54 



DISPLACEMENT INTERFEROMETRY BY 



consequence. But there is a much more serious consideration. In long rods 

 the stresses distribute themselves equally throughout the sectional area ; but 

 in short rods this is probably not the case. There will be lines of longitudinal 

 stress and part of the area A will be relatively unstressed. Hence the values 

 of E will come out too small and the question is rather to what degree such 

 a method can be made trustworthy. With the optical method there will be 

 no difficulty, if the achromatic fringes are used. The observed displacement 

 of these is adequate if a reasonably thin rod is used and fringes need not 

 be counted. It is not even necessary to make the method very sensitive. 

 Fringes of moderate size suffice. 



Should the method of end-thrust fail, the method of flexure is more liable 

 to be successful , since the measurement of the sag at the middle of two rods 

 placed at r and r' parallel to FF offers less serious difficulties. But such flexure 

 involves continuously decreasing stresses from the middle to the ends of each 

 rod and is thus fundamentally less interesting. For very short rods, moreover, 

 theoretical difficulties similar to the preceding would be encountered. 



28. Apparatus. The apparatus used is shown in front (normal to the 

 interferometer rays) and side elevation in figures 28 and 29. F is the bar 

 carrying the mirrors m,m', already mentioned, and compressing the rods r,r' 



1 



30 



p 



r t\ I n| 



c/'r 1J 



PL I 



U^xJ U 



CS7 



to be tested. The forces are applied (shown in detail in figure 30) by means 

 of a set of weights ww', the pulleys cc' and the rigid brass offset 55 acting 

 on the cone at q, which fits into a depression in F. The lines p q pass through the 

 axes of the rods r or r'. In this way any flexure of the bar F is obviated. 



The rigid part (abutments of the system) is made up of the cast-iron bricks 

 A, A', B bolted together firmly. These rest upon the plate C provided with 

 three leveling-screws g,h,i, g,i being parallel to the barF. These leveling-screws 

 are of special importance in controlling the size and inclination of the 

 achromatic interference fringes, gi being the horizontal axis. The whole 

 apparatus may be rotated around a vertical axis on the horizontal base GG, 

 which is itself movable on slides nearer or further from the interferometer. 



