The problem of measuring low values of slip of induction machines is present in most Electrical Engineering laboratories. A rather high accuracy of the determination is necessary for theoretical calculations of motor loading, using equivalent circuit techniques. Conventional methods of determining slip fail to give high accuracy at low values of slip. This thesis presents a solution to this problem by describing the design and construction of a precision slip frequency detector of nominal cost. The principle employed is that of beat frequency comparison of the rotor shaft speed against line frequency. The output of the beat frequency detector actuates a multivibrator-differentiating circuit, the output of which triggers an electro-mechanical counter. A photoelectric pickup device for determining shaft speed is also discussed.