Glass thermometers Glass thermometers with mercury have been in use since 1870. While they are the least expensive thermometers, they record temperature slowly and are often hard to read. To read a glass thermometer, find where the mercury line ends by rotating the thermometer slightly until the line appears. If you have difficulty reading a mercury thermometer, buy one with color zones. Where to take the temperature Rectal temperatures are the most accurate.
Oral temperatures are also accurate if taken properly.
Armpit temperatures are the least accurate, but they are better than no measurement. For children 5 years of age and younger, rectal temperatures are preferred. Armpit temperatures are a second choice. For children over 5 years, oral temperatures are the usual method. F. Also, always clean the thermometer with rubbing alcohol after each time it is used.