limiting value of the current it is well 

 known that the sensitiveness of any method for comparing 

 two resistances is proportional to the current employed in 

 making a measurement, and hence a lower limiting value is 

 fixed by the sensibility of the galvanometer. The upper 

 limiting condition is that none of the arms of the bridge 

 shall change appreciably in resistance owing to the increase of 

 temperature produced by the current, or that the changes 

 shall be capable of accurate estimation. 



We now consider the heating effect of a current through 

 two substances of different specific resistances place! in 

 series. When a current i is passed through a straight wire 



