Shaft A of the tachometer is driven by the shaft whose speed is to be measured. Mounted on shaft A is an impeller with radial vanes 1. When shaft A rotates, the impeller sets up a whirling air stream which, owing to friction, tends to rotate light metal cylinder 2. Secured to the cylinder is hand 3 which stops in a position indicating the angular velocity of rotation of shaft A. Spring 4 prevents rotation of cylinder 2 and returns hand 3 to the zero position when shaft A stops.
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