548 BELL SYSTEM TECHNICAL JOURNAL 



cathode particle — there was, nevertheless, full confidence that these 

 relationships would be confirmed by more exact measurements. And 

 this indeed proved to be true. The anticipated details of the picture 

 as then blocked in have since been supplied by a series of precision 

 experiments in which Millikan's measurement of the absolute magni- 

 tude of the electronic charge is preeminent. 



The turn of the century was a time of high hope. The key had 

 been found, it appeared, to an understanding of vast ranges of phen- 

 omena; given the electron, electrodynamics and sufficient mathematics, 

 all electrical and magnetic phenomena must become explicable. It 

 seemed not too daring even to have thoughts concerning the structure 

 of the atom. But this, as it turned out, was mostly illusion ; every suc- 

 cess of the electron theory of this period was matched by an equally 

 conspicuous failure. Metallic conductors were pictured as containing 

 atmospheres of free electrons with the properties of a monatomic gas. 

 The drift of this electronic gas under the influence of an impressed 

 field constituted the electric current. The form of Ohm's law was 

 neatly explained, but not so the direct proportionality between the 

 resistivity of a pure metal and its absolute temperature. The ther- 

 mionic emission of electrons could be explained, apparently, in all its 

 details, but the distribution of energy in the black body spectrum 

 could not. The explanation of the simple Zeeman elifect was most 

 gratifying and reassuring, but the simple numerical relationships 

 among the frequencies of line spectra remained as baffling as ever — and 

 this, in spite of the considerable success which Drude and others had 

 achieved in explaining the optical properties of materials in terms of 

 electrons elastically bound within atoms. 



The impasse was finally breached by Planck who showed, in 1905, 

 that the black body spectrum could be explained if one were willing 

 to assume that materials contain electric oscillators which emit and 

 absorb energy only in amounts proportional to their frequencies. 

 The conception of the electron was unaltered — not even involved, 

 perhaps — but an oscillator, which might be a vibrating electron, was 

 conceived to behave in a manner contrary to electrodynamical princi- 

 ples. A success had been achieved at the cost of violence to classical 

 ideas regarding the production of electromagnetic radiation. 



The next assault — a brilliant tour de force by Bohr — achieved its 

 first objectives at a stride, but at a sacrifice of electrodynamical prin- 

 ciples greater even than Planck's. Bohr showed in 1911 that by com- 

 bining the idea of a concentrated atom nucleus required by Ruther- 

 ford's experiments on the scattering of alpha rays with the heterodox 

 idea of Planck, and with new devices of his own invention, one could 



