218 ELECTRICAL MEASUREMENTS 



the points a and b may be made zero; when this adjustment is 

 complete the deflection of the galvanometer G is zero and 'the 

 current which then flows through x, is given by the galvanom- 

 eter Gz. It follows that 



_E _ Er 2 

 X *~h ~ Ei' 



r Mance's method, originally 

 designed for measuring the in- 

 ternal resistance of batteries, 

 may be so modified that it can 



FIG. 124.-Connections for measuring also be used for measuring in- 

 resistance to ground. sulation resistances while under 



the working voltage- 



THE TEMPERATURE COEFFICIENT OF ELECTRICAL RESISTANCE 



The effect of change of temperature on the electrical resistance 

 of various materials is shown in Fig. 125. It is seen that, with 

 few exceptions, the resistance increases with increase of tem- 

 perature. For pure metals an approximate figure is 0.4 per cent, 

 per degree C. It may be noted that metals which, at certain 

 temperatures, undergo changes of structure, for instance, iron 

 at about 780C., show alterations of curvature in the temperature- 

 resistance curve at those points. 



From this it is obvious that if a statement of an electrical 

 resistance is to possess definiteness, the temperature at which the 

 measurement was made must be given. Also, when a resistance 

 is measured, it is necessary to know the temperature and tem- 

 perature coefficient of the coils with which it is compared, as 

 well as their value at some particular temperature, in order that 

 their true resistance at the time of use may be known. 



Experiments show that, in general, when the temperature is 

 altered, the change of the electrical resistance of a conductor to 

 which terminals are rigidly attached, and which therefore pos- 

 sesses a constant mass, is not quite linear; that is, the plot con- 

 necting resistance and temperature is not exactly a straight line. 

 For the ordinary ranges of atmospheric temperatures the curva- 

 ture is very slight, but may be considerable if the temperature 

 be carried to high values. 



