98 BELL SYSTEM T EC EI NIC A L lOURNAL 



fications of the Stationary Slates are founded, and the notations by 

 which they are named. 



The atom-model to which this article is devoted, the atom-m(xlel of 

 Rutherford and Bohr, is designed to interpret these facts of the 

 Stationary States, l)ut not these alone. It is designed also to in- 

 terpret certain experiment's — chiefly, though not altogether, experi- 

 ments on the deflections suffered by minute flying charged particles 

 when they pass through matter — which indicate that an atom consists 

 of a positively-charged nucleus with a congeries of electrons around it. 

 Specifically, the results of these experiments agree with the notion 

 that the iVth element of the Periodic Table consists of a nucleus with 

 positive charge Ne and N electrons surrounding it; and this is the 

 simplest and most satisfying notion with which they do agree. Yet 

 there is something paradoxical about this atom-model; for electrons 

 could neither stand still nor yet revolve permanently in orbits around 

 a nucleus, if they conformed to the laws of electrostatics. Also 

 there must needs be something paradoxical about any attempt to 

 interpret the Stationary States by this model, for there is nothing 

 inherent in it to make any energy-value preferable to any other. 

 Under these circumstances Bohr's procedure was, resolutely to accept 

 both paradoxes at once, and to say that the electrons can re\oK"e 

 permanently in those and just those particular orbits, whereby the 

 energy of the atom assumes the particular values which are those of 

 the observed Stationary States. This is easy to say; but it is not 

 important, unless one succeeds in showing that those and only those 

 particular orbits are set apart from all others by some peculiar feature, 

 are distinguished by conforming to some particular principle, which can 

 be exalted into a "Law of Nature" to complement or supersede the 

 laws of electrostatics. Otherwise the atom-model would be of no value. 



Thus in order to make the test of the atom-model, it is necessary 

 to trace these orbits. One is confronted with this problem of orbit- 

 tracing: Given: the observed energy-values of the Stationary States; 

 required: to trace the orbits such that, when the electrons travel in 

 them, the energy of the atom has these observed values. If this 

 problem cannot be solved, it is impossible to take the next and es- 

 sential step of ascertaining whether these particular orbits are dis- 

 tinguished in any particular way from all the other conceivable ones. 



In the case of a single electron re\'olving iibout a nucleus, this 

 problem is sometimes soluble. If the mass of the electron is regarded 

 as invariable, and no outside influences are supposed to act upon the 

 atom, then the solution is comparatively easy to attain. It was 

 performed in the Second Part of this article. If an external magnetic 



