6 ELECTRICAL MACHINERY 



Strip made of copper ........ 100 units of current 



11 " iron .......... 16 



" " mercury ....... 1.6 



' ' " carbon ........ - unit of current 



.2500 



uum ....... 40000000000 



All of these substances are called conductors. If now 

 we used other strips made of mica, glass, gutta percha, etc., 

 some current would flow but it would be so small that all 

 ordinary measuring instruments would fail to detect it; 

 these strips would be called insulators. 



Resistance Depends upon Temperature. The resistance 

 of a body varies greatly with its temperature. In all 

 metallic bodies the resistance increases with the tempera- 

 ture but in a few substances, notably glass and carbon 

 and solutions of acids and salts, the resistance decreases 

 greatly with increase in temperature. At all ordinary 

 temperatures glass is a very good insulator; when red hot 

 it becomes a good conductor. 



The variation of resistance with temperature in all pure 

 metals is found to closely follow the law given by the 

 equation, 



(1) 



where RQ = resistance of the conductor at Centigrade. 

 Rt = resistance of the conductor at t Centigrade. 



t = temperature of conductor, above Centigrade. 

 .004 = the temperature coefficient of resistance. 



Resistance Depends upon Size and Shape of Conductor. 

 For a body made of any given material it is found that the 

 resistance varies directly with its length and inversely with 

 the area of its cross-section; thus a piece of iron one foot 



