A barometer is used to measure atmospheric pressure. A barometer
is composed of a tube filled with mercury that opens into a mercury
reservoir. Atmospheric pressure exerts force on the reservoir,
causing the mercury in the tube to rise or fall depending upon
the amount of pressure on the reservoir. Using a Vernier scale,
the atmospheric pressure can be measured, usually in units of
millimeters mercury (mmHg).
Using a Barometer
Before the atmospheric pressure can be read from the scale,
the mercury reservoir must be properly adjusted.
Video: Adjusting the reservoir (
1.65 M ) Text description
Next, slide the movable scale piece until its bottom edge
is aligned with the meniscus of the mercury
in the tube.
The mercury level in the reservoir should be adjusted:
A. so that the tip of the needle is 1 cm below the mercury
B. so that the tip of the needle just touches the mercury
C. so that the mercury fills the entire reservoir.
D. only if it is raining outside.
Taking and Adjusting Readings
Many barometers utilize a Vernier scale.
A vernier scale is an auxiliary sliding scale used to more
easily read the values on a fixed main scale. Its purpose is to
allow accurate readings, rather than estimations, between the
smallest graduations on the fixed scale. A vernier scale commonly
has ten graduation marks. Each division on the Vernier scale is
nine-tenths of the size of the finest division on the main scale.
When the sliding scale piece has been aligned with the meniscus,
read the main, fixed scale to the last certain digit. The last
certain digit on the main scale is the graduation just below the
zero on the Vernier scale. The mark on the Vernier scale that
directly lines up with a graduation mark on the main scale is
the last digit in your reading.
Video: Reading the Vernier scale
( 3.54 M )
After obtaining a pressure reading, this measurement should
be adjusted for the effect of temperature
on the expansion of the mercury, brass, and glass that compose
A student records a pressure of 735.4 mmHg using the barometer
shown in the picture above. What should the reading have been?
Copyright © 1995-1996 NT Curriculum Project, UW-Madison