830 The Farmsfpod 



expensive, has so much more conductivity that 

 to get rid of a certain charge of electricitj^ 

 requires mucli less of it. So with ahmiinum, 

 which has slightly less conductivity and which 

 costs more than copper, but which is so light 

 that a rod having the same conducting ability 

 when made of this metal actually costs less than 

 one made of copper, and the price of aluminum 

 is constantly lessening, while that of copper can- 

 not fall much on account of the limited supply. 

 To compare actual figures, call the conductivity 

 of copper 100, then that of steel or iron will be 

 about 18, and that of aluminum about 60. As 

 to relative weights, copper weighs about 550 

 pounds per cubic foot, iron or steel 480, and 

 aluminum 160. As the prices of these ma- 

 terials are constantly varying, it would be im- 

 possible to say at this time what the relative 

 costs would be at any other time; but it can be 

 said that on the score of cost there is little 

 choice among them. For a number of reasons 

 aside from cost, copper is at present the best 

 material, and these reasons are: That it is 

 smaller than the others for a given conducting 

 ability, and thus is more sightly ; that it is 

 easier to support on account of this small size, 

 and that it can be readily soldered to the 

 ground plate, which will be considered later. 

 In addition to the material of the lightning 



