168 



Introduction to the Study of Science 



follow in order. It must be assumed that all materials are of 

 the same length, the same thickness or area of cross section, 

 and at the same temperature, as 32 F. 



On the same scale, such materials as glass, rubber, and 

 paraffin, which are commonly classed as insulators, would have 

 several million times as much resistance as silver. 



Resistance and temperature. Resistance changes with tem- 

 perature. Glass, for example, is an excellent insulator at 

 ordinary temperatures, but at 200 C. or 392 F. it becomes a 

 fairly good conductor. The carbon filament in the incan- 

 descent lamp has a high resistance at room temperature ; but 

 at operating temperature it loses fifty per cent of its resistance. 

 Certain other substances have the same property. The glower 

 of the Nernst lamp is made of certain rare earths of volcanic 

 origin, and at usual temperature has so great resistance that it 

 is a non-conductor. But when heated to a given high tem- 

 perature, as it is in this kind of lamp by a special device, it 

 allows the electric current to pass and is heated sufficiently to 

 radiate light. 



Certain other substances, such as tungsten, copper, aluminum, 

 and silver, behave in the opposite way. The tungsten filament 

 of a lamp of candle power equivalent to that of a carbon lamp 

 has at room temperature only one twelfth the resistance that 

 the carbon filament has ; but at the temperature of operation 

 its resistance increases more than twelve times its original 

 amount, so that it then has more than twice the resistance of 



