3io 



ELEMENTARY LESSONS ON [CHAP. vi. 



TABLE OF SPECIFIC RESISTANCE. 



It is found that those substances that possess a high 

 conducting power for electricity are also the best con- 

 ductors of heat. Liquids are worse conductors than the 

 metals, and gases are perfect non-conductors, except 

 when so rarefied as to admit of discharge by convection 

 through them (Art. 283). 



349. Effects of Heat on Resistance. Changes 

 of temperature affect temporarily the conducting power 

 of metals. Forbes found the resistance of iron to 

 increase considerably as the temperature is raised. The 

 resistances of copper and lead also increase, while that 



