the Ultra-Micrometer. 637 



It was very soon found that the apparatus just described 

 was most extraordinarily sensitive to small changes in tin* 

 value of P. Although, in anticipation, the whole arrangement 

 was placed in a solid basement room of the University, the 

 note emitted from T instead of being pure was rough, owing 

 to the continuous small vibrations of the building. 



It was only, in fact, in the small hours of the morning that 

 the purity of the note could be maintained, and even then 

 distant traffic proved troublesome at times. 



Before explaining the methods used for determining the 

 sensitiveness of the instrument, it may be mentioned in 

 passing that the bending of the solid looking table shown 

 in the photograph, produced by a penny placed on its edge, 

 was clearly indicated by a change in the note from T. 



A series of experiments was now carried out to find what 

 was the smallest change in x that could be detected by this 

 arrangement. 



This involved preliminary experiments on the application 

 of comparatively large known bending moments to the 

 left-hand rod in the photograph*, and measuring by means 

 of a micrometer the consequent lateral shift of the plate. 

 A micrometer capable of measuring to 10~ 4 inch was used, 

 contact with the plate being determined by use of a sensitive 

 galvanometer and battery. 



Two different mechanical schemes were tried shown in 

 tig. 2 ; of these, the one marked (a) was very soon abandoned 

 as it was found that when very small weights were used the 

 law of proportionality between shift of plate and applied load 

 broke down. 



This was doubtless due to the frictional forces introduced 

 at the junction J even when the finest unspun silk was used. 



A method free from this defect is that of fig. 2 (6), in 

 which the bending couple is applied by placing suitable 

 weights on the graduated quartz rod LM. 



As a result of a large number of experiments it was found 

 that Hookers law was obeyed to the limits of accuracy set by 

 the micrometer. 



By extrapolation it was concluded that 1*4 grains placed 

 one inch along the rod would produce a lateral shift at the 

 centre of the plate of exactly lO -6 inch. 



This was a convenient number to remember from which to 

 calculate the shift produced by any other weight at any 

 distance. 



After a long series of trials it was found that the smallest 



* [ am indebted to Mr. J. Gilchrist, M.Sc, for great help in this part 

 of the work. 



