O Level Physics Notes: the Fortin Barometer
The zero of the scale is at the tip of an ivory pointer fixed to
the lower end of the brass tube. The mercury reservoir is a leather
bag which can be raised or lowered by a screw. Before taking a
reading the screw is adjusted until the tip of the pointer just
touches it's image in the surface of the mercury. The pressure due to
a column of liquid of density
and
height
is
given by
where
If
we ignore the variation of the density of mercury with temperature
and the variation of
over
the surface of the Earth, the pressure is proportional to the height
of the mercury column, It is sufficient to give the pressure in terms
of the height of the mercury column.