The
old quantum theory is a collection of results from the years 1900–1925 which predate modern
quantum mechanics. The theory was never complete or self-consistent, but was a set of
heuristic prescriptions which are now understood to be the first quantum corrections to
classical mechanics. The
Bohr model was the focus of study, and
Arnold Sommerfeld made a crucial contribution by quantizing the z-component of the
angular momentum, which in the old quantum era was called
space quantization (Richtungsquantelung). This allowed the orbits of the electron to be ellipses instead of circles, and introduced the concept of
quantum degeneracy. The theory would have correctly explained the
Zeeman effect, except for the issue of electron
spin.