Re: I have the elusive dangerous substance in my possession!

John G wrote:

"DoN. Nichols" <dnichols@xxxxxxxxxxx> wrote in message
news:f3vjc301e2n@xxxxxxxxxxxxxxxxxxxx
According to John G <Greentest@xxxxxxxxxxxxxx>:

"Rich Grise" <rich@xxxxxxxxxxx> wrote in message
news:pan.2007.05.31.21.09.23.650221@xxxxxxxxxxxxxx

[ ... ]

Pardon me if this is a stupid question, but couldn't you just get a
piece of glass tube and make a replacement?

[ ... ]

The tube is quite a small bore with thick wall.
I have not figured out how to refill it even if I could get the glass
sealed and sealed to the top of the mercry resevoir.
It seems the mercury was just in the steel (iron) pot but what stops
it
spilling thru the small breathng hole I have not worked out.

The very atmospheric pressure which you are measuring. The only
thing which needs to be sealed is the top of the tube (done by slowly
rotating it in a flame until it closes, and then letting it cool
slowly,
by backing off the flame while you keep rotating it).

The "seal" to the top of the reservoir is only to keep it from
rattling. The breathing hole is there to let the atmospheric pressure
changes make their way to the top of the mercury reservoir.

I know where ther is an identical one in a museum here in Sydney and
I
should go and talk to th curator one day.

You should not need that -- just get some glass tubing from a
chem lab supply place, seal it as I described, and fill it with
mercury.

There are some poor pictures at
http://members.ozemail.com.au/~john.griffiths/others/Barometer/

It looks like well worth rebuilding.

If the glass is as thick as it appears, you will have to be slow
and careful about the sealing of the end of the tube, since the glass
will crack if you cool (or heat) it too quickly. And it may be more
of
a trick finding it. Something closer to heavy gauge capillary tubing,
perhaps. :-)

Good Luck,
DoN.

--

I quite understand how it all works and even though I am over 70 I can
remember a school teacher building a fortin barometer in a science class
with just a tube and a beaker of mercury and dipping his finger in the
mercury as he inverted the full tube. (All this before the do gooders
and workplace safety.)

I have a short glass tube full of mercury in the window so the barometer
looks real but is only correct when the pressure is about average. ( I
sealed the bottom of the tube myself and it has a plastic stopper at the
top.)

I understand the reason for the hole but the one in the museum is lying
horizontal in a fitted case seeming to imply it was transported that way
and I am not sure if I have all the bits as the person I bought it from
did not even know what it was.

One day I will get some better photos and also photograph the works of
the banjo barometer which has a tube, a reservoir and a system of
pullies and string to transfer the position of a weight floating on the
reservoir to the dial on the front.
--
John G.



In junior high school we made a barometer out of a milk carton, a metal
can that fit inside of the milk carton, some string, a penny, a baloon,
a rubber band and a large paper clip. The balloon was stretched over
the end of the metal can. The string was attached to the center of the
balloon. The can was placed inside of the milk carton with the top cut
off. The paper clip was opened up and pushed horizontally though both
sides of the milk carton. The string was wrapped around the paper clip
several times and the free end attached to the penny. The end of the
paper clip was bent at 90 degrees so that as the shaft of the paperclip
turned as the balloon moved with changing pressure, the end of the paper
clip would rotate in a circle. The thing worked real good once you had
it calibrated with the local weather forcast. A piece of paper glued on
the side of the milk carton to make a face for the paper clip pointer.


John
.

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