27 



function of the iron is to add strength to the rod, its conducting power being but 

 small as compared with that of the sheath. 



Aluminum. 



Aluminum is about half as good a conductor as copper, the size of wire being 

 the same in the two cases. But if aluminum wire is twice as large in cross-section 

 as the copper, then their conducting power is equal. Aluminum, so far as we can 

 see, should prove just as durable as copper, although we must remember that it is 

 a peculiar metal, and experience may bring out some practical weaknesses that as 

 yet may not have developed. 



So many questions are asked regardiiiu' the resistance of iron, copper and 

 aluminum that it is thought wise to give a few comparisons. Figure 35 shows 

 samples of six copper rods on the market in Ontario. In Figure 36 the two lono- 

 samples, Nos. 7 and 8, are of iron, and the short ones, Nos. 9 and 10, of aluminum. 

 These ten rods are compared in table below: 



CoMPAEisox OF Some Cables. 



