A remote position measuring device with electrical coupling to a computer has been developed for use in the elementary-physics laboratory. Light from a moving object and a fixed-reference lamp is focused by a lens on a hollow, rotating drum with equally-spaced slits. The light passing through a slit is detected with a phototube. The time difference of amplified phototube signals from the object and reference-light sources is a measure of the position coordinate of the object at the time of the object signal. The model constructed has a precision of about 0.03 cm, better than a 3% differential linearity for a 40-cm field of view in the object plane and a sampling rate of30  sec−1. The device can track more than one object simultaneously and has been successful in providing position samples for two colliding gliders on an air-track.

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