This method, originally devised by Margules, consists of measuring the viscous drag or torque on the inner of two concentric cylinders when the outer cylinder holding the molten glass is rotated at constant velocity. Hawkes, assisted by other laboratories co-operating in the project, have adapted the rotating cylinder method ( 1) to the determination of the viscosity of glass above its softening temperature. While at work on this problem for the U.S. Therefore there is a need for standard samples of glass of known viscosity against which process samples can be compared. When high speed mass production techniques are employed the glass must conform closely to viscosity tolerances so that products will have uniformity of thickness, shape and strength. The viscosity of glass has a considerable effect on its other physical properties such as its melting and crystallisation characteristics and the temperature and pressure ranges within which the glass can be worked.
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