How to Choose and Use a 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:

  1. The ability to measure temperature without coming in contact with the body;
  2. 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
  3. 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: 7/7/2004




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