Together with H.G. Moseley, Rutherford developed the atomic numbering system in 1913. Rutherford and Moseley's experiments used cathode rays to bombard various elements with streams of electrons and observed that each element responded in a consistent and distinct manner. Their research was the first to assert that each element could be defined by the properties of its inner structures – an observation that later led to the discovery of the atomic nucleus. This research led Rutherford to theorize that the hydrogen atom (at the time the least massive entity known to bear a positive charge) was a sort of "positive electron" – a component of every atomic element.
It was not until 1919 that Rutherford expanded upon his theory of the "positive electron" with a series of experiments beginning shortly before the end of his time at Manchester. He found that nitrogen, and other light elements, ejected a proton, which he called a "hydrogen atom", when hit with α (alpha) particles. In particular, he showed that particles ejected by alpha particles colliding with hydrogen have unit charge and 1/4 the momentum of alpha particles.
Rutherford returned to the Cavendish Laboratory in 1919, succeeding J. J. Thomson as the Cavendish professor and the laboratorys director, posts that he held until his death in 1937. During his tenure, Nobel prizes were awarded to James Chadwick for discovering the neutron (in 1932), John Cockcroft and Ernest Walton for an experiment that was to be known as splitting the atom' using a particle accelerator, and Edward Appleton for demonstrating the existence of the ionosphere.