1112 Prof. N. Bohr on the Selection 



Note. — Mr. H. F. Biggs has pointed out an extremely 

 simple way of viewing the action of the condenser, which 

 immediately leads to the conclusions already obtained. Since 

 there can be no conduction-current in a perfect insulator, 

 the total charge that passes into the network at the junction 

 X during a complete period of the tuning-fork must be zero. 

 The condenser-plates attached to the vibrating reed may 

 be then regarded as conveying a charge C(V a — Vj) from A 

 to B n times per second. 



On this ^view, it immediately follows that the permanent 

 connexion may be made with any part of the network with- 

 out affecting the galvanometer " balance " when once made. 



The Electrical Laboratory, Oxford. 

 June, 1921. 



CX VII. On the Selection Principle of the Quantum Theory. 

 To the Editors of the Philosophical Magazine. 



Gentlemen,— 



IN a note titled U A Significant Exception to the Principle 

 of Selection/' and published in the April number of this 

 magazine, P. 1). Foote, F. L. Mohler, and W. F. Meggers 

 describe some interesting experiments on the excitation of 

 the lines of the arc spectrum of potassium, and conclude 

 that the results of these experiments throw doubt on the 

 general principles, on which the laws of the series spectra 

 are interpreted on the basis of the quantum theory. As I 

 shall try briefly to explain, the results in question do not 

 seem, however, to offer sufficient basis for such a conclusion. 

 According to the ideas of the quantum theory each of the 

 Hues of a spectrum like the arc spectrum of potassium is 

 emitted by the atom during a process of transition between 

 two among a multitude of stationary states, in which one 

 electron moves in an orbit, the dimensions of which are 

 large compared with the orbits of the other electrons in the 

 atom, which together with the nucleus may be said to form 

 the inner system. In first approximation the orbit of the 

 outer electron will be a plane central orbit, which may be 

 described as a plane periodic orbit, on which is superposed 

 a uniform rotation in its plane. In the stationary states of 

 the outer electron the motion is therefore in first approxi- 

 mation fixed in the well-known way by means of two quantum 

 numbers, which may be denoted by n l and n 2 . While n x 



