[en] A timing device for use with the Farmer Dosemeter 2502/3 has been constructed and tested. The device offers significant advantages over commerically available timers, by replacing manual termination of the timer by an electronic sensing system. The null position of the Farmer analogue meter is used to activate a high impedance Schmitt Trigger which through suitable circuitry terminates a digital stopwatch. The timer is started by the available contacts incorporated in the Farmer zero/measure switch. The timer has proved an invaluable aid in the calibration of high output x-ray equipment