The basic principles of the nuclear free precession magnetometer are briefly described, and full details given of the coils, switching circuits and transistorized amplifying equipment of such a magnetometer. Accuracy of absolute measurement of the earth's total field is ±10 μG, and even better when only relative measurements are required. Setting-up of the instrument, which requires no calibration, and taking a reading takes only a few seconds. Suggestions are put forward for possible further applications of the free precession technique.