Geiger counters are devices to detect and measure ionizing radiation, as emitted by radioactive sources. The heart of a geiger counter is the Geiger-Mueller-Tube. This is a gas filled tube, to which a voltage of several 100V is applied. Normally, the gas insulates and no current is drawn. When a radiation particle passes the tube, it triggers a gas discharge, i.e. gas becomes conducting. The resulting current impulse can be amplified and made visible or hearable ("clicking").
There are two main kinds of tubes:
A typical Geiger counter circuit looks like this:
Any good Geiger counter should be calibrated against a known radiation source . The calibration insures that your measurements are correct and your Geiger counter is working properly.