Ask A Scientist General Science Archive

Author:     Virginia Reid
Text:       How does a barometer work?

Response #:  1 of 1
Author:      Ronald Winther
Text:        The type of barometer most commonly found today is called an 
aneroid (meaning "without air") barometer.  The "heart" of such a barometer is 
a metal box that has been evacuated (i.e., the air inside it has been pumped 
out, that is the "without air" part) and sealed.  The top and bottom of the 
box are thin corrugated plates, and inside the box are one or more springs 
that push the top and bottom of the box against the outside air pressure, 
which tries to crush the box.  When the outside air pressure increases, the 
springs are compressed a bit; when the pressure decreases, the springs can 
expand.  This causes the thin metal plates of the box to flex a bit in or out, 
respectively.  This movement of the plates is conveyed through a system of 
levers to the pointer on the front of the barometer indicating the air 
pressure.  Technically, a barometer detects only changes in pressure, not the 
actual pressure, so it has to be "set"; that is, you need to find out the 
current barometric pressure at some time (from the local weather news, for 
example) and turn the needle on the barometer to that value (just as, when you 
first plug in a clock, you need to find out what time it is and set the clock 
to that time).  This information was obtained mostly from the book  "The Way 
Things Work:  An Illustrated Encyclopedia of Technology," an excellent source 
for finding out how a lot of everyday devices work.