An instrument is described which gives a rapid indication of the peak factor (peak/r.m.s.) An electrostatic voltmeter is connected to give readings proportional to the r.m.s. voltage. When the reading is steady, operation of a press-key introduces a rectifier into the circuit for peak voltage, and in the case of alternating current also introduces by a tapping 1/√2 times the applied voltage, so that the voltage applied to the voltmeter is the same as for the r.m.s. position if the wave form is sinusoidal. In this case no pointer movement occurs and the scale may be calibrated as for 1.41 peak factor. Other peak factors may be added directly from the voltage scale.The application to rectified and smoothed direct current is also proposed, “pure” direct current having a peak factor of unity. It is further proposed that “impure” direct current may be specified by its peak factor rather than by the “ripple,” as no standard is laid down for the term “ripple.”Conditions of use are considered and possible errors examined. The instrument is inherently accurate, as the voltmeter scale is used for both peak and r.m.s. readings and scale errors thus tend to cancel out.Tests have borne out the theoretical considerations.