Definition of Thermometer
Thermometer: A device used to measure the temperature of
gaseous, liquid or solid matter or of a chemical reaction such as
fire. Temperature measurement is important to a wide range of
activities, including industry, scientific research, and health care.
In health care, thermometers are used to measure the temperature of
the human body. They include the following types:
Oral thermometers (those placed under the tongue)
Rectal thermometers (those placed within the rectum)
Multi-purpose thermometers (those that can be placed under the
tongue, in the rectum or under the armpit)
Eardrum thermometers (those placed inside the ear).
Basal thermometers (highly sensitive thermometers placed under
the tongue or in the rectum to measure slight temperature changes
indicating that ovulation has taken place in a woman).
Oral and rectal thermometers: A conventional oral or rectal
thermometer consists of a sealed glass tube containing a liquid such
as mercury. Imprinted on the tube is a temperature scale. When the
temperature rises or falls, the
mercury expands or contracts, causing it to move up or down the scale
inside a tiny passageway. A narrow point along the passageway allows
the mercury to move up the scale but prevents it from moving down
until the thermometer is "shaken down" after
use. The thermometer must maintain contact with the body for about
four (4) minutes to yield an accurate reading. A rectal reading is
usually slightly more accurate than an oral reading.
New-fangled thermometers: Thermometers are also available
with digital displays that facilitate easy reading of the
temperature, beepers to signal when it is time to withdraw the
thermometer, and flexible tubes to resist
breakage. Manufacturers provide instructions for their use.
Ear thermometers: Ear or more precisely eardrum
thermometers measure temperature by reading infrared
radiation emanating from eardrum tissue. They have three key
advantages:
- The ability to measure
temperature without coming in contact with the body;
- The ability
to give a close estimate of the temperature of the brain (because of
their proximity to the brain) as a true gauge of body temperature;
and
- Their ability to provide a readout in two or three seconds.
However, the accuracy of the eardrum thermometer has been questioned
by some physicians. Whether it will supplant the usual oral or rectal
thermometer remains to be seen.
Basal thermometers: Basal thermometers measure minor
temperature changes in women to
indicate whether ovulation (the release of an egg for fertilization
by sperm) has taken place. The temperature of women normally rises
slightly when ovulation occurs and does not return to normal until
menstruation begins. Basal thermometers are sensitive enough to
monitor the slight changes in temperature.
Taking temperature: Normal body temperature is considered
to be 98.6 degrees
Fahrenheit, although it may range between 97 and 99 degrees during
the day. The temperature at the beginning of the day, when a person
awakens, is called the basal temperature. A doctor should always be
consulted if the temperature reaches 103 degrees Fahrenheit or more
in a child and 104 to 105 degrees or more in an adult.
Thermometers are available not only with Fahrenheit temperature
scales, but also Celsius scales. The Fahrenheit scale registers the
freezing point of water at 32 degrees and the boiling point at 212
degrees. The Celsius
scale (formerly called the centigrade scale) registers the freezing
point of water at 0 degrees and the boiling point at 100 degrees.
Fahrenheit temperature can easily be converted to Celsius by
subtracting 32 from the reading, then multiplying by 5/9. Example:
98.6-32 =66.6. Next, 66.6x5/9 = 37. Thus, a normal temperature of
98.6 F is 37 on the Celsius scale. Celsius can be converted to
Fahrenheit by multiplying the Celsius temperature by 9/5 and adding
32. Example: 37x9/5 = 66.6. Next, 66.6+32 = 98.6. Thus, a normal
temperature of 37 on the Celsius scale is 98.6 on the Fahrenheit
scale.
The invention of the thermometer: This feat is
conventionally credited to the renowned (and, in his lifetime,
reviled) Italian astronomer and physicist Galileo (1564-1642). In
Galileo's instrument, built about 1592, changes in temperature within
an
inverted glass vessel caused the expansion and contraction of the
air within it and this changed the level of the liquid with which the
vessel's long, openmouthed neck was partially filled.
The German physicist Daniel Gabriel Fahrenheit (1886-1736)
developed accurate mercury thermometers which he calibrated to the
standard scale that now bears his name. The first centigrade scale
(made up of 100 degrees) was developed in 1742 by a Swedish
astronomer Anders Celsius (1701-1744). It was known simply as the
centigrade scale until in 1948 when the name was changed to honor
Celsius.
The history of the thermometer has been recently been red-hot. A
professor at the University of Arkansas, Jacob Adler, has found
what appears to be the first description of a liquid-in-glass
thermometer. He stumbled upon it in a book Ma'yan Ganim (A
Fountain of Gardens) published in 1629 by the physician and rabbi
Joseph Solomon Delmedigo. Delmedigo's thermometer was filled not with
mercury but with brandy.
Last Editorial Review: 1/11/2000 7:36:00 AM
Back to MedTerms online medical dictionary A-Z List
