A long time ago I serviced videotape machines among other things, both open reel and cassette systems. There is a specification for the amount of tension the tape is supposed to have as it's pulled along the transport mechanism by the capstan and take-up motors. This tension is measured with a device called a tentelometer. If the tape tension was too high or too low, the video wouldn't play back properly. If the tape was recorded with incorrect tension, it wouldn't play back properly on anyone else's machine, a condition called "out of interchange." To use it, the tentelometer is placed along the tape path while it's running, and the tension pushes on the middle pin of the gauge, producing the reading.
Haven't done this in years but I still have the gauge. (This is an example picture, not mine).
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