Last night, I took my temperature with a standard fever thermometer and it read 101.4. I woke up this morning in a 76 degree room, and the thermometer still reads 101.4. I understand basically how a thermometer works- liquid expands and contacts due to temperature- why wasn’t it effected by the room temperature (even if the room temperature was well below it’s accurate measuring capability)? Oddly enough, I put the thermometer in my mouth, and viola! it became a thermometer again and read 99.8. What gives?
Thermometers you use for yourself are designed to not contract again after they display the temperature - this is so you can take it out of your mouth and read it and it won’t be inaccurate.
The way it is done is that there is a vacuum inside the tube (except for mercury vapor) so there is no pressure to push the mercury back down. You have to shake the mercury back down into the bulb before you use it, otherwise the accuracy is suspect.
As to why it went down when you put it in your mouth, well, it could be lots of things. Perhaps you held the other end in your hand and the temperature drove the mercury down. In any case, if you read the directions for the thermometer it should say to shake it down before use.