A mercury thermometer is a tool used to measure temperature. There are a number of different model types, sizes, and specifications, but the general operating principle is about the same in all cases. The thermometer usually has a small reserve of liquid mercury in its base, which is most often pressurized; above it is usually a glass or synthetic tube with a calibrated temperature scale. Mercury, which is a chemical element, rises and falls in response to temperature changes, and watching where it stops along the scale provides a reading of how hot or cold a particular substance is. Many of these tools are used in medicine and around the home to gauge body temperature, though they can also be used outside and in more professional meteorological situations. In some instances they’re also used in kitchens, usually as a way of monitoring warm liquids, but most modern chefs prefer more food-specific gauges. There are usually a number of different options and models available depending on the intended use.
The basic instrumentality is usually pretty simple. Mercury, which is also sometimes known as quicksilver, is a silvery liquid that responds very well to atmospheric changes, particularly temperature and barometric pressure. The German physicist Gabriel Fahrenheit is credited with inventing the first mercury thermometer in 1714, which he made with a glass tube attached to a small well of mercury. That mercury was secured in a metal tip. Metal is usually regarded as a good conductor of heat, and Fahrenheit noticed that when that tip was placed into liquids of varying temperatures, the mercury rose and fell in the glass chamber. He then developed a system of calibration to create a scale for easy reading. Most modern models work this way, and though advancements have been made in terms of presentation the basics are usually about the same.
Mechanics and How They Work
Most thermometers hold between 0.02 ounces (0.5 g) and 0.1 ounces (3 g) of mercury. Thick glass generally encases the temperature-sensitive liquid metal. When the tool is placed outdoors or in the body, the metal tip heats and mercury stretches along the length of hollow space within the tube. Higher temperatures cause mercury to grow longer, while low temperatures cause it to fall or shrink back down. These thermometers are usually marked with temperature readings on the Fahrenheit or Celsius scale.
Most Common Uses
These sorts of thermometers are most commonly used in medicine and meteorology. Medical doctors often use a version known as the maximum mercury thermometer to measure human body temperature. This type can be used either orally, which is to say slipped under the tongue, or rectally.
A maximum mercury-style thermometer works in much the same way as the standard version, but the mercury does not shrink back into the tip when removed from a heat source. This allows temperature readings to be kept until mercury is forced back into the tip by centrifugal force, or shaking the thermometer with the head pointed toward the floor. Nitrogen gas is what usually holds the mercury in place, and usually has to be inserted during the manufacturing process.
Mercury-driven thermometers may also be used by meteorologists to measure air temperature. Standard mercury freezes at -37.89°F (-38.83°C). In order to measure temperatures below this point, meteorologists will often mix mercury with thallium alloy. Thallium reduces the freezing point to -78°F (-61.1°C). If mercury freezes, nitrogen can be caught behind the metal causing the thermometer to stop working.
Mercury can be toxic if consumed, and it can also cause skin and eye damage if touched or respiratory distress if inhaled. This has led some people to question the wisdom of having mercury so readily accessible in homes. Within the confines of the thermometer there isn’t usually any safety risk, but should the glass break, exposure is likely. Spilled mercury can be very challenging to clean, since the loose liquid often forms rounded “balls” that can bounce and scatter across a floor or room.
Most modern thermometers are made of much stronger glass then their original counterparts, and the amount of mercury contained in each is quite low in any event. Just the same, due to the dangers associated with mercury poisoning, many countries around the world, most notably in Europe, have banned mercury-based thermometers for medical use. It is also difficult to purchase mercury thermometers for home use in many of these countries.
The United States has not banned mercury in medical settings, though many advocacy groups have recommended a number of safety precautions. The American Academy of Pediatricians and the United States Environmental Protection Agency, for instance, have each suggested alternative temperature-measuring devices for personal use in order to reduce the risk of accidental exposure.