Why Does Shaking the Mercury Thermometer Temporarily Lower the Mercury Level?

Introduction

Have you ever shaken a thermometer and noticed that the mercury level seemed to drop? This is a common practice, especially with traditional mercury thermometers, but does it actually alter the temperature reading? This article explores the science behind the shaking effect of mercury thermometers, explains the reasons why this happens, and clarifies whether it affects the temperature measurement.

Thermometer Structure

A mercury thermometer consists of a long, thin glass tube with mercury (mercury, also known as quicksilver) inside. The key principle behind a mercury thermometer is based on the thermal expansion and contraction of mercury. As the temperature of the substance being measured changes, the mercury inside the thermometer expands or contracts accordingly, resulting in a visible change in the mercury column.

Shaking Effect

When you shake a mercury thermometer, you introduce kinetic energy to the mercury, which causes the mercury to move. This movement can affect the visible mercury level in the thermometer. Specifically, when shaken, the mercury may move up the tube and break the column, creating a gap. As a result, the visible mercury level may drop momentarily, giving the illusion that the temperature has changed.

Resetting the Thermometer

In some instances, shaking the thermometer is used as a way to reset it to a lower temperature reading. This is particularly useful with thermometers that have previously measured high temperatures. The shaking action can help bring the mercury down, allowing for a new, more accurate measurement. This is a common practice with mercury thermometers that are designed to remain at their highest indication until shaken down.

Temperature Measurement

The actual temperature reading is a result of the thermal equilibrium between the mercury and the surrounding environment. Once the thermometer is allowed to stabilize, the visible mercury level will accurately reflect the temperature of the substance being measured. The shaking only temporarily affects the mercury level and does not change the actual temperature being measured until the thermometer stabilizes again.

It is crucial to understand that while the visible mercury level may drop after shaking, this does not mean the temperature of the substance being measured has changed. It is merely the mercury column that momentarily drops due to the shaking action.

Shaking vs. Swinging

It is important to clarify that when you shake a thermometer, you are not just shaking it; you are actually swinging it back and forth. This action allows the air around the mercury to "flow" and take away some of the heat energy from the thermometer. Mercury, being highly sensitive to heat, expands when heated and contracts when cooled. This means that any heat energy in the thermometer is transferred to the container or substance being measured, and while the visible mercury level may drop, the temperature of the substance itself is not significantly affected.

Conclusion

Shaking a mercury thermometer temporarily lowers the mercury level by creating a gap in the mercury column but does not change the actual temperature being measured until the thermometer stabilizes again. There are some thermometers, particularly medical ones, that are designed to leave the mercury at its highest indication until shaken down. Other thermometers do not typically form a gap in the mercury as the temperature goes down. Understanding the science behind this phenomenon is crucial for ensuring accurate temperature readings.

Acknowledgement

This article provides an in-depth look at the common practice of shaking mercury thermometers and clarifies its effects on temperature measurement. By understanding the principles of thermal expansion and contraction, you can better appreciate the importance of allowing thermometers to stabilize before reading the temperature accurately.