What is this thing [telecom]

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So I recently got this with some telecom stuff and a Western Union loop supply on ebay. I know its a gauge or dial of some sort; there was a similar one on ebay showing a pointer attached to the coil floating in the center, but what is this dial used for? Figured this group [might] know.

Fyi, there were some 1920s radio parts included [in the purchase] as well, so possibly a radio part.