A thermometer is a device used to measure temperature. The thermoscope, developed by Galileo around 1592, was the first instrument used to measure temperature qualitatively. It was not until 1611 that Sanctorius Sanctorius, a colleague of Galileo, devised and added a scale to the thermoscope, thus facilitating quantitative measurement of temperature change. By this time the instrument was called the thermometer, from the Greek words therme ("heat") and metron ("measure"). About 1644 it became obvious, however, that this instrument—comprising a large bulb flask with a long, open neck, using wine to indicate the reading—was extremely sensitive to barometric pressure. To alleviate the problem, Grand Duke Ferdinand II of Tuscany developed a process to hernetically seal the thermometer, thereby eliminating outside barometric influence. The basic form has varied little since.
There are many types of thermometers in use today: the recording thermometer uses a pen on a rotating drum to continuously record temperature readings; the digital readout thermometers often coupled with other weather measuring devices; and the typical household types hung on a wall, post, or those used for medical purposes.
With a thermometer, temperature can be measured using any of three primary units: Fahrenheit, Celsius, or Kelvin. At one point during the eighteenth century, nearly 35 scales of measure had been developed and were in use.
In 1714 Gabriel Daniel Fahrenheit, a Dutch instrument maker known for his fine craftsmanship, developed a thermometer using 32 (the melting point of ice) and 96 (the standard temperature of the human body) as his fixed points. It has since been determined that 32 and 212 (the boiling point of water) are the scale's fixed points, with 98.6 being accepted as the healthy, normal body temperature.
Swedish scientist Anders Celsius, in 1742, assigned 0 degrees as the point at which water boiled and 100 degrees as the point at which ice melted. These two figures were eventually switched—creating the scale we know today—with 0 degrees as the freezing point of water and 100 degrees as the boiling point. Use of this scale quickly spread through Sweden and to France, and for two centuries it was known as the centigrade scale. The name was changed in 1948 to Celsius to honor its inventor.
In 1848 another scientist, Lord Kelvin (William Thomson), proposed another scale based on the same principles as the Celsius thermometer, with the fixed point of absolute zero set at the equivalent of -273.15 degrees Celsius (the units used on this scale are called Kelvin [K]). The freezing and boiling points of water are registered at 273 K and 373 K respectively. The Kelvin scale is most often used in scientific research studies.
The operating principle of a thermometer is quite simple. A known measure of liquid (mercury, alcohol, or a hydrocarbon-based fluid) is vacuum-sealed in a glass tube. The liquid expands or contracts when air is heated or cooled. As the liquid level changes, a corresponding temperature scale can be read to indicate the current temperature.
Thermometers are designed according to predefined standards identified by the National Institute of Standards and Technology (NIST, formerly the National Bureau of Standards) and standard manufacturing practices. Within the regulatory guidelines there are provisions for the custom manufacture of thermometers. Custom thermometers can be as varied as those who use them. Different sizes exist for the amount, weight, and length of glass used, the type of liquid filled into the glass, the frequency of gradations laid onto the glass tube or enclosure, and even the color of the gradation scale marks.
A design engineer will look at the travel limits for the liquid to be used in the thermometer. Once precise limits are established, the dimensions of the glass tube and size of the glass bulb can be determined.
Use of electronic components in thermometers has grown. Many of today's broadly used thermometers contain digital readouts and sample program cycles to feed back the current temperature to a light-emitting diode (LED) or liquid crystal display (LCD) panel. For all the electronic wizardry available, a thermometer must still contain a heat-cold sensitizing element in order to respond to environmental changes.
Thermometers consist of three basic elements: spirit-filled liquid, which responds to changes in heat and cold; a glass tube to house the temperature-measuring liquid; and black ink to color in the engraved scale marks with legible numbers. In addition, other elements are necessary for the manufacture of thermometers, including a wax solution used to engrave the scale marks on the glass tube; an engraving engine that makes permanent gradations on the glass tube; and a hydrofluoric acid solution into which the glass tube is dipped to seal the engraving marks.
The glass material forming the body of the thermometer is usually received from an outside manufacturer. Some thermometer products are made with an enclosure, which can be made of plastic or composites and may contain scale gradations as opposed to having these on the glass tube itself. The enclosure also serves to protect and mount the thermometer on a wall, post, or in a weather shelter box.
Although there are numerous types of thermometers, the production process for the most common of these—the classic household variety—is described below.
The vacuum is then gradually reduced, forcing the fluid down near the top of the tube. The process is the same when mercury is used, except heat is also applied in the vacuum chamber.
The manufacturing process is controlled by widely adopted industry standards and specific in-house measures. Manufacturing design considerations include quality control checks throughout the production process. The equipment used to perform fabrication tasks must also be carefully maintained, especially with updated design protocol.
Waste materials accrued during manufacturing are disposed of according to environmental regulatory standards. During the manufacturing cycle, equipment used to heat, evacuate, and engrave the thermometer must be checked and calibrated regularly. Tolerance tests are also performed, using a known standard, to detennine the accuracy of the temperature readings. All thermometers have a tolerance for accuracy. For the common household, this tolerance is usually plus or minus 2 degrees Fahrenheit (16 degrees Celsius). For laboratory work, plus or minus 1 degree is generally acceptable.
Although the longstanding simple glass thermometer is unlikely to change, other thermometer
Gardner, Robert. Temperature and Heat. Simon & Schuster, 1993.
McGee, Thomas D. Principles and Methods of Temperature Measurement. John Wiley & Sons, 1988.
Pavese, F., ed. Modern Gas-Based Temperature and Pressure Measurements. Plenum Publishing, 1992.
Alderman, Lesley. "Stick It In An Ear," Money. January, 1993, p. 19.
"Fever Thermometers," Consumer Reports. December, 1988, p. 214.
DiChristina, Mariette. "Thermometer You Swallow," Popular Science. March, 1990, p. 113.
"Taking the Heat from Inside," Discover. June, 1988, p. 12.
Joyce, Mary E. "Thermometer Assists in Cancer Therapy," Design News. September 21, 1992, p. 46.
— Matthew Fogel