On the coefficient of viscous traction and its relation to that of viscosity

Abstract
When experiments are made on the viscous flow of pitch and other substances of similar character, in the form of rods or cylinders, by the torsional method, it is found that the rate of turning under torsion of these rods is not strictly proportional to the driving couple. Thus the rate of flow of the material under shearing stress cannot be in simple proportion to stress. If it is wished to investigate the exact law connecting the rate of flow with the shearing forces, by means of the torsional method, a complication is at once met with, arising from the fact that the rate of flow in a twisting rod is not of the same value everywhere, but necessarily varies from nothing at the centre to a maximum at the surface of the rod. With the view of developing a more suitable way of investigating the phenomenon, trials were made with different methods of observing the flow of such bodies, under conditions in which the said objection does not apply. The results obtained in these ways exhibit the same departures from linearity as was suggested by the results obtained by the method of torsion.

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