28 



Eod No. 5, the woven ribbon, is just on the 3-ounce limit for copper, and we 

 may take its resistance and heat capacity as the standard requirements in these 

 two respects. iSTo. 3 is too light. Nos. 2 and -1 are almost identical with No. 1 

 in resistance and heat capacity because almost identical in weight. No. 6 has 

 about 11/2 times the standard amount of copper and is unquestionably the best of the 

 six copper rods. Its resistance is lowest, meaning it is the best conductor. Its 

 heat capacity is highest, its durability greatest. However, judging by data at 

 hand it is probably a needless expense to use such a heavy cable. 



The two steel (iron) rods are both much heavier than the standard copper, 

 but their resistances are 3 1-3 and 4 1-3 times as great, showing that they are 

 only about 1-3 and 1-4 as good conductors respectively of steady current as the 

 standard. In heat capacity they, of course, head the list. 



The aluminum rods have the lowest resistance, that is, they are the best con- 

 ductors of steady current. The large aluminum has about twice the heat capacity 

 of the standard copper, so that this cable, like No. 6, seems needlessly large. No. 

 10, however, is just a shade small, being rather low in heat capacity. So far as we 



12 3 4 5 6 



Fig. 35. — ^Six different makes of copper rod. For properties see Table 



are aware, no specifications have heretofore been laid down as to the minimum 

 weight for aluminum rods. To have the same heat capacity as the standard copper 

 would require 2 1-7 ounces per foot, but in view of the fact that the meltiag point 

 of aluminum is only 1,157 degrees F., while that of copper is 2,012 degrees F., it 

 would seem prudent to require a little greater heat capacity than in the copper. 

 Consequently I should be inclined to specify at least 2 1-4 ounces per foot for 

 aluminum rods. Extensive use will be necessary to determine practically whether 

 this will be ample, though so far as we can see it should be. 



Iron has a melting-point of 2840*'F, the highest of the three. That's one reason 

 why the iron has such high heat capacity. The other reason is the weight of the 

 rod. The aluminum has the lowest melting point of all. Whether this will be 

 any practical detriment to rods of this metal also remains to be established by 

 extended use. 



