Thermometers can measure fever orally, rectally, under the armpit, in the ear and on the forehead. The medical community no longer recommends glass thermometers due to the potential for mercury exposure, according to the Mayo Clinic. Most thermometers now work without mercury and have digital readouts. In children, the type of thermometer used is determined by age. But adults can use any type of fever thermometer -- the only considerations are convenience and accuracy.
Oral or Rectal Digital Thermometers
In the past, the tips of oral and rectal thermometers were different from each other. Today's plastic digital thermometers have only one type of tip. You can use them orally, rectally or under the arm, though you should use separate thermometers for taking oral and rectal temperatures. Most adults will use them orally. Digital thermometers use electronic heat sensors to measure temperature and, in less than a minute, provide the most accurate readings compared to other types of thermometers, according to Larry D. Jones, health director for the City of Independence, Missouri. An oral temperature reading is generally 1 degree Fahrenheit lower than a rectal reading, and an axillary, or underarm, reading is 2 degrees Fahrenheit lower than rectal, according to the Mayo Clinic.
Tympanic Thermometers
Tympanic thermometers measure body temperature inside the ear using infrared technology. These thermometers also have digital readouts. They are quick and accurate as long as you position them properly in the ear canal and are an effective choice for those who cannot tolerate an oral thermometer under the tongue, says Jones. A tympanic thermometer provides a reading in less than 2 seconds.
Temporal Artery Thermometers
Temporal artery thermometers also use infrared technology to measure the temperature of the temporal artery across the front and side of the forehead, according to the Nebraska Department of Education. Temperature measurement takes only a few seconds and is non-invasive and accurate.
Liquid Crystal Thermometers
The heat-sensitive liquid crystal in these plastic strip thermometers changes color to indicate temperature. You may have to wait a few minutes after pressing the plastic strip on your forehead before you have a reading. The Nebraska Department of Education says liquid crystal fever thermometers are not as accurate as other types of thermometers and can leave fever undetected.
