These past days I had a fever and I have taken my temperature many times with the thermometer in the photo. I think this is just like a mercury thermometer, even if it's a newer one that does not contain mercury, but my question applies also to mercury thermometers.
Every time I take the temperature with a thermometer like this I have to shake it to make it go back, otherwise it stays to the higher temperature it reached for a very long time. If it works just by thermal expansion, when I'm done taking my temperature and I put it at ambient temperature shouldn't it go back? Why doesn't it?
Best Answer
This looks to be an alcohol thermometer, designed solely for measuring body temperature.
Near the reservoir there's some kind of a 'soft' one-way valve. This prevents the alcohol from flowing back into the reservoir just after measurement, as this would lead to erroneous readings (underestimates) of the body temperature. The latter could have very serious consequences, of course.
Shaking then forces the alcohol back into the reservoir through the valve, after the themometer has cooled back to RT.
The reservoir is made of shiny metal, possibly to emulate the now defunct mercury thermometers.
In response to OP's comment, here's a possible idea for a 'soft' one way valve, for this device:
The assymetric constriction would allow the expanding alcohol through but severely restrict flow in the other direction. The forces of repeated shaking would force the alcohol back into the reservoir.