A method is described whereby high‐resolution measurements of the derivative of the electron current transmitted through a gas may be made without the use of deflection or magnetic fields. The method consists of modulating a retarding element and the interaction region with signals which are 90° out of phase. It is shown that when the transmitted current is measured by a phase‐sensitive detector which is referenced to twice the modulation frequency, the output signal is proportional to the derivative of the transmitted current and is characteristic of a monoenergetic beam. Results of performance tests on helium are presented and discussed. The tests show that the resolution is essentially equal to the average of the two modulation amplitudes. In the case of the 22Sresonance in He−, a resolution of 29 meV with a signal‐to‐noise ratio of better than 400 has been obtained. Resolutions as good as 10 meV have been obtained with poorer signal‐to‐noise ratios.