A system for measuring the cumulative amount of radiation received by a person includes a radiation monitoring electret (13). This electret (13) is supported on a first conductive plate and preferably is positioned within a protective cavity such that it is not discharged by non-radiation sources. Measurement of the charge decrease on the electret over time is achieved by a null method. In particular, a second plate (11) in the monitoring device is located over the electret, or the electret itself, is vibrated and an A.C. amplitude modulation detector (16) produces an output signal created by the alternating electric field due to the vibration. A D.C. voltage of opposite charge is then applied to the first conducting plate on which the electret is supported. This field is increased until it cancels the A.C. field above the electret. When this null occurs, the applied D.C. voltage is equal to the charge on the electret. Using this null technique the electret charge can be accurately measured without contacting the electret.