End-Effect in Rotational Viscometers

Abstract

In the classical equations for the traction on concentric cylinders by a viscous substance, it is assumed that traction on the top and bottom of the inner cylinder (bob) is negligible in comparison with that on the curved surface. In viscometers of practical dimensions, however, it is necessary to compensate for the end-traction or end-effect by adopting one of several expedients in design or by determining the magnitude of the effect and accounting for it in terms of increased length of bob.In the experiments reported in this paper, the method of multiple bobs has been used, and the variation of endeffect with the following factors studied: (1) radius of bob;(2) clearance between cylindrical surfaces; (3) separation between bottom of bob and cup; and (4) viscosity.The magnitude of the end-effect increases with radius and with clearance. For separations at the bottom greater 1 See, for example, H. Lamb, Hydrodynamics (Cam-

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Rheometry

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