592 Sir J. Larmor : Escapements and Quanta. 



which throws some light upon the processes of the charac- 

 teristic radiation from gases, and the X-rays from solids, and 

 the beta rays emitted when X-rays strike a target, as well as 

 the photoelectric effect, and possibly the elusive phenomenon 

 of fluorescence. In each case the model affords a picture of 

 just what is vibrating with the characteristic frequencies 

 observed, and also a reason for the existence of Planck's 

 rule in radiation. 



The Bohr model affords no picture of anything that really 

 vibrates with the observed frequencies, since nothing is said 

 about the model during the very time when the radiation is 

 taking place, but merely about its state before and after such 

 radiation. 



The Bohr model, the Lewis-Langmuir model, and the 

 model above suggested each throw light upon different 

 sets of physical phenomena, and each seems likely to be 

 useful in assisting the development of a more general 

 comprehensive model. 



Nela Research Laboratories, 



Cleveland, Ohio. 

 March 1921. 



LXYI. Escapements and Quanta. 

 By Sir Joseph Larmor *. 



MIGHT not an atom be a clock? It is asserted on 

 sufficient evidence that when an electron is fired into 

 an atom, it sometimes is retained, but its energy is emitted 

 again in radiation of Rontgen rays, mostly forming a group 

 of very definite periods but in part irregular. The former 

 periods are so sharp that a train of many thousands of 

 undulations perfectly regular in amplitude and phase, must 

 be sent out, in order to produce such narrow lines in their 

 Fourier spectrum as appear in crystal analysis. Yet an 

 unsustained electric vibrating system radiates so intensely 

 that if left to itself it would be quenched in a few vibrations. 

 How is all this to be imagined ? Such equivalence between 

 the impulse of an absorbed projectile and a long train of 

 regular waves is unfamiliar and must therefore be significant* 

 Consider, however, a common pendulum clock, or better, 

 a clock with a compound pendular system having a group of 

 sharp periods in its complex mode of oscillation. If vibrating 

 free the pendulum wall soon slow down to rest ; but if it is 

 engaged through an escapement (the great invention of 



* Communicated to the British Association, Sept. 1921. 



