Abstract
A source of the irreproducibility in measuring low pressure with a McLeod gauge is described. Particular attention has been paid to the capillary phenomena in narrow tubes. The form of a meridional section of a mercury meniscus in cylindrical tube has been calculated with a high-speed digital computer from the Laplace's differential equation which has an entirely theoretical basis. Experimental data for capillary tubes differing in material, bore, and condition of inner surface have been compared with the computed values. It is observed that the capillary depression extensively varies, depending presumably on the surface condition of glass. Cancellation of the capillary depression, therefore, can be hardly expected even in the conventional McLeod gauges having twin capillaries. Tapping is found to be effective in reducing the irregular variation of the capillary depression.
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