A Precision Determination of the Viscosity of Air

The rotating cylinder method has been used for making a precise determination of the viscosity of air. The inner cylinder was rotated, which made possible a higher degree of accuracy in the construction of the apparatus and in its adjustment than was possible if the outer cylinder had been rotated. Two rotating cylinders were used, and the error introduced into the viscosity by inaccuracies in machining and measurements was for the best cylinders 2.01×${10}^{-3\mathrm{}}$ percent. Torsion wires were studied in detail and a method of treatment used which removed the defects of zero drift and variability of torsion constant for static and dynamic conditions. A precision telescope and calibrated comparator were used in measuring the angular deflection of the deflecting cylinder. The timing and driving mechanism consisted of a tuning fork controlled d.c. motor whose speed was constant within one part in ${10}^6$, as checked by N.A.A. time signals. The viscosity of air at 20.00°C is (1819.20±0.06) ${10}^{-7\mathrm{}}$ c.g.s. unit.