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The purpose of this report is to describe a source of the ambiguity in measuring a low pressure by using the McLeod gauge. It has been found that the capillary depression is obstructive of the accurate measurements in pressure. Particular attentions have been paid to the inner surface of glass capillary, because the reproducibility of the capillary depression strongly depends on the surface conditions of capillary.
The computation, of which the results theoretically predict the shape of meniscus, has been carried out with a high-speed digital computor, IBM-7090, according to Eq. 4. A table capable of obtaining the capillary depression, the meniscus height, and the contact angle for fine capillaries of 4 to 0.1 mm in inside diameter, has been made. Consequently, it has become possible to separate the ambiguity due to the contact angle from that due to the surface tension as shown in Fig. 6.
Comparing the observed values with this table, the following results have been revealed that :
(1) All the observed values are in good agreement with the theoretical curves, independent of the capillary species,
(2) The capillary depression is different depending on the glass species and the surface condition as shown in Fig. 6,
(3) The contact angles are in 119°-130° for the untreated Pyrex capillary, being in 138°-160° for the Pyrex one roughened by Rosenberg's method, as in Fig. 6 and 7,
(4) Tapping is effective of reducing the extent of capillary depression, and
(5) The surface tension seems to be vatiable.

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