mercury thermometer

Close up of a maximum thermometer. The break in the column of mercury is visible

A mercury-in-glass thermometer, invented by German physicist Daniel Gabriel Fahrenheit, is a thermometer consisting of mercury in a glass tube. Calibrated marks on the tube allow the temperature to be read by the length of the mercury within the tube, which varies according to the temperature. To increase the sensitivity, there is usually a bulb of mercury at the end of the thermometer which contains most of the mercury; expansion and contraction of this volume of mercury is then amplified in the much narrower bore of the tube. The space above the mercury may be filled with nitrogen or it may be a vacuum.

History

The thermometer was used by the originators of the Fahrenheit and Celsius temperature scales.

Anders Celsius devised the Celsius scale, which was described in his publication the origin of the Celsius temperature scale in 1742.

Celsius used two fixed points in his scale: the temperature of melting ice and the temperature of boiling water. This wasn't a new idea, since Isaac Newton was already working on something similar. The distinction of Celsius was to use the melting temperature and not the freezing temperature. The experiments for reaching a good calibration of his thermometer lasted for 2 winters. By performing the same experiment over and over again, he discovered that ice always melted at the same calibration mark on the thermometer. He found a similar fixed point in the calibration of boiling water vapour (when this is done to high precision, a variation will be seen with atmospheric pressure). At the moment that he removed the thermometer from the vapour, the mercury level climbed slightly. This was related to the rapid cooling (and contraction) of the glass.

The air pressure influences the boiling point of water. Celsius claimed that the level of the mercury in boiling water is proportional to the height of the barometer.

When Celsius decided to use his own temperature scale, he originally defined his scale "upside-down", i.e. he chose to set the boiling point of pure water at 0 °C (212 °F) and the freezing point at 100 °C (32 °F)[1]. One year later Frenchman Jean Pierre Cristin proposed to invert the scale with the freezing point at 0 °C (32 °F) and the boiling point at 100 °C (212 °F). He named it Centigrade [2].

Finally, Celsius proposed a method of calibrating a thermometer:

1. Place the cylinder of the thermometer in melting pure water and mark the point where the fluid in the thermometer stabilises. This point is the freeze/thaw point of water.
2. In the same manner mark the point where the fluid stabilises when the thermometer is placed in boiling water vapour.
3. Divide the length between the two marks into 100 equal pieces.

These points are adequate for approximate calibration but both vary with atmospheric pressure. Nowadays, the triple point of water is used instead (the triple point occurs at 273.16 kelvins (K), 0.01 °C).

Maximum thermometer

A special kind of mercury thermometer, called a maximum thermometer, works by having a constriction in the neck close to the bulb. As the temperature rises the mercury is pushed up through the constriction by the force of expansion. When the temperature falls the column of mercury breaks at the constriction and cannot return to the bulb thus remaining stationary in the tube. The observer can then read the maximum temperature over a set period of time. To reset the thermometer it must be swung sharply. This is similar to the design of a medical thermometer.

Physical properties

Mercury will solidify (freeze) at -38.83 °C (-37.89 °F) and so may only be used at higher temperatures. Mercury, unlike water, does not expand upon solidification and will not break the glass tube, making it difficult to notice when frozen. If the thermometer contains nitrogen the gas may flow down into the column and be trapped there when the temperature rises. If this happens the thermometer will be unusable until returned to the factory for reconditioning. To avoid this some weather services require that all mercury thermometers be brought indoors when the temperature falls to -37 °C (-34.6 °F). In areas where the maximum temperature is not expected to rise above -38.83 °C (-37.89 °F) a thermometer containing a mercury-thallium alloy may be used. This has a solidification (freezing) point of -61.1 °C (-78 °F).

Phase out in North America

Today mercury thermometers are still widely used in meteorology, however in other usage they are becoming increasingly rare, as many countries have banned them outright from medical use^{[citation needed]}. Some manufacturers use a liquid alloy of gallium, indium, and tin, galinstan, as mercury replacement.

The typical "fever thermometer" contains between 0.5 to 3 g (.3 to 1.7 dr) of elemental mercury.^[1] Swallowing this amount of mercury would, it is said, pose little danger but the inhaling of the vapour could lead to health problems.^[2]

In the United States both the American Academy of Pediatrics^[3] and the United States Environmental Protection Agency^[4] recommend that alternative thermometers be used in the home.^[5]

In Canada, the Federal agency, Environment Canada since 2002 has been working with groups to exchange mercury thermometers^[6]

What to do if you break a mercury thermometer

The United States Environmental Protection Agency gives detailed guidelines ^[7] for cleaning up the site in case you break a mercury-in-glass thermometer. Please read the guidelines there; here, we will only give a short overview of what you should never do in case you break a mercury thermometer:

• Never walk through the mercury spills nor let anyone else do it.
• Don't touch the spills with bare hand; use latex gloves instead.
• Never use vacuum cleaner to clean up the spills. Doing so will only increase the amount of dangerous mercury vapor in the air.
• Never use a brush or broom to clean up the spills. Doing so will only make them smaller and spread them.
• Don't wash mercury contaminated clothes in your washing machine. They may spread contamination to your washing machine and pollute water.
• Don't walk around in contaminated clothes or shoes.
• Mercury is hazardous waste - don't throw it in a regular waste container. Contact your local authorities to find sites and procedures of hazardous waste disposal.

References

1. ^ Environment Canada - Mercury and the Environment
2. ^ Health Canada - Mercury and Human Health
3. ^ AAP Supports Elimination of Mercury-Containing Thermometers
4. ^ Things are Heating Up with Mercury in Thermometers
5. ^ Technical Report: Mercury in the Environment: Implications for Pediatricians
6. ^ Environment Canada - "Mercury Fever Thermometer Take Back"
7. ^ EPA - guidelines for mercury spills removal

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