80 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



Successive alternations were now made by twisting the wire between fixed 

 stops, using the gram weight specified as a counterpoise. Table 3 is a summary 

 of these results. The whole of the 10 series for the blue state are further shown 

 in figure 103, which also contains the graph for the hard-drawn state. 



As a rule, the curves for tempered wire, so obtained, differ but little. When 

 they do, some irregularity in twisting the wire, or an accidental delay in finding 

 the first fringes or different periods of application of the preceding stress, 



40 60 so ICQ 



#0 160 iso 200 



differences of temperature, etc., are in question. In the last series, No. n, the 

 wire had been previously twisted to and fro upwards of 100 times to test the 

 stops. The effect is some decrease of viscosity. In general, the odd series 

 show less yield than the even series, probably because the stress is not quite 

 symmetric on the two sides. 



69. Equations. Equations for the description of the yield in the lapse of 

 time have been proposed by different authors, in particular by Kohlrausch, 

 the original investigator. They are usually rather complicated higher expo- 

 nentials. It is interesting to test a tentative form. Let it be supposed that 

 the rate of yield dy/dt is proportional to the number dn/dt of instabilities van- 

 ishing per second, so that 



(1) dy/dt = Cdn/dt or A y = Cn 



Furthermore, let the decay of instabilities vary as the square of the effective 

 number n present; *'. e., 



(2) dn/dt = a'n- or = a't 



n n 



If equation (i) is inserted, (2) takes the form 

 (3) 



A-y 



i 



'A 



