THE AID OF THE ACHROMATIC FRINGES. 



59 



exceedingly complex. There may be some instrumental error; but it is 

 interesting to note that in figure 37 the rod subjected to variations of loads 

 "between i and 4 kg. shows gradual expansion at the lower pressures. Just 

 as the loops are evidences of hysteresis, so the grad- 

 ual upward trend of successive loops between the 

 same end-loads is evidence of viscosity. 



Finally I collected the values of the modulus E, 

 obtained under different loads P in the successive 

 series of experiments. In each series the stable value 

 was reached between given alternating end-pres- 

 sures, gradually, as already explained. The mean 

 values are 



Loads=i4.2 23.6 33.0 42. 4 kg/cm 2 . 



io- 10 E=i.8i 2.82 4.00 5.11 



results which are exhibited in figure 39. They hap- 

 pen to lie on a straight line. Although these mean 

 values are unequally influenced by the character 

 and number of the experiments made, the result as 

 a whole disarms suspicion. It is improbable, in 

 other words, that data which have shown such 

 detailed consistency as appears in figures 35 to 39 



should be seriously influenced by imperfections of apparatus or of method, 

 though it is possible that at the higher pressures the sides of the hard-rubber 

 rods may have expanded into and been sustained by the walls of the rigid 

 sheath b (fig. 34). In the above estimates the rate R at which the modulus 

 E increases per kg./cm. 2 of pressure would be, nearly, 



5.1- 1.8 



40 



:=io 10 - 



IO 



10 



42.4-14.2 

 The rate R is excessive as compared with subsequent values. 



3 1 . Same. Thinner rods, hard rubber. The suspicion left in the preceding 

 experiments that the marked increase of E was possibly due to the lateral ex- 

 pansion of the hard-rubber rod, so that it more or less filled the rigid sheath 

 at the highest loads, induced me to repeat the work with slightly thinner rods. 

 The former case would make E approach the bulk modulus. The rods last 

 used were therefore turned down on the lathe until their dimensions were 



2L = 4.9ocm. 2r = o.35cm. A =0.098 cm. 2 



They were then tested on the interferometer for cyclically increasing and 

 decreasing loads P, as shown in table 3, where AW is the micrometer equiv- 

 alent of elongation. A few extra triplets were added. The results are also 

 given in figure 40 and show a beautiful case of hysteresis with gradually 

 increasing loops. This hysteresis is in no way less accentuated when com- 

 pared with the preceding cases. 



