let's consider a classic mercury thermometer.
I do not understand why it does not behave like a "normal" thermometer which exploits volume dilatation. In a normal thermometer, I'd say that the mercury length would be proportional to its temperature.
Therefore, I should be able to measure, for instance, 37 of body temperature, also starting with the thermometer at 38: there would be a contraction, but the measure would be correct! Why does this not happen? And why if I measure for instance, 38, and I try to cool the thermometer by putting it inside cold water, it does not become cooler? Why should I cool it by shaking it?
It seems a very not ideal thermometer... but what are the causes of these non-idealities?