You searched for: “electroradiometer
1. An electroscope (device for detecting the presence of an electrostatic charge) of any of several designs suitable for the measurement of ionizing radiation arriving through the air.

Typically a microscopic, electrically conducting fiber (for example, silvered quartz) is suspended in a chamber, close to and connected with an insulated wire support where an electrostatic charge is applied, and similar charges deflect the fiber away from the support, the movement being monitored with a microscope and scale.

As the radiation of interest ionizes the enclosed gases, a slow discharge occurs, and the fiber retreats toward its resting position, the rate of movement indicating the intensity of the radiation.

2. A modified electroscope designed for the differentiation of radiant energy or energy contained in light rays or any other form of radiation; such as, X-rays or gamma rays.

An electroscope is an instrument for detecting the presence of electricity, or changes in the electric state of bodies, or the species of electricity present, by using pith balls, etc.