634 Prof. JR. Whiddington on 



Summary. 



Further experiments with the mass-spectrograph yielding 

 provisional analyses of the elements B, F, Si, P, S, As, and 

 Br are described. 



Of these B, Si, and Br are definitely complex, the others 

 apparently simple. 



The atomic weights of all conform to the whole-number 

 rule. 



Some anomalous fractional lines are mentioned and their 

 possible origin discussed. 



Cavendish Laboratory, 

 August 1920. 



LXXIII. The Ultra-Micrometer ; an application of the Ther- 

 mionic Valve to the measurement of very small distances. 

 By P. Whiddington, M.A., D.Sc; Cavendish Professor 

 of Physics in the University of Leeds *. 



[Plate XV.] 



SINCE the discovery and application of the various forms 

 of interferometer the direct and accurate measurement 

 of small distances has been regarded, with some reason, as a 

 problem solved. 



All interferometer methods, however, are limited in 

 accuracy of measurement by the wave-length of the light 

 used in the production :pf the fringes. The -present paper 

 is an account of a new method which is not limited in this 

 way, and which is easily capable of measuring small distances 

 of the same order of magnitude as the atomic diameter. 



The theory of the method may be put very simply. 



If a capacity C be connected to an inductance L, the 

 frequency of oscillation N natural to the circuit is given by 



N= * . 

 2tjVLC 



If the condenser be composed of two parallel plates area A, 

 separated by distance x, 



p A 



4:7TX 



Whence 



\ttLAJ 



Clearly a change in x involves a change in N, so that we 

 may take the change in N as an indication of a change in x. 



* Communicated by the Author. 



