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XVIII. 



A DIEECT EEADING ULTRA-MICEOMETER. 

 By JOHN J. BOWLING, M.A., M.E.I.A., F.Inst.P. 



Read January 25. Published March 4, 1921. 



In a recent commimicationi to the Eoyal Dublin Society I described an 

 ■electric valve method for the measurement of capacity, which could also be 

 applied to the measurement of extremely small displacements ; but, inasmuch 

 as a very constaiat source of alternating e. m. f. was necessary for its opera- 

 tion, further work was promised with a view to overcoming this drawback. 

 In the course of this work, the present method suggested itself, and, on trial, 

 was found to be even more sensitive than that previously described. It has 

 the great advantage of simplicity, and, although capable of extraordinary 

 sensitiveness, is singularly free from that unsteadiness to be expected in a 

 highly sensitive apparatus. Furthermore, with a suitable adjustment of the 

 apparatus, it is possible to maintain a high and almost constant degree of 

 sensitiveness over a total displacement of, perhaps, one-tenth of a milli- 

 metre. 



In the present communication I shall confine myself to a short descrip- 

 tion of the apparatus in the form first set up, together with an account of 

 some trial measurements made with it. I shall reserve for another occasion 

 a fuller consideration of the principles involved, and of the conditions of 

 ■operation of the device. 



It is well known that for sustained oscillations to occur 'in a three- 

 •electrode valve circuit, the values of the inductances and capacity must fall 

 within certain limits. If the self-induction and the coupling of the valve 

 •circuit shown in the figure be left unaltered, while the capacity is gradually 

 changed, two effects are produced : the frequency of the oscillations changes 

 and their amplitude is also, in general, affected. The former phenomenon was 

 that availed of by Whiddingtou in the construction of an ultra-micrometric 

 ■device." The latter may be regarded as the effect upon which the present 

 ■device depends. As the amplitude of the oscillations of potential varies, a 

 ■corresponding change takes place in the plate current, and it is this property 

 of the oscillating circuit which is utilized. 



"A Sensitive Valve Method of Measurement of Capacity." Scient. Proc. Roy. 

 Dub. Soc, vol. xvi. No. 17, p. 175. 1921. 



2 Phil. Mag., Nov., 1920, vol. xl, pp. 634-639. 



SOIENT. PKOC. K.D.S., VOL. XVI,, NO. XVIII. 2 F 



