1884.] Heating Effects of Electric Currents. 



467 



The above figures are the average of numerous experiments, and 

 they are plotted out in fig. 2. 



The general law governing the ratio between the current strength 

 and the diameter of the wire when the latter is raised to a definite 

 temperature, and where radiation is free and unchecked by any ather- 

 manous envelope, appears to be that the current should vary as the 

 diameter X ^/diameter;* this may be proved as follows. 



The heat developed in any cylindrical wire is by Joule's law — 



H = C 2 IU, 



where R,= i^l 



x being the specific resistance, I the length, and d the diameter, then — - 



■>-<%"> 



or if we maintain the ratio between the current and the diameter 

 constant, the same amount of energy will be wasted in the conductor. 



But the temperature of the conductor is dependent on the rate 

 at which the heat is radiated and conducted away, and when the 

 heat imparted by the current just balances that lost by radiation 



* " The Electrical Eeview," June 24, 1882, p. 454, and Prof. George Forbes, 

 Brit. Assoc., 1882. 



VOL. XXXVI. 2 K 



