XV. I] 
THE ZEEMAN EFFECT 
207 
of which pass through an axis. Suppose the system subjected 
to the action of a uniform magnetic field H parallel to that axis, 
then the motion of the system is such that it may be represented 
as a possible motion for which H = 0 combined with a preces- 
sional rotation as a whole around the axis. When the other 
forces are central, this applies to any direction of H as axis. 
It may be noted that Larmor’s theorem does not justify the 
assumption that the orbits before and after the imposition of the 
magnetic field are identical. It can, however, be shown from 
classical electro-dynamics that, to the first order in terms involv 
ing the field, the two sets of orbits are the same.* 
In the case of the simplest atomic model consisting of a 
Fig. 28.—Model illustrating the Zeeman Resolution. 
single electron and a massive nucleus, the angular velocity of 
precession is given by 
1 e H 
o = — 15 
2 m 0 c 
The classical value for the change in frequency of a vibrating 
electron, due to a magnetic field, follows at once from Larmor’s 
theorem. The effect of the magnetic field is to superimpose an 
angular velocity of amount o on the original motion of the 
electron. This means a change in frequency numerically equal 
to —, so we mav write