Below the surface the pressure increases, due to the depth of mercury, but the weight of mercury inside the tube below the level in the bowl counteracts the weight of that outside so that the upward pressure per square inch at the surface line is 15 pounds per square inch inside the tube no matter how much or little it is submerged. In the upper end of the tube the mercury has dropped away, leaving a complete vacuum.
Note.—Moreover it has the advantage over a scientifically graduated gauge, which would be graded at 0 for a perfect vacuum and 15, or more nearly 14.7, for atmospheric pressure, that the inch indication increases as the vacuum is more complete while the absolute pressure decreases. The inch of mercury has also the advantage over the pound as a unit for measuring the degree of vacuum or the difference between the pressure in the condenser and that of the atmosphere that there are twice as many inches in a perfect vacuum as there are pounds so that the gauge can be read more closely without fractional units. It is easier to say 23 inches than eleven and a half pounds.
The 15 pounds will force the mercury up into the tube until the column is high enough to balance that pressure. One cubic inch of mercury weighs about half a pound. It would take two cubic inches to weigh a pound and a column two inches high to exert a pressure of one pound per square inch of base, or a column 30 inches high to balance the pressure of 15 pounds.
Fig. 335.
Fig. 336.
If instead of a perfect vacuum there was a pressure of two pounds in the upper end of the tube the column would have to balance a pressure of 15-2 = 13 pounds and would be 26 inches high. As the absolute pressure in the top of the tube gets greater, that is to say, as the difference between that pressure and that of the atmosphere or the so-called vacuum gets less, the column of mercury gets lower, and its height is a measure of the completeness of the vacuum.