The torque produced by a short circuit or other transient will produce a corresponding acceleration of the rotor. If the rotor is not connected to a load the acceleration of the rotor will be directly proportional to the torque. A small instrument is described which can be attached to the end of the shaft of the machine to be tested. This instrument records the instantaneous acceleration of the rotor, the corresponding torque being calculated. The acceleration is measured by two separate methods. The first method gives points on an acceleration — time curve and the second gives a continuous record of the torque. The acceleration is recorded on the oscillogram so that by using a six element oscillograph a simultaneous record can be obtained showing both the acceleration and the short-circuit currents. The mechanism for closing the short circuit at the desired point of the voltage wave and the method of checking the accuracy of the instrument are also described. A record from an actual short-circuit test is shown. The results of the tests will be discussed in a later paper. The instrument can also be used for measuring sudden shocks on motors and other rotating machinery.