The Victoreen 500 high-precision, MOS-FET, dual polarity electrometer permits the optimum use of many commercially-available ion chambers with coaxial or triaxial terminations. It accepts triaxial BNC and coaxial UHF connectors. PET, MICRO-DOT and TNC adapters are available. The instrument's innovative design permits these chambers to be connected with the chamber bias potential placgd automatically and correctly . . . without potential dangerous shock hazards. In addition, the instrument assures compliance with modern dosimetry protocols requiring precision, polarity reversal and saturation studies. Electron beam dosimetry and other special studies are facilitated through the use of the reversed chamber bias switch, 40% normal bias switch and alternate bias connector. The reverse bias switch allows immediate reversal of the chamber bias which is displayed on the front panel. The 40% normal bias switch facilitates saturation studies and is spring loaded to prevent accidental operation at the lower potential.