The design and construction of a vibrating capacitor‐type electrometer is described. Experimental data are presented to show that this electrometer reaches, within a small factor, the theoretical limit of sensitivity. The principle of the electrometer is a time‐varying capacitor at the input of the device, which inverts the low frequency signal to a relatively high frequency, and raises its energy level, thereby simplifying the amplification of the voltage being measured, permitting the use of ordinary vacuum tubes, and producing an electrometer that is mechanically rugged and free from long period drift. Stabilization is obtained by negative feed‐back. The electrometer was originally developed for radiation well‐surveying in the petroleum industry, but has found other application in general radiation measurements.