A proportional counter is a measurement device to count particles of ionizing radiation and measure their energy. A proportional counter is built the same way as a Geiger counter, but uses a lower high voltage: any incoming ionizing particle ionizes the gas inside and so allows for a current to flow between the two electrodes (a metal wire in the middle of a metal can is positive electrode and the metal can itself is grounded, i.e. it is the zero voltage electrode).