THE AID OF THE ACHROMATIC FRINGES. 



15 



The fringes found without difficulty were small, though adequate for the 

 purpose. Measurements were made in cycles, care being taken to repeatedly 

 tap the movable parts of the apparatus before each reading, and these came out 

 remarkably smoothly at once. The first results were (initial load about 2 kg.) : 



No. of turns of screw, 7*1234543 2 i 



io 3 XAA/" 24.7 26.9 29.0 30.9 32.8 31.1 29.1 27.3 25.1 22.6 cm. 



These data are also given at a, figure 9. The outgoing branch is nearly 

 straight and regular, though the first observation was lost. The return branch 

 is less regular, as would be expected, but the departures lie within 5 X io~ 4 cm. 

 If we take the differences between observations n = 3 turns apart, in the out- 

 going series the results would be 



io 3 AAT 



10 



-10 j 



Loads about 



6-45 

 2.18 



2-5 



6.i S 



2.29 



4-6 



5.85 cm. 

 2.41 

 5-8 kg. 



The rod therefore grows steadily more rigid as the load increases, as shown 

 at b, figure 9, a result also to be anticipated. Of hysteresis there is no certain 

 indication, since the higher results in the return may be otherwise explained. 

 If the means of advance and return series had been taken, the values of io 3 A7V 



would have been 6.47, 6.10, 5.67, and their mean difference from the above is 

 within the errors of the screw micrometer reading to io- 4 cm. 



TABLET. Hard rubber rod. =2.47 cm. .4 =0.112 cm 2 . = io 10 X4.69n/AJV. 

 Initial load about 2 kg. Additional load about 1.8 kg. per turn (n = i). 



* Later. 



