An integrator is described which accepts positive voltages in the range from 0.7 to 7 V and provides a photographic record of the integrated output at intervals of 0.01 sec, with an accuracy of 0.1%. The integrator requires an input current of the order of 0.5 mA at the maximum voltage. The design can easily be modified to accept higher voltages, subject to the same current requirement and the same 10 to 1 range of inputs. The application of the integrator to the measurement of a rising magnetic field is described.