66381 str (111)

B / B LI OGRA PHY

BCcDowrll and Usher . Proc. Roy. Soc. (A), jn,. csyyt g|| i 1932, CXXXVIIL, 133.    '

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Schofield and Scott Blair. Proc. Roy. Soc. (A), 1933, CXL1

|§

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Scott Blair. Rheol., 1930,1., 424 ; Koli. Zeits., 1937, LXXVIII,₁

19.

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CHAPTER VI

Fading Sphere Viscometer. Corrections. Plasticlty Definltions and Measurement. Flow-plastlcity. Influence of Electrolytes. Flocculatlon and Syneresis. Turbulence. Speclal Applications—Ceramics, Coal.

If a sphere falls through a fluid, it attains and maintains a constant velocity (v), which depends on the densities of the sphere and of the fluid (a and p), on the radius of the sphere (R), and on the viscosity of the fluid (17). Newton knew this, but it was Stokes who first evaluated the eąuation

7)=- kjZfte . RS .... (i)

This is only an approximation, and even for the case where the containing vessel is very large compared with the size of the sphere, Oseen has shown that a morę complex expression is reąuired.

I f the sphere is at all big compared with the vessel in which it falls, fairly large corrections are needed. For an experiment of this type, it is usual to consider 1 cylindrical vessel, with the sphere in the middle. Faxćn points out that for these finite conditions Oseen’s modifications of Stokes’ law are unnecessary. Ladenburg gives, as a first approximation for the correction to be madę to v

v(i + 2-Ąd/D)

when d and D are the diameters of the sphere and of the cylinder. Faxćn points out that Ladenburg madę a slip in his calculations, and that the constant should be 2-z instead of 2-4.