Geiger-Muller tube. Illustration of a Geiger-Muller tube (upper left, blue) measuring radiation from a source (far left), with a scaler and loudspeaker (right). The Geiger-Muller (G-M) tube is an instrument for detecting and measuring ionizing radiation. The tube contains a gas that ionizes and produces an electrical pulse in a wire when ionizing radiation passes through it. The scaler counts the pulses and the loudspeaker produces a sound whenever a pulse occurs.