Eighty-five years ago this month, the May 1940 issue of Radio Service Dealer provided this handy stroboscope for testing the speed of record players.

The included instructions explained how to place it on a piece of cardboard, or an old record. If a record was used, it was critical to make sure the hole was exactly in the center. Then, it was placed on the platter, and illuminated with a neon bulb powered by 60 Hz. At exactly 33-1/3 RPM, the outer segments would remain stationary, and at 78.26 RPM, the inner ones would do the same. If the speed were too fast or too slow, the segments would appear to move forward or backward.

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