THE AID OF THE ACHROMATIC FRINGES. 



17 



TABLE 2. Thin hard rubber rod. .,=2.55 cm.; diameter 0.215 cm -. ^4 =0.0363 cm. 2 ; 

 Xlo 10 = 1 4.957 io 3 A./V. Initial load about 0.5 kg.; additional load 1.8 kg per turn (w = r). 



difficult to interpret ; for the reversal of the twist may imply some dislocation 

 of the spring, like a twist around its axis, for instance. In table 2, E has there- 

 fore been computed from the data of AvV for n = i , leaving out the irregularities 

 specified. The values of E in a given branch (ascending or descending) of the 

 cycles is virtually constant. The mean values in series 3 are (ascending) 2.34, 

 (descending) 2.50; in series 4, (ascending) 2.41, (descending) 2.48. The rod 

 is always apparently more rigid in the case when stress is removed than when 

 it is added. But this difference decreases and would probably vanish if the 

 experiments were indefinitely continued. It eventually vanishes from triplets. 

 The mean of the whole of the third series is XicT 10 =2.42; of the fourth, 

 EXicr 10 = 2.44, which may be regarded as satisfactory for the body in ques- 

 tion. We may note that a dimension-ratio of 10/1 has been required to obtain 

 this result. 



koniit, JJUtt^ ^- 



~~% 7% S.tUtiru,ek^ 



4 



8. The same. Brass. A brass rod was now inserted to test the rigidity 

 of the apparatus. The dimensions of the rod were , = 2.50 cm.; diameter = 

 2 



