A thermometer is a device used to measure temperature.
A thermometer is used in health care to measure and monitor body temperature. In an office, hospital or other health care facility, it allows a caregiver to record a baseline temperature when a patient is admitted. Repeated measurements of temperature are useful to detect deviations from normal levels. Repeated measurements are also useful in monitoring the effectiveness of current medications or other treatments.
The patient's temperature is recorded to check for pyrexia or monitor the degree of hypothermia present in the body.
All health care professionals use thermometers. All health care facilities have thermometers. Most homes also have thermometers.
A thermometer can use any of several methods to register temperature. These include mercury; liquid-in-glass; electronic with digital display; infrared or tympanic; and disposable dot matrix. A thermometer can be used in a clinical or emergency setting or at home. Thermometers can record body temperatures in the mouth (oral), armpit (axillary), eardrum (tympanic membrane), or anus (rectal).
A mercury thermometer consists of a narrow glass stem approximately 5 in (12.7 cm) in length with markings along one or both sides indicating the temperature scale in degrees Fahrenheit, Centigrade or both. Liquid mercury is held in a reservoir bulb at one end and rises through a capillary tube when the glass chamber is placed in contact with the body. Mercury thermometers are not used in modern clinical settings.
Electronic thermometers can record a wide range of temperatures between 94°F and 105°F, (35°C and 42°C) and can record oral, axillary, or rectal temperatures. They have temperature sensors inside round-tipped probes that can be covered with disposable guards to prevent the spread of infection. The sensor is connected to a container housing the central processing unit. The information gathered by the sensor is then shown on a display screen. Some electronic models have such other features as memory recall of the last recording or a large display screen for easy reading. To use an electronic thermometer, the caregiver places the probe under the patient's arm or tongue, or in the patient's rectum. The probe is left in place for a period of time that depends on the model used. The device will beep when the peak temperature is reached. The time required to obtain a reading varies from 3–30 seconds.
A tympanic thermometer has a round-tipped probe containing a sensor that can be covered with a disposable guard to protect against the spread of ear infections. It is placed in the ear canal for 1 sec while an infrared sensor records the body heat radiated by the eardrum. The reading then appears on the unit's screen.
Digital and tympanic thermometers should be used in accordance with the manufacturer's guidelines.
Disposable thermometers are plastic strips with dots on the surface that have been impregnated with temperature-sensitive chemicals. The strips are sticky on one side to adhere to the skin under the armpit and prevent slippage. The dots change color at different temperatures as the chemicals in them respond to body heat. The temperature is readable after two to three minutes, depending on the instrument's guidelines. These products vary in length of use; they may be disposable, reusable, or used continuously for up to 48 hours. Disposable thermometers are useful for children, as they can record temperatures while children are asleep.
Caregivers should be given training appropriate for the type of device used in their specific clinical setting.
The patient should sit or lie in a comfortable position to ensure that temperature readings are taken in similar locations each time and to minimize the effects of stress or excitement on the reading.
The manufacturer's guidelines should be followed when taking a patient's temperature with a digital, tympanic, or disposable thermometer. Dot-matrix thermometers are placed next to the skin and usually held in place by an adhesive strip. With the tympanic thermometer, caregivers should ensure that the probe is properly inserted into the ear to allow an optimal reading. The reading will be less accurate if the sensor cannot accurately touch the tympanic membrane or if the ear canal is clogged by wax or debris.
A mercury thermometer can be used to monitor a temperature in three body locations:
- Oral or sublingual. This placement is never used with infants.
- Rectal. This method is used with infants. The tip of a rectal mercury thermometer is usually colored blue to distinguish it from the silver tip of an oral/axillary thermometer.
Before recording a temperature using a mercury thermometer, the caregiver shakes the mercury down by holding the thermometer firmly at the clear end and flicking it quickly a few times with a downward wrist motion toward the silver end. The mercury should be shaken down below 96°F (35.5°C) before the patient's temperature is taken.
In axillary placement, the silver tip of the thermometer is placed under the patient's right armpit, with the patient's arm pressing the instrument against the chest. The thermometer should stay in place for six to seven minutes. The caregiver can record the patient's other vital signs during this waiting period. After the waiting period has elapsed, the caregiver removes the thermometer and holds it at eye level to read it. The mercury will have risen to a level indicating the patient's temperature.
The procedure for taking a patient's temperature by mouth with a mercury thermometer is similar to the axillary method except that the silver tip of the thermometer is placed beneath the tongue for four to five minutes before being read. In both cases, the thermometer is wiped clean and stored in an appropriate container to prevent breakage.
To record the patient's rectal temperature with a mercury thermometer, a rectal thermometer is shaken down as described earlier. A small amount of water-based lubricant is placed on the colored tip of the thermometer to make it easier to insert. Infants must be positioned lying on their stomachs and held securely by the caregiver. The tip of the thermometer is inserted into the rectum no more than 0.5 in (1.3 cm) and held there for two to three minutes. The thermometer is removed, read as before, and cleansed with an antibacterial wipe. It is then stored in an appropriate container to prevent breakage. This precaution is important as mercury is poisonous when swallowed.
Liquid-in-glass thermometers contain alternatives to mercury (such as colored alcohol), but are used and stored in the same manner as mercury thermometers.
Many digital and infrared thermometers are self-calibrating and need relatively little care. To ensure accuracy, mercury thermometers should be shaken down prior to every use and left in place for at least three minutes. They require careful storage to prevent breakage and thorough cleaning after each use to prevent cross-infection.
As of early 2003, there is a nationwide initiative to ban the sale of thermometers and blood pressure monitors containing mercury. Health activists are concerned about mercury from broken or unwanted instruments contaminating the environment. A mercury thermometer contains 0.7g (0.025 oz) of mercury; 1 g of the substance is enough to contaminate a 20-acre lake. Several states have banned the use of products containing mercury. Most retail stores have stopped selling mercury thermometers. In October 1999, the Environmental Protection Agency (EPA) advised using alternative products to avoid the need for increased regulations in years to come and to protect human health and wildlife by reducing unnecessary exposure to mercury. According to a 2001 study by the Mayo Clinic, mercury-free devices can monitor information without compromising accuracy.
A thermometer should be cleaned, disinfected and placed in an appropriate container for storage.
Breakage of a glass thermometer creates a risk of cuts from broken glass and possible mercury poisoning. Improper operation of a tympanic thermometer can cause injury to the middle ear. As digital devices have replaced glass thermometers, however, the number of injuries has declined.
An additional risk is that old or broken thermometers may give inaccurate results.
A normal body temperature is defined as approximately 98.6°F (37°C). Body temperature is not constant throughout a 24-hour period. Some variation (0.3°F) is normal. Individuals also vary in their basal temperatures (0.3°F). A fever is defined as a temperature of 101°F or higher in an infant younger than three months or above 102°F for older children and adults. Hypothermia is recognized as a temperature below 96°F (35.5°C).
Morbidity and mortality rates
Injuries caused by properly inserted and normally functioning thermometers are extremely rare.
There are no convenient alternatives to using a thermometer to measure body temperature.
See also Health history; Physical examination; Vital signs.
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dowding, d., s. freeman, s. nimmo, et al. "an investigation into the accuracy of different types of thermometers." professional nurse 18 (november 2002): 166-168.
drake-lee, a., i. mantella, and a. bridle. "infrared ear thermometers versus rectal thermometers." lancet 360 (december 7, 2002): 1883-1886.
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american academy of family physicians. 11400 tomahawk creek parkway, leawood, ks 66211-2672. (913) 906-6000. <www.aafp.org>. e-mail: firstname.lastname@example.org.
american academy of pediatrics. 141 northwest point boulevard, elk grove village, il 60007-1098. (847) 434-4000; fax: (847) 434-8000. <www.aap.org>. e-mail: kidsdoc @aap.org .
american college of physicians. 190 n. independence mall west, philadelphia, pa 19106-1572. (800) 523-1546, x2600 or (215) 351-2600. <www.acponline.org>.
american medical association. 515 n. state street, chicago, il 60610. (312) 464-5000. <www.ama-assn.org>.
american nurses association. 600 maryland avenue, sw, suite 100 west, washington, dc 20024. (202) 651-7000 or (800) 274-4262. <www.nursingworld.org>.
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L. Fleming Fallon, Jr., MD, DrPH
WHO PERFORMS THE PROCEDURE AND WHERE IS IT PERFORMED?
Most health professionals are trained in the proper operation of thermometers used in clinical settings. Most families have and use thermometers in the home.
QUESTIONS TO ASK THE DOCTOR
- What is the patient's temperature?
- In the opinion of the professional, is the recorded temperature abnormal?
- Based on the recorded temperature, what actions would the professional recommend?
"Thermometer." Gale Encyclopedia of Surgery: A Guide for Patients and Caregivers. . Encyclopedia.com. (December 16, 2017). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/thermometer
"Thermometer." Gale Encyclopedia of Surgery: A Guide for Patients and Caregivers. . Retrieved December 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/thermometer
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 glass bulb
- 1 First, the raw glass material is received from an outside manufacturer. The tube is made with a fine passage, or bore, throughout its length. The bored tubes are checked for quality; any rejected parts are sent back to the manufacturer for replacement.
- 2 The bulb reservoir is formed by heating one end of the glass tube, pinching it closed, and using glassblowing and the application of an air-driven torch to complete it. Alternately, the bulb can be made by blowing a separate piece of lab material that is then joined with one end of the glass tube. The bulb is sealed at its bottom, leaving an open tube at the top.
Adding the fluid
- 3 With the open end down in a vacuum chamber, air is then evacuated from the glass tube, and the hydrocarbon fluid is introduced into the vacuum until it penetrates the tube about 1 inch (2.54 centimeters). Due to environmental concerns, contemporary thermometers are manufactured less with mercury and more with a spirit-filled hydrocarbon liquid. Such a practice is mandated (with tolerance for a limited use of mercury) by the Environmental Protection Agency (EPA).
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.
- 4 Once full, the tube is placed upon its bulb end. A heating-out process is then conducted by placing the thermometer into a warm bath and raising the temperature to 400 degrees Fahrenheit (204 degrees Celsius). Next, the temperature is reduced to room temperature to bring the residual liquid back to a known level. The open end of the thermometer is then sealed by placing it over a flame.
Applying the scale
- 5 After the tube is sealed, a scale is applied based on the level at which the fluid rests when inserted into a water bath of 212 degrees Fahrenheit (100 degrees Celsius) versus one at 32 degrees Fahrenheit (0 degrees Celsius). These reference points for the desired scale are marked on the glass tube before engraving or silkscreening is done to fill gradations.
- 6 The range lengths vary according to the design used. A scale is picked that best corresponds to even marks between the reference points. For accuracy purposes, engraving is the preferred method of marking. The marks are made by an engraving engine after the thermometer is placed in wax. The numbers are scratched onto the glass and, once complete, the thermometer is dipped in hydrofluoric acid to seal the engraved markings. Ink is then rubbed into the marks to highlight the scale values. When enclosures are used on the scales, a silkscreening process is used to apply the marks.
- 7 Finally, the thermometers are packaged accordingly and shipped to customers.
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 forms continue to evolve. With technological advances and the more widespread use of lighter and stronger materials, manufacturers of electronically integrated temperature instruments can provide more accurate measurements of temperature with minimal equipment bulk and at an affordable price. Analog box thermometers, for example, were once used with a long wire and probe tip for in-ground temperature measurements, among other uses. Today, the probe tips are made of lighter materials, and the boxes, loaded with digital electronics, are not as bulky and square. Looking ahead, further work with the microchip may provide the impetus to fully digitize the temperature measuring process. Also, it may eventually be possible to direct an infrared beam into soil and extract a temperature reading from a target depth without even touching the soil.
Where To Learn More
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.
"Thermometer." How Products Are Made. . Encyclopedia.com. (December 16, 2017). http://www.encyclopedia.com/manufacturing/news-wires-white-papers-and-books/thermometer
"Thermometer." How Products Are Made. . Retrieved December 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/manufacturing/news-wires-white-papers-and-books/thermometer
In the earliest days of the medical profession, no device existed to measure a patient's body temperature. Evaluating a patients' body temperature depended totally on the doctor's personal methods of observation. It was not until the late sixteenth century that scientists devised an instrument able to detect changes in air temperature. It was many years later that a medical thennometer was constructed.
The first thermometers were created to measure changes in air temperature. The most famous of these was invented by Galileo (1564-1642) in 1592. Called an air-thermoscope (or air thermometer), it consisted of a long glass tube with a wide bulb at one end. When the tube was first heated the air inside expanded and some of it was naturally expelled. While still warm, the open end of the tube was placed into a flask of water. As the tube cooled, the warm air contracted, drawing water into the tube. Once the tube-and-water system reached a state of balance, any change in air temperature would cause the level of the water within the tube to rise or fall.
There were two major hindrances to the acceptance of the air-thermoscope. First, the varying sizes of the tubes made it very difficult to graduate the device and therefore establish a degree scale. Second, it was soon discovered that the air-thermoscope was unreliable. It gave widely varying readings for apparently identical temperatures.
In the 1660s it was realized that the open-ended system would react to air pressure as well as temperature. This meant that the instrument per-formed as a barometer as well as an air thermometer. This realization came several years after a solution was found in 1654 by Duke Ferdinand II of Tuscany. The duke constructed a sealed liquid-in-glass thermometer that did not vary with changes in air pressure. Coupling this new thernometer with graduations devised in 1612 by Italian doctor Santorio Santorre, the device was complete.
Perfecting the Design
European scientists quickly began working to perfect the design of the thermometer. One of the issues addressed was the need for an instrument that could travel by sea. This was because ordinary liquid thermometers were rendered unreliable by a ship's rocking motions. In 1695 French physicist Guillaume Amontons designed a thermometer composed of a tube filled with compressed air and capped with a layer of mercury. As the temperature increased the air would expand, causing the mercury level to rise. Conversely, as the temperature decreased, the mercury would fall. Another Frenchman, Rene de Reaumur (1683-1757), sought to improve upon Amontons's design by replacing the air-and-mercury system with a mixture of alcohol and water. Reaumur's thermometer was remarkable in that he devised an 80-degree temperature scale based upon the freezing and boiling points of water. These were to be the same points that would become the basis for the more widely accepted scales of Celsius and Fahrenheit.
At the turn of the eighteenth century the most important issue for scientists and instrument makers was the lack of a standard temperature scale. Because the level of glass-blowing technology was too poor to make identical thermometer tubes, every scientist's temperature scale was different. In 1717 a Dutch instrument maker Daniel Fahrenheit introduced a line of mercury-filled thermometers of nearly identical proportions. His use of mercury in very thin tubes allowed him to graduate the scale into many degrees. He used the boiling (212 degrees) and freezing (32 degrees) points of water as reference points. Fahrenheit developed the first scale to be accepted as a worldwide standard.
The Fahrenheit scale enjoyed global popularity for many years, until the introduction of the 100-degree scale by Anders Celsius in 1746. Several scientists had attempted to popularize a 100-point scale, but Celsius was the first to also utilize water's freezing and boiling points as the 0-and 100-degree marks. Originally, Celsius placed the freezing point at 100 degrees and the boiling point at 0 degrees. This was reversed in 1747, at which time the centigrade (meaning "500 steps") scale began to increase in popularity. In 1946 the Celsius scale was adopted by most of the world as the official temperature scale.
The Medical Thermometer
Probably the most familiar thermometer is that found in a doctor's office, or the "clinical thermometer." The clinical thermometer was invented in 1866 by Sir Thomas Clifford Allbutt, an English physician. The important features of this thermometer were that it was relatively short, usually no longer than six inches, and it responded quickly to the patient's temperature. Previous instruments required nearly 20 minutes to get an accurate reading, while Allbutt's thermometer could reach equilibrium in less than five minutes. This made it easier for doctors to follow the course of a fever, since temperatures could be taken more quickly and more often.
Modern thermometers come in many different varieties. New thermometers are being designed that can read a patient's temperature using infrared technology. These devices can determine a person's temperature in about one minute, and take the reading from inside the ear, rather than the mouth.
"Thermometer." Medical Discoveries. . Encyclopedia.com. (December 16, 2017). http://www.encyclopedia.com/medicine/medical-journals/thermometer
"Thermometer." Medical Discoveries. . Retrieved December 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/medicine/medical-journals/thermometer
thermometer, instrument for measuring temperature. Galileo and Sanctorius devised thermometers consisting essentially of a bulb with a tubular projection, the open end of which was immersed in a liquid. Heating or cooling the bulb affected the height of the column of liquid in the tube, on which a scale was marked. Over a century later appeared the three thermometers now most widely used—the Fahrenheit, the centigrade (Celsius), and the Réaumur (used to some extent in parts of Europe). The first, invented by Fahrenheit c.1714 in Danzig, initiated the use of mercury as a heat-measuring medium; the thermometer of Réaumur, invented c.1730, used alcohol; the Celsius, invented by Anders Celsius at Uppsala (probably 1742) is now most used in laboratory work. The clinical thermometer is a small tubular instrument of rather thick glass. It consists essentially of a small vacuum tube of uniform bore closed at one end and connected at the other with a mercury chamber (either a bulb or a short tube of larger bore). A Celsius or a Fahrenheit scale (or both) is etched on the front of the thermometer; opposite this the glass is milky or semiopaque, to facilitate reading the temperature. When heat is applied, the mercury expands and rises from the chamber past a narrowed point and up the small tube. This narrowed point prevents the mercury from sinking back until shaking forces it down. A thermocouple can be used as a thermometer for measuring temperatures outside the range of liquid-in-glass thermometers. It is based on the thermoelectric effect occurring when the two junctions of a closed loop made of two different metals are at different temperatures (see thermoelectricity).
"thermometer." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (December 16, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/thermometer
"thermometer." The Columbia Encyclopedia, 6th ed.. . Retrieved December 16, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/thermometer
ther·mom·e·ter / [unvoicedth]ərˈmämitər/ • n. an instrument for measuring and indicating temperature, typically one consisting of a narrow, hermetically sealed glass tube marked with graduations and having at one end a bulb containing mercury or alcohol that expands and contracts in the tube with heating and cooling. DERIVATIVES: ther·mo·met·ric / ˌ[unvoicedth]ərməˈmetrik/ adj. ther·mo·met·ri·cal / ˌ[unvoicedth]ərməˈmetrikəl/ adj. ther·mom·e·try / -trē/ n. ORIGIN: mid 17th cent.: from French thermomètre or modern Latin thermometrum, from thermo- ‘of heat’ + -metrum ‘measure.’
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