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From the Toolbox, episode five

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Before there was a Hemmings Motor News at 222 Main Street in Bennington, Vermont, there was a Sibley Machine Shop. Today, the Sibley Shop houses Hemmings own automotive museum, which also contains a host of curiosities related to motoring and mechanics. On a dusty shelf in the back corner, I stumbled across this strange tool from the 1930s or early 1940s.

Measuring about 11 inches tall, with roughly a seven-inch base, the conical device is painted in a textured blue finish, though the end with the meter is bare metal. At the bottom is an arrow and a sign that reads, “Engine must be stopped and battery stabilized before testing,” while the gauge at the top reads from 0 to 1000 in increments of 50. Oddly enough, there’s no indication of what the device is actually measuring.

Inside the cone, roughly two inches from the bottom, is a translucent pane that appears to be made of plastic or Plexiglass. Now hardened by time (and aren’t we all), a rubber seal surrounds the bottom edge. The device has no external adjustments and doesn’t appear is if it can be calibrated. It also does not require external power to operate.

Without consulting Google, can you tell us what this is and what it measures?