To look at while you search for printed sources, here is a link to a
collector's site that has many pictures -- I won't vouch for the accuracy
of the text but the illustrations are good.

http://www.insulators.com/


On a related subject, one of the proposed themes for this year's CNEHA
meeting is 20th Century Archaeology.  It would be interesting to have
papers about dating all of these early 20th century electrical etc.
artifacts.  The meetings are in Wilmington, Del., which is very easy to get
to from most places.

Meta Janowitz