78 



DISPLACEMENT INTERFEROMETRY APPLIED TO 



A weight of the order of i gram was selected and the double twist produced 

 by this was A0= 138.2 88. 2 = 50.0. The weight was passed from panto 

 pan without difficulty and the fringes set to coincide with the shadow of the 

 cross-wire on the slit. Great care had to be taken to avoid twisting the frame 

 of the carrying wire and torsion heads. If the beam is of steel, fluctuations 

 of the magnetic field are also a menace. 



The data of the interferometer being identical with the above, we may write 

 dg_ dd_ AN cosi _ _io~ 4 Xo.7i 

 g A0 



= i.6Xio- 6 



\e &A0/2 10X25X0.0175 



and as AAT may be read within io~ 4 , the sensitiveness is about io~ 6 , the gram 

 counterpoise in question presupposed. 



The error to be apprehended from viscosity may be obtained from the 

 following observations (table 2 , series 0) of the yield of the wire with a gram 

 excess on the scale pan : 



TABLE 2. Viscous deformation of hard-drawn steel wire. 



* Room colder. 



The observations marked [*] were made in a very cold room and give evidence 

 of increased rigidity. The table shows that if measurement is made within 3 

 minutes after twisting, the error from viscosity, if ignored, would not exceed 

 dg/g = 33Xicr 6 ; within 10 minutes it would not exceed 7oXio~ 5 . If, however, 

 the coefficient of viscosity is known, from a preliminary examination of the 

 wire, an error of dg/g = ^X icr 5 is improbable. 



Hand-drawn steel, though admirably resilient, is a metal of somewhat low 

 viscosity, particularly when subjected to additional tensile stress between the 

 torsion-heads. To remedy this, the wire must be tempered. Samples were 

 taken and tempered in molten lead. It was found that they had lost but little 

 of their resilience, contrary to expectations; but their viscosity is necessarily 

 increased, owing to the elimination of molecular instabilities. 



