The CD 782 - What Is It?

Reprinted from "Crown Jewels of the Wire", January 1996, page 30

Since the discovery of the CD 782 several years ago, collectors have been puzzled about its unique design and practical application as an insulator. To further complicate the puzzle, when the insulator was viewed normal (skirt down) the embossing M.T.Co. was upside down! (See photo below.)

To date, only two specimens have been unearthed, both with considerable damage to the flat disc of glass above the skirt. When I first examined one, I was baffled. Because of the embossing and standard threadless pinhole, I was confident that it was a piece of telegraphic equipment, but at the same time I was reasonably certain that it was not a line insulator for two reasons: 

First, it was excavated at the site of an old railway station; and second, I had never heard of any specimens of shards found along a telegraph line. 

When I began to ask collectors for their opinions, one possibility that arose was that it was a bureau knob style, but the flatness of the top along with the tiny rim along the edge of the disc suggested a different use to me. At that time, I thought that it had some specific use in a telegraph office, perhaps to insulate the line upon entering the building somehow.

Well, I believe the mystery was solved this summer at the National in Marlborough. I talked to Paul Plunkett, a knowledgeable and curious collector, about this strange piece of glass. To my amazement, the very next day, Paul came up to me with a smile on his face and a photocopy in his hand.

He had been visiting with Dario DiMare, an avid researcher of early telegraph history and collector of early catalogs. In a telegraphic supply catalog of the 1870's vintage was advertised a very similar item as a battery insulator. Well, there was no doubt now as to the CD 782' s intended design and use. Probably there was a rack of threadless pins on a frame in the telegraph office upon which a series of these units could be set up to accommodate battery jars, the ridge around the rim there to keep them from sliding off. Thanks, Paul and Darion -- the puzzle is solved.

A Robertson's Patent Battery Insulator