It is possible to have mercury 76 cm high in a tube that is straight and
open at the bottom, but not likely. When you start with a mercury
filled tube that is longer than 76 cm and you remove your finger and the
mercury starts to come out, it is likely that some air will bubble up
through the mercury. The main thing you accomplish by putting the open
tube in a pan of mercury before removing your finger is the prevention
of air entering the tube.
You can dispense with the pan of mercury if the tube is bent into a U so
that the open end faces up. If the sealed end extends more than 76 cm
above the open end, and the whole tube is filled with mercury and your
finger is covering the open end, then when you remove your finger
mercury will come out until the mercury in the sealed end is 76 cm above
the open end. Air cannot get in the open end because it would have to
"float downward" which it cannot do.
Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu