RESISTANCE AND ELECTROMOTIVE FORCE. 



17. Variation of resistance with temperature. The electrical 

 resistance of a wire, or of a liquid column which forms a portion 

 of an electrical circuit, varies with temperature. Consider, for 

 example, (a) an iron wire, (b] a copper wire, (c) a platinum wire, 

 (d) a german-silver wire, (i) a carbon rod, and (/) a column of 

 dilute sulphuric acid, each of which has a resistance of 100 ohms 



ohms 



200 

 1 80 



j6o 

 140 



t20 



80 



60 



o 20 40 60 .80 ioo 120 140 160 180 

 degrees centigrade 



Fig. 14. 



at o C. The values of the resistance of (a), (b\ (c\ (d\ (e) and 

 (/) at other temperatures are shown by the ordinates of the 

 curves in Fig. 14. It is evident from Fig. 14 that iron and cop- 

 per increase very greatly in resistance with rise of temperature, 

 and that german silver increases slightly, whereas the carbon and 

 sulphuric acid decrease in resistance with rise of temperature. 

 All pure metals increase in resistance with rise of temperature in 

 approximately the same ratio, alloys usually increase in resistance 

 with rise of temperature but to a much smaller extent than pure 



