the Ultra-Micrometer. l>39 



There is not the slightest doubt that an apparatus built on 

 these lines could be made to indicate much smaller changes 

 than that experimentally obtained here. 



I have not so far attempted to push it to its practical limit, 

 as for the purpose I have in immediate view its present sensi- 

 tiveness is sufficient; there seems little doubt, however, that 

 a sensitiveness of 100 times that just attained could be got 

 without much difficulty. 



In the previous paragraph it has been suggested how the 

 apparatus may be used to detect extremely small distance 

 changes, of the order of -^} U) millionth of an inch. We 

 there kept every thing constant except the variation in x 

 and observed the consequent change in the number of beats. 



When, however, we are dealing with a much larger shift 

 in x — say * part of an inch or so — it is possible to 

 adopt a rather simpler scheme. 



With such comparatively large changes in x a very large 

 change in N occurs, so that a considerable change in C must 

 be made to recover the standard note issuing from T. 



With the actual apparatus shown in the photograph it was 

 found that over quite a large range of the condenser scale 

 — viz. from 20 deg. to 60 deg. pointer readings — a change 

 of 1 deg. meant a change in * inch in x. As it was 



possible to read y 1 ,, degree quite easily, this scale-reading 

 allows of a direct determination of a change in x within 

 one three-millionth of an inch. 



It is hoped to apply this extremely delicate method of 

 observing small distance changes to various problems in the 

 near future. 



Summary. 



If a circuit consisting of a parallel-plate condenser and 

 inductance be maintained in oscillation by means of a therm- 

 ionic valve, a small change in distance apart of the plates 

 produces a change in the frequency of the oscillations which 

 can be accurately determined by methods described. It is 

 shown that changes so small as 2 oo millionth of an inch can 

 easily be detected. The name " ultra-micrometer " is tenta- 

 tively suggested for the apparatus. 



The Physics Laboratories. 

 The University, Leeds. 



