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5° RELAXATION

eąuation. For further details, reference should be madę to Hatschek's book.

The ratę of decay of stress in plastic materials is very complex. Phillips proposes a law of the typ_e I S == a — b log t,

which works well enough for rubber and glass, but I not for metals. Freundlich and Seifriz have studied the rigidity of gelatine sols by observing the move- I ment of tiny nickel splinters in the sol, while a magnet I was brought near and then removed. On the I removal of the magnet the metal splinters re. I bound.

The phenomenon of flow-elasticity will be eon- I sidered in a later chapter.

Schofield and Scott Blair show that MaxwelTs I treatment can be applied fairly satisfactorily to I flour doughs. Here, however, on account of elastic I after-effect (which will be dealt with in Chapter IX), I a term (dctfdt) has to be introduced. This term is I large when abrupt changes of stress have recently I occurred ; but smali, as a rule, during the application I of the stress. The eąuation is

de _ /i dS _ da\ . ig    H I

dt ~ \ń" dt dt) t]    I

elastic part    viscous part

(where dejdt is the ratę of fractional elongation).

McDowell and Usher did similar experiments to I those of Hatschek and Jane, but with copper I ferrocyanide and sodium chloride. They find that, I when the dispersed particles are clustered together, a I rigidity is produced which is not present when this I network formation is prevented.

With pasty materials, there is considerable slip of I the cylinder (equivalent to pług fłow in capillaries). I McDowell and Usher discuss the effect of ratę of I

OSTWALD TYPE VISCOMETERS 51

shear in breaking down structure, and so lowering viscosity.

Bingham and Robertson caused a smali amount of ammonium oleate sol to oscillate in a capillary, by applying forces at opposite ends altemately. They observed the movement of a speck of dust and noted that the oscillation is of the same freąnency, but out of phase with the applied oscillation. This difference in phase gives a measure of the elastic modulus of the sol.

Speakman shows that wool relaxation obeys a law of the Maxwell type for moisture contents not exceeding 75 per cent. A morę complex eąuation is suggested for very wet conditions and for chemically treated wool.

Before considering technical viscometers, some attention should be paid to the Ostwald viscometer, sińce this is much used for true fluids in industry. Since considerations of this kind do not really fali within the scope of this book, the subject cannot be dealt with in detail, and those who reąuire further information are referred to Barr’s book. A great many varieties of the Ostwald viscometer have been developed, using the same fundamental idea, namely, the measurement of the time reąuired for a given volume of liąuid to flow through a vertical capillary tubę, under the influence of gravity. It should be noted that, during such a process, the shearing stress is changing all the time as the head falls. Viscosities are generally obtained by com-paring the time of the fali of the surface of the liąuid in ąuestion, with that of water at the same temperaturę, and if necessary this is converted into absolute units by use of the known absolute figurę for water. It should be remembered that the time H is proportional not only to the viscosity but also