Hmmmmmm. I was under the impression that sea level pressure
(28.2 or 28.5 not sure) was the most vacuum you could get. I am
sure I must be wrong.. JB...
What’s a vacuum? (Revisited)
G’day; it’s nothing. But that isn’t helpful, so lets start
G’day; Take a piece of thick walled glass tube at least a
metre long. Seal one end completely. Fill the tube completely
with mercury. Put your thumb over the end and invert it into a
vessel of mercury. After removing your thumb, the mercury column
in the tube will fall to a certain height; measure the height
(inches, centimetres,- anything you like) of that column from the
surface of the pool and that is the present barometric pressure.
What’s that? There is a mountain of air (atmosphere) above and
all around us, which like you and me, has mass because gravity is
acting upon it (and us). So it is exerting a pressure, just as
we exert a pressure on the floor… So we can now measure the air
pressure acting on the surface of the mercury pool and express it
in inches and it will be whatever the barometric pressure happens
to be today. In cyclone conditions it can fall to 28 inches; in
anticyclonic conditions it can rise to 31 inches. The average is
around 30 inches. So can see that the higher up a mountain you
go, the lower is the air pressure around you. You’re not asleep
at the back, you lot? No? well, let’s get on.
Now, instead of a sealed tube dipping into mercury, suppose you
get another similar, unsealed tube and put that in the mercury
pool too. Now attach your vacuum pump to the end that is open.
When your pump has drawn out as much of the air as it is capable
of, your second mercury column has risen a height of 29 inches.
Today the atmosphere is calm, and the barometer (the sealed tube)
reads, say, 30 inches. So, you don’t have a perfect vacuum.
Unlike the space at the top of the sealed tube. (If you want to
argue that it isn’t really a perfect vacuum, but holds mercury
vapour, come and see me after class. especially if you’re female
and good looking) I could go on, but I see eyes drooping all
over the place so I will leave it at that, and besides, I’ll only
cause worse confusion. Things can get quite complicated;
scientists would tell you that you have a vacuum of one inch or
25.4mm. (see what I mean?) So, ENOUGH! Cheers, and drive
/ / Johnb@ts.co.nz
(_______) In sunny temperate Mapua NZ -