The A B C of atoms

by Bertrand Russell

, say

. The second quantum condition determines how much the ellipse departs from a circle; it states that there is a second whole number

(the second quantum number), such that

is the amount of flattening in the sense defined a moment ago. The second number

′ may be zero; we then obtain Bohr’s case of circular orbits. If it is not zero, the electron moves in a more or less eccentric orbit.