Barometer

Taking and Adjusting Readings

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A Description

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 surface.

B. so that the tip of the needle just touches the mercury surface.

C. so that the mercury fills the entire reservoir.

D. only if it is raining outside.

Answer

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 the barometer.

Barometer

A Description

Using a Barometer

Taking and Adjusting Readings

Before the atmospheric pressure can be read from the scale, the mercury reservoir must be properly adjusted.

Next, slide the movable scale piece until its bottom edge is aligned with the meniscus of the mercury in the tube.

A. so that the tip of the needle is 1 cm below the mercury surface.

B. so that the tip of the needle just touches the mercury surface.

C. so that the mercury fills the entire reservoir.

D. only if it is raining outside.

Many barometers utilize a Vernier scale.

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 the barometer.

A student records a pressure of 735.4 mmHg using the barometer shown in the picture above. What should the reading have been?

Answer