CHAPTER II. 



ELASTIC DEFORMATION IN SMALL BODIES, MEASURED BY THE PRECEDING 



CONTACT LEVER. 



4. Introductory. In the preceding report I communicated a series of 

 experiments on the traction modulus of small bodies, using an interferometer 

 design which worked admirably so far as the optic measurements were con- 

 cerned. The mechanical part of the contrivance showed an apparent yield, 

 the nature of which I was unable to detect, but which seemed to be in some 

 way associated with the flexure of parts of this massive apparatus. In fact, 

 pulleys and weights were used for imparting stress. It may be argued that 

 any contrivance of this kind, however convenient in other respects, is danger- 

 ous because of the force couples introduced, even when the rigid parts of the 

 apparatus are nearly 2 inches thick, as in the case in question. 



In the present apparatus all this is completely avoided by the use of springs 

 to impart stress, and the above contact lever to measure strain. True, fric- 

 tion enters into functioning of such an apparatus to a menacing degree. It 

 thus becomes an experimental question to determine how far it can also be 

 eliminated by judicious tapping, etc. 



5. Apparatus. The simplest of the apparatus designed is shown in figure 7. 

 The rod to be tested, 2 to 3 cm. long, is at r held in a brass sheath s, loosely 

 fitting it. This is screwed into the middle of the massive brass cross-piece A. 

 A little disk of glass has been attached at a, and the end e of the contact lever 

 touches it, to indicate the small displacements. The longitudinal displace- 

 ments Ax of the pin e are observed by the interferometer, as explained in the 

 preceding paper. 



a 



B is a cast-iron brick, about 10 inches high, 2 inches thick, and 3.5 inches 

 broad, provided with two horizontal (0.25 inch) perforations, parallel to each 

 other and normal to the large face. Through these pass the 0.2 5-inch brass 

 rods bb and cc loosely, rigidly connecting the cross-piece A with the similarly 

 massive cross-piece C (screws and locknuts m, n). The rectangle AC is thus 

 free to slide in B, except so far as it is limited by the contact of the rod r with 

 the smooth face of the brick B. 



To apply stress, the system d, w, S, f has been provided, consisting of the 

 stiff open spring S encircling the brass rod df, firmly screwed into the brick 



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