A physicist built a device that clicks each time radiation passes through it

Hans Geiger, working under Ernest Rutherford, built the first practical radiation detector in 1908 to help count alpha particles fired at gold foil, an experiment that ended up revealing the atom's nuclear structure. In 1928, Geiger and his student Walther Muller refined the device into the sealed tube still used today, which produces an audible click each time a single ionizing particle passes through it, one of the most recognizable instruments in physics.

— Hans Geiger and Walther Muller, Das Elektronenzahlrohr — Physikalische Zeitschrift, 1928