Sunday, October 7, 2007

Port Tobacco Lightning Rod

While excavating shovel test #216, Scott recovered a glass insulator. This is a part of the site in which late 19th-century maps revealed no buildings. The insulator, along with architectural and domestic debris from surrounding units clearly indicate that a building site is present in this location. The thing that made the insulator particularly interesting is the embossed text on its side: "PATENTED" and "AUGUST 26, 1851."

A check of US Patent Office records revealed that George W. Otis of Lynn, Massachusetts, had patented (Letters Patent No. 8,316) an improvement in insulators for lightning rods. The drawings Otis submitted describe how a glass and wood fixture might be attached to the wall of a building to support a lightning rod, while keeping it a safe distance from the structure.

In truth, the object Scott recovered from the unit bears little resemblance to Otis' drawings; but he patented a specific idea about anchoring lightning rods, the manufacturers developed their own specifics about how to construct and assemble the device. As manufactured, the insulator is a truncated cone, about 2 inches high, 1.3 inches in diameter at the top, 1.8 inches at the base. It is pressed glass (likely manufactured by the Sandwich Glass Company of Massachusetts, an early leader in pressed glass manufacturing), colorless with numerous bubbles in the metal. There is a deep, broad V in the top of the piece and a small hemispherical depression in the base.

We do not know when or why the item was acquired by the building's occupants. The patent date provides only a date after which the insulator was manufactured, not when it was made and bought. Were there catastrophic fires ignited by lightning strikes elsewhere in the community, or was this merely a prudent installation replicated on scores of buildings in the area? Further testing in town might provide answers. No doubt the Port Tobacco team will have frequent recourse to the US Patent Office records as we unearth additional objects bearing patent dates or numbers.


P.S. For more on the history of insulators, click here.

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