An electromagnetic viscometer for molten silicates at temperatures up to 1800 C

Previously reported measurements have been marked by an upper temperature limit of 1650° C and by the poor agreement obtained between different authors working on the same system. The present paper shows how, by the use of molybdenum apparatus, the temperature range may be increased to an upper limit of 1820° C and a method for the measurement of torque, not previously used in this field, permits a higher degree of accuracy than previously attained. It is shown that, of the methods available for room temperature viscosity measurements, a modification of the concentric cylinders method is most suited to adaptation for high temperature measurements on molten silicates, for which the range 0.1-10 000 P is the most important. By the use of an electromagnetic method of torque measurement, it has been found possible to cover the range 0.05-14 000 P without modification of the apparatus and an accuracy of ±0.5% has been obtained with the apparatus at room temperature. A method of calibration is demonstrated whereby the appratus constant is obtained for any depth of liquid and an accuracy of ±6.9% with a reproducibility of ±3.0% has been obtained at 1800° C. The construction of the apparatus is briefly described and details are given of the operation and method of calibration.