Overview
A clinical thermometer, also called a medical or doctor's thermometer, is a device designed to measure a person's body temperature. It is calibrated to cover the narrow range typical for humans and provides readings used to assess fever, monitor illness, or check for hypothermia. For further general information see clinical thermometer.
Design and main parts
Traditional clinical thermometers were mercury-in-glass instruments: a slender glass tube with a bulb containing mercury and a finely graduated scale. The tube often includes a constriction (a kink) that prevents the mercury column from falling back into the bulb so the reading remains visible after removal. Modern equivalents use electronic sensors, infrared detectors, or liquid crystals rather than elemental mercury. For details about mercury thermometers see mercury-in-glass thermometers.
Types and measurement sites
- Oral thermometers: placed under the tongue; commonly used in adults and cooperative children.
- Rectal thermometers: give core temperature estimates and are frequently used for infants.
- Axillary (underarm) thermometers: less invasive but usually produce lower readings.
- Tympanic (ear) infrared thermometers: measure infrared emissions from the eardrum and are fast.
- Forehead (temporal artery) and strip thermometers: noninvasive and quick, with varying accuracy.
Scales, ranges and interpretation
Clinical thermometers typically show both the Celsius and Fahrenheit scales. A commonly cited average body temperature is about 37 °C (98.6 °F), but normal values vary by person, time of day, and measurement site. Most clinical instruments are designed to measure in the approximate range of human health, so they display a restricted span rather than the wide limits of laboratory thermometers. See scale references such as Celsius and Fahrenheit for more on the temperature scales.
History and development
Early medical thermometers were large and slow to respond. In 1714 Gabriel Fahrenheit introduced a more practical mercury thermometer and the Fahrenheit temperature scale, improving speed and reliability. Through the 19th century devices became smaller and more portable. In the 20th century innovations produced tympanic infrared devices and, later, widespread adoption of electronic digital thermometers toward the end of that century.
Use, cleaning and safety
Proper technique and hygiene are essential. A thermometer should be placed according to manufacturer guidance and, if reused, cleaned before and after each use. Common practice includes wiping or disinfecting with solutions such as isopropyl alcohol; see cleaning guidance at alcohol-based disinfectants. Mercury thermometers are accurate but pose a poisoning and environmental hazard if broken; many regions now limit their use and recommend disposal through hazardous-waste programs or replacement with non-mercury models.
Accuracy, advantages and caveats
Mercury-in-glass thermometers were historically prized for stability and precision, but modern digital and infrared thermometers provide faster readings, safer handling, and convenience for clinical and home use. Accuracy depends on sensor quality, measurement site, user technique, and device calibration. When precise core temperature is critical, clinical protocols typically specify the preferred measurement method. For additional technical and clinical context consult general resources such as medical device guides or manufacturer instructions; broader background can be found via technical summaries and scale explanations at Celsius and Fahrenheit.