An apparatus designed by Ernest Rutherford and his assistant Thomas Royds (1884–1955) in 1909 to confirm the nature of alpha particles. The specific charge of alpha particles was determined soon after their discovery. It was found to be the same as that of a doubly ionized helium atom (helium nucleus). Rutherford and Royds’ apparatus (see diagram) was designed to allow alpha particles, which were products of a radon-gas decay, to pass through a thin glass partition into a partially evacuated chamber X. Alpha particles trapped in X were then able to acquire electrons either by ionization of traces of air or by collisions with the glass. The apparatus was left for about a week before the gas trapped in X was analysed. An electrical discharge through the gas produced the characteristic spectrum of helium. Alpha particles were therefore confirmed to be helium nuclei.

Rutherford and Royds’ apparatus.