– Where is the temperature measured in thermometers ?

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I was interested in making an experiment and started to see what thermometer is best to use, when I became confused to where exactly is the measurement comes from.
For example, If I dip a mercury thermometer into a cup of water & oil, where is the measurement being received from?
The tip, only with the mercury sits? Or the temperature of the area dipped in both
Another example would be the prong style thermometers with a metal rod or whatever.
Is the reading an average of the temps throughout the rod? Or just the tip?

In: Other

Mercury expands and contracts with the temperature change. The glass tube is thin enough that the top of the mercury rises to mark the temperature at the very top of its level in the tube.

The bulb, well technically the whole thing but the thicker glass in the stem kinda insulates the mercury thats in the stem, the glass is thinner in the bulb where most of the mercury is.

As for the prong thermometer, that’s likely to be the whole prong that heats up, the prong might even be a heat conduit, a thin pipe designed to quickly conduct heat to the what’s probably a thermostat or some sort of digital thing.