Synopsis
W{th flow birefringence measflremerfts; carried 0u±in" the usual Couette t3~pe appafa-tus, a -well-known optical disturbance effect appears,, which is~caused by frictional heat. (Bj 6rnstAhl-1)).. On starting the experiment, however, the development. 0f,this effeq.t requires a certain time, In .the beginning of this starting period undisturbed measurements have been carried out by J. L era y 2). According t O Y. B j 6 r n s t Ah 1, the length of the starting period is proportional to the square of the gap widtH. As a 6~snseqfience, with a narrow gap (d < 1 mm) the starting period becomes imprkct~cably short.
As the frictional heat evolved in '.the streaming liquid is conducted_through the cylinder walls, the length of the starting .period depends also on. the heat conductivity of the cylinder walls. So far. however, only metal, cylinders with an extremely high heat conductivity had been used. In this paper the usefulness of cylinders with a low heat conductivity is discuss6d witl~ reference to a si~ecial case.
From the calculations the conclusion may be drawn that the obtainable increase of star~ing time is not considerable. Therefore no decisive improven~nt, over the use of metal cylinders, can be advised. (Elsewhere in this journal 4) the opposite conclusion is drawn for the'convection method. Cf. also s)s)).
Only with a' relatbcely wide gap (d > 2 mm), the starting period will be sufficiently great. The method will therefore only be practicable for viscous liquids (concentrated solutions, polymer melts etc.).