Medical thermometers are used for measuring human body temperature, with the tip of the thermometer being inserted either into the mouth under the tongue (oral or sub-lingual temperature), under the armpit (axillary temperature), or into the rectum via the anus (rectal temperature). CLASSIFICATION BY TECHNOLOGY
The traditional thermometer is a glass tube with a bulb at one end containing a liquid which expands in a uniform manner with temperature. The tube itself is narrow (capillary) and has calibration markings along it. The liquid is often mercury, but alcohol thermometers use a colored alcohol. Medically, a maximum thermometer is often used, which indicates the maximum temperature reached even after it is removed from the body.
To use the thermometer, the bulb is placed in the location where the temperature is to be measured and left long enough to be certain to reach thermal equilibrium—typically three minutes. Maximum-reading is achieved by means of a constriction in the neck close to the bulb. As the temperature of the bulb rises, the liquid expands up the tube through the constriction. When the temperature falls, the column of liquid breaks at the constriction and cannot return to the bulb, thus remaining stationary in the tube. After reading the value, the thermometer must be reset by repeatedly swinging it sharply to shake the liquid back through the constriction. Mercury
Mercury-in-glass thermometers have been considered the most accurate liquid-filled types. However, mercury is a toxic heavy metal, and mercury has only been used in clinical thermometers if protected from breakage of the tube.
The tube must be very narrow to minimize the amount of mercury in it—the temperature of the tube is not controlled, so it must contain very much less mercury than the bulb to minimize the effect of the temperature of the tube—and this makes the reading rather difficult as the narrow mercury column is not very visible. Visibility is less of a problem with a coloured liquid.
In the 1990s it was decided [by whom?] that mercury-based thermometers were too risky to handle; the vigorous swinging needed to "reset" a mercury maximum thermometer makes it easy to accidentally break it and spill the moderately poisonous mercury. Mercury thermometers have largely been replaced by electronic digital thermometers, or, more rarely, thermometers based on liquids other than mercury (such as galinstan, coloured alcohols and heat-sensitive liquid crystals). Electronic or Digital
Since compact and inexpensive methods of measuring and displaying temperature became available, electronic thermometers (often called digital, because they display numeric values) have been used. Many display readings to great precision (0.1 °C or 0.2 °F, sometimes half that), but this should not be taken as a guarantee of accuracy: specified accuracy must be checked in documentation and maintained by periodical recalibration. A typical inexpensive electronic ear thermometer for home use has a displayed resolution of 0.1 °C, but a stated accuracy within ±0.2 °C when new. The first electronic clinical thermometer, invented in 1954, used a flexible probe that contained a Carboloy thermistor.  Contact
Some electronic thermometers may work by contact (the electronic sensor is placed in the location where temperature is to be measured, and left long enough to reach equilibrium). These typically reach equilibrium faster than mercury thermometers; the thermometer may beep when equilibrium has been reached, or the time may be specified in the manufacturer's documentation. Remote
Other electronic thermometers work by remote sensing: an infrared sensor responds to the radiation spectrum emitted from the location. Although these are not in direct contact with the area being measured, they may still contact part of the body (a thermometer which senses the temperature of the eardrum without touching it is inserted into...
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