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In article <14816@topaz.rutgers.edu>, ron@topaz.rutgers.edu (Ron Natalie) writes:
> You ought to be able to fabricate one out of some mercury filled
> speaker cables.

	Haha!  That was a cheap shot, but was quite amusing.

	Actually, there is a grain of truth in what you said.  If you take
a glass tube about 34 inches long and sealed at one end, fill it with mercury
and carefully invert it into a small cup of mercury (say, 25 cc), you can
create a "traditional" mercury barometer.
	It is rather easy to measure the height of the mercury column using
capacitance measurement (I'd consider using a 1 MHz measurement frequency).
To create such a variable capacitor, carefully cover about 6 inches of the
outside of the glass tube with tightly wrapped aluminum foil.  The aluminum
foil should be tightly wrapped around the glass tube to assure uniform
contact, and then fixed in position with cement or varnish at its seam and
at the ends.  The aluminum foil should be centered around the "measurement
range" (say, +/- from 28 inches).  The other end of the capacitor is formed
by placing a wire in the mercury reservoir.  It would be a good idea to shield
the completed mercury barometer using metal screening (keep out RFI, and
prevent the radiation of the capacitance bridge excitation frequency).
	For practical purposes, barometric pressure is a linear function
of measured capacitance using _this_ technique (there are other ways to
create a capacitance barometer using mercury, but this is the simplest).
	There are a number of bridge circuits available to measure capacitance
which will give you a voltage output; there are also circuits which operate
on other principles, such as changing the frequency of an oscillator which
in turn feeds a frequency-voltage converter, etc.

	Concerning the hazards of mercury: Mercury is a toxic substance if
ingested or inhaled as vapor.  However, there is nothing inherently unsafe
about building such a mercury barometer, PROVIDED:

1.	You work with the mercury within a large, shallow pan to entrain
	any accidental spillage.

2.	You are careful not to spill any mercury, and you thoroughly wash
	your hands after handling any items which may have been in contact
	with mercury.

3.	You keep the mercury barometer in a ventilated, but secure location
	where children or pets have no direct access.

4.	You place the mercury barometer within another vessel of sufficient
	capacity to contain the mercury should the glass column or reservoir
	be broken.

	Since you are measuring barometric pressure, the safest place to use
such a barometer is to place it outdoors in a locked container attached to
the ground or to a building.  The container should be ventilated, but not
enough allow precipitation to enter the container and create capacitance
surement artifacts.

	Of course, if you feel that mercury is too hazardous, you can always
use water instead.  But then you'd need a tube about 35 feet high... :-)

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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