A radiation detection and measuring instrument. It consists of a gas-filled tube containing electrodes, between which there is an electrical voltage, but no current flowing. When ionizing radiation passes through and ionizes the gas within the tube a short, intense pulse of current passes from the negative electrode to the positive electrode and is measured or counted. The number of pulses per second is an indication of the rate at which ionizing events are occurring within the tube. It was named for Hans Geiger and W. Mueller, who invented it in the 1920s. It is sometimes called simply a Geiger counter or a GM counter, and is the most commonly used portable radiation instrument.