but for all values of
. In addition, for a stationary orbit
is equal to the mean value of the total kinetic energy
of the particles; from (10) we therefore get
In using the expressions (6) we have assumed that the motion of the particles in the stationary states of the system can be determined by help of the ordinary mechanics. On this assumption it can be shown generally that the conditions (8) and (11) are equivalent. Consider a particle moving in a closed orbit in a stationary field. Let
