Paul,

There are several short items in CREGJ on the topic of earth rods, and I can 
send you a list, but the best paper I have found is the one by David Gibson: 
"The Resistance of Ground-Electrode Arrays", CREGJ 29 pp26-27, September 1997.  
Please let me know if you do not have ready access to this publication.  Under 
fair dealing provisions I can probably send you a scan of the article, but if 
we wish to make it generally available by posting it here, there will be a 
copyright question to resolve.  I will check with the author.

73
John F5VLF