THE AID OF THE ACHROMATIC FRINGES. 



57 



ferometer. This tube is closed at t with a tightly -fitting screw-plug provided 

 with a conical depression to receive the thrust P from the point q of the offset 

 5 s (fig. 30) . The rod r is thus compressed between q and the rigid abutment A 

 of the apparatus, and projects but slightly (i or 2 mm.) beyond the tube b. 

 Special sheaths b are provided, fitting neither too tightly nor too loosely, 

 for different diameters of rod r ; or a number of coaxial tubes, b, may be tel- 

 escoped for the purpose. 



This device gave more satisfactory results at once, and with such bodies 

 as hard rubber showed the change of modulus with stress, the occurrence 

 of hysteresis, and viscous deformation. It is particularly interesting, inas- 

 much as it gives the apparent value of the modulus under each of these 

 conditions. 



The successive contractions (AA r ) and modulus values for hard rubber as 

 found in several successsive series are exhibited in table 2 and figures 35 

 and 36. In the first series the relatively large micrometer displacements, A7V, 



are probably due to crushing or to fitting the unstressed rod to the abut- 

 ments of the apparatus under increasing pressure. Thereafter these large 

 contractions do not again occur. The moduli obtained from triplets of obser- 

 vation between 3 and 4 kg. gradually increase to a fixed value. In the 

 second series the rod which had been loaded for some time (see table) with 

 about 40 kg. per centimeter shows the limiting value of moduli found, 

 E = 4-4X io 10 . We may contrast with this the small modulus when the load 

 is but i to 2 kg. 



In the third series, beginning with a rod but slightly stressed, a low value of 

 the modulus at first appears, but it soon reaches the limiting values again. In 

 figures 35 and 36 the contractions (the numerals show the loads) are given in 

 succession. With the exception of the necessary break at the beginning of 

 the second series, the work is continuous without modification of adjustment. 

 As contractions are positive the rod is gradually becoming shorter and more 

 viscous. The data are throughout consistent, much more so than was ex- 

 pected. For instance, the effects of the removal of weights at b and c are 

 practically identical. The triplets in figure 35 all show an upward slope or 

 continuous viscous contraction of the rod under large end-thrust. In series 



