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What is a stroboscope?
It's a calibrated flashing light used to measure the rotational speed of, say, a spinning motor shaft. Like the timing light used by mechanics on a car. The strobe is variable…, and it can be "run up" until the flashing light on the spinning object causes the object to "stand still" and not appear to move. Then a peek at the readout of the strobe, and the user will know how fast the object is turning..
Note that the shaft doesn't stop turning, but the strobe flashes and it illuminates the same spot on the shaft with every rotation, and that's what makes it (the shaft) appear to stop moving. All the operator need to be concerned about is avoiding a harmonic. If the strobe is flashing at exactly half the speed of the shaft, it will be illuminating the same spot on the shaft each time it flashes, but the shaft will actually have turned twice instead of once. But a good operator can avoid that. (MORE)