39 



In this comparison no attention has been paid to "surface," although much 

 stress has from time to time been laid upon this by makers of various types of rods. 

 Sir Oliver Lodge's experiments, which show that a thin broad strip has less self- 

 induction than a wire of the same material and weight have been taken as an indica- 

 tion that we should have great surface of conductor, and durability and con- 

 venience of installation have sometimes been sacrificed to surface. We should not 

 forget that when it came to the practical side of the question Lodge recommended 

 the more rugged types of conductor, such as solid rod or strong cable, in explanation 

 whereof he said : " It is because I doubt whether decently substantial conductors 

 are in any real danger from heat that I have asserted the advantage of greater sur- 

 face to be but small." Consequently shape of rod is immaterial, so far as efficiency 

 is concerned, and a tape or other form designed for great surface has by reason 

 thereof no practical pre-eminence over other shapes' of rod. 



Side Flashes. 



Sometimes part of a lightning-charge will side-flash from the rod, jumping 

 several feet of air gap in doing so. Why? The total obstruction to the current 



7 8 9 10 



Fig. 36. — Two iron rods (long) and two aluminum rods (short). 



is made up of two parts, the resistance of the wire, and the self-induction. The 

 more suddenly the current comes on and the higher the frequency of alternation 

 the greater the self-induction, and this is the larger factor when a stroke occurs. 

 The self-induction of a short air circuit is sometimes less than that of a long 

 metallic circuit, hence the side-flash in such cases. 



Groundings. 



2. Ground Connections. — For an ordinary building, not an L or T, at least two 

 groundings should be made, preferably at corners diagonally opposite, though tlii- 

 may be modified to meet conditions, e.g., to run near conductor pipe from eave- 



