40 seconds.
1 Use a clean thermometer, one that has been washed in cold water, cleaned with rubbing alcohol, and then rinsed to remove the alcohol. 2  Do not eat or drink anything for at least five minutes before you take your temperature. 3  You should keep your mouth closed during this time. 4  Place the thermometer tip under the tongue. Normal body temperature is about 98.6 degrees Fahrenheit (°F) or 37 degrees Celsius (°C). Your temperature often varies from 1 to 2 °F or ½ to 1 °C. Your temperature is usually lower in the morning and increases during the day. Confidence votes 248. You must hold your mouth closed and keep the thermometer in place for a certain period of time. If you have a digital thermometer, it will tell you when it's done. If you are using an old mercury thermometer, the thermometer must stay under the tongue for at least a minute to 2 minutes. NOTE: Mercury thermometers are being phased out. The mercury inside is highly toxic and you should dispose of all mercury thermometers by taking them to a recycling center that handles that.  The general rule of thumb is to keep an oral thermometer under the tongue and off to one side in a gently closed mouth for three minutes. Always use a disposable cover and dispose of it promptly when finished. Wash both the thermometer and your hands thoroughly. Digital thermometers can be used orally, in the rectum or under the armpit to take a temperature. Wait until about a half hour after eating or drinking before inserting a digital thermometer in the mouth. The very head of the device should be placed under the tongue. Temperature: Oral, Rectal, and Axillary Page 2 of 2. ORAL TEMPERATURE 1. Use a thermometer with a long bulb (Picture 3). 2. After shaking the mercury down, place the thermometer under the child's tongue, with the bulb toward the back of the mouth. Tell your child to keep the lips firmly closed, but not to bite the thermometer. 3. Leave the thermometer in place for 3 minutes. 4. Remove the thermometer and read it Taking Oral Temperatures. 1  Be sure your child has not taken a cold or hot drink within the last 30 minutes. 2  If using a glass thermometer, shake it until the mercury line is below 98.6° F (37° C). 3  Place the tip of the thermometer under one side of the tongue and toward the back. 1 Be sure your child has not taken a cold or hot drink within the last 30 minutes. 2  If using a glass thermometer, shake it until the mercury line is below 98.6° F (37° C). 3  Place the tip of the thermometer under one side of the tongue and toward the back. 4  An accurate temperature depends on proper placement. 1 If using a glass thermometer, shake it until the mercury line is below 98.6° F (37° C). 2  Place the tip of the thermometer under one side of the tongue and toward the back. 3  An accurate temperature depends on proper placement. 4  Ask a physician or nurse to show you where it should go. 1 Be sure your child has not taken a cold or hot drink within the last 30 minutes. 2  If using a glass thermometer, shake it until the mercury line is below 98.6° F (37° C). 3  Place the tip of the thermometer under one side of the tongue and toward the back. 4  An accurate temperature depends on proper placement. If you have a digital thermometer, it will tell you when it's done. If you are using an old mercury thermometer, the thermometer must stay under the tongue for at least a minute to 2 minutes. NOTE: Mercury thermometers are being phased out. The mercury inside is highly toxic and you should dispose of all mercury thermometers by taking them to a recycling center that handles that.  1 You should keep your mouth closed during this time. 2  Place the thermometer tip under the tongue. 3  Hold the thermometer in the same spot for about 40 seconds. 4  Readings will continue to increase and the F (or C) symbol will flash during measurement. Normal body temperature is about 98.6 degrees Fahrenheit (°F) or 37 degrees Celsius (°C). Your temperature often varies from 1 to 2 °F or ½ to 1 °C. Your temperature is usually lower in the morning and increases during the day. 1 Oral temperature. 2  Turn on the digital thermometer. 3  Place the tip of the thermometer under your child's tongue and ask your child to keep his or her lips closed. 4  Remove the thermometer when it signals that it's done and read the number. 1 Rectal temperature. 2  Turn on the digital thermometer and lubricate the tip of the thermometer with petroleum jelly. 3  Lay your baby or child on his or her back, lift his or her thighs, and insert the lubricated thermometer 1/2 to 1 inch (1.3 to 2.5 centimeters) into the rectum.
http://my.clevelandclinic.org/health/diseases_conditions/hic_Fever/hic_How_to_Take_Your_Temperature http://www.answers.com/Q/How_long_should_you_keep_the_thermometer_under_your_tongue http://www.ehow.com/way_5343508_long-should-leave-oral-thermometer.html http://kidzdoc.com/wp-content/uploads/2013/12/temperature-taking-glass-thermometer.pdf http://www.cpmc.org/advanced/pediatrics/patients/topics/temperature.html http://www.cpmc.org/advanced/pediatrics/patients/topics/temperature.html https://answers.yahoo.com/question/index?qid=20091018065502AAjSdF1 http://my.clevelandclinic.org/health/diseases_conditions/hic_Fever/hic_How_to_Take_Your_Temperature http://www.mayoclinic.org/healthy-lifestyle/infant-and-toddler-health/in-depth/thermometer/art-20047410?pg=2
how long should you leave a thermometer under your tongue