But the inaccuracy is so small as to be
practically negligible.
A Pressure Recorder.
[Transcribers note: Even with the precautions used in this project, health
standards of 2004 would consider any exposure to mercury dangerous. Water
could be substituted and the column lengths scaled up by about 13.5.]
If mercury is poured into a vertical tube closed at the bottom, a pressure
is exerted on the bottom in the proportion of approximately one pound per
square inch for every two inches depth of mercury. Thus, if the column is
30 inches high the bottom pressure is slightly under 15 lbs. per square
inch.
This fact is utilized in the pressure recorder shown in Fig. 167, a
U-shaped glass tube half filled with mercury. A rubber tube is attached to
the bent-over end of one of the legs, so that the effects of blowing or
suction may be communicated to the mercury in that leg. Normally the
mercury stands level in both tubes at what may be called the zero mark. Any
change of level in one leg is accompanied by an equal change in the
opposite direction in the other. Therefore, if by blowing the mercury is
made to rise an inch in the left leg, the pressure exerted is obviously
that required to support a two-inch column of mercury--that is, 1 lb.
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