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Torricelli



The thread about siphoning made me
think about a Torricelli tube. And I have
a question. Suppose a one-meter long
tube (closed at one end) is filled with
mercury. I cover the open end of the tube
with a finger, turn the tube up side down,
and immerse my finger into a dish with
some mercury. Then I remove the finger
and see that some mercury goes into the
dish. The length of the column becomes
76 cm and vacuum is formed in the upper
part of the tube. This is what Torricelli did.

Here is my question. Suppose that I do
the same without a dish. I simply slide the
finger to the side and allow the excess
mercury to fall into a box with sand. I do
this very slowly (to make sure that the
acceleration of mercury inside the tube
is negligible). Will I observe the 76 cm
column of mercury standing above the
open end of my tube (after the finger is
finally removed)? If not then why?
Ludwik Kowalski


On Sunday, Mar 14, 2004, at 20:12 America/New_York, Josip Slisko wrote:

It is logically possible that there are two mecchanism for siphon -
effect:

1. Atmospheric pressure minus hidrostatic pressure in the arms of
different LENGHTS give rise to two different PRESSURES at the mouths of
short and long arms, and the liquid moves in the direction of the LOWER
PRESSURE.

The comment of Hero from Alexandria shows that it is not necessarly the
direction of the BIGGER WEIGHT.

2. In the absence of external pressure, the possible mecanism is that
there are two arms of different WEIGHT and that the liquid moves
in the direction of the BIGGER WEIGHT. This mechanism, of course,
depends
on the existence of, at least, of one liquid being resistent to
boiling in
vacuum and able to sustain positive stress necessary for pulling.

It would be nice to know if anybody has seen a published result of an
experiment which show that such a siphon does work.

If the existence of two mechanisms for siphoning were an experimental
fact, than we should teach siphon-effect more carefully, avoiding to
suggest that there is only one mechanism (either atmospheric
pressure based or gravity-cohesion based).

Josip Slisko