Prof. J. Ferry on Liquid Friction. 445 



Two methods of determining the torsional constant of the 

 wire were employed : — 



First Method. — A fine cotton thread was wound round the 

 outside of the suspended cylinder and passed over a nearly 

 frictionless pulley (the p alley of an Attwood's machine) to a 

 scale-pan. The thread was nearly horizontal as it left the 

 cylinder. In this way it was found that the twisting moment 

 required to produce a pointer-rotation of one degree was 

 1581 dyne-centimetres. In making the measurement as the 

 weight of the scale-pan and its contents was gradually 

 increased, the steel wire was drawm away from the vertical, 

 and therefore from the middle of the scale; but the stand 

 was tilted to counteract this effect. 



The effects due to solid friction were eliminated by taking 

 the mean of the limiting weights for equilibrium. When the 

 weight was 30 grams, one tenth of a gram either added to or 

 taken from the scale-pan produced a perceptible change in the 

 position of the pointer ; so that the solid friction was small. 



Second Method. — The suspended cylinder was allowed to 

 vibrate, twisting and untwisting the wire ; and its times of 

 oscillation were noted. The observations were repeated when 

 a known moment of inertia had been added. Unloaded, it 

 made 40 complete oscillations in 583 seconds, or one oscillation 

 in 14*575 seconds. We then attached to the cylinder an iron 

 bar of rectangular section, whose own moment of inertia had 

 been determined accurately by previous experiments (found 

 to agree with calculation on the assumption that it was homo- 

 geneous), this moment of inertia being 566*2 (in gram- 

 centimetre 2 units). The time of a complete oscillation was 

 now found to be 21*425 seconds. It follows that the moment 

 of inertia of the suspended cylinder is 487*72, and the tor- 

 sional constant of the wire is readily obtained. This constant 

 being corrected on account of the position of the pointer, it 

 follows that to produce a rotation of the pointer of one degree 

 requires a torque of 1552 dyne-centimetres. This is greater 

 than the constant derived from direct measurement by 1£ per 

 cent. ; but, on the whole, we are rather inclined to accept the 

 number obtained directly, as we are not quite sure that tho 

 mean position of the iron bar was at right angles to the mag- 

 netic meridian. 



Eight quite independent measurements of the diameter of 

 the wire were made by men experienced in making such 

 measurements ; and the mean value was '0371 inch, the 

 greatest and least being '0373 and *0369. Using this mean 

 value, and the directly measured torsional constant, it would 

 seem that the modulus of rigidity of the steel is 7*71 x 10 u . 



