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I remember when I was in primary school, the science teacher put me in charge of a mercury thermometer. I do not quite understand the mechanics behind except that mercury expands when it is hot and contracts when it is cold, and that this could be read off a temperature scale along the stem of the thermometer.

At the back of my mind, there is this doubt on how gravity affects the movement. Having seen most thermometers being hanged vertically on a wall, I did likewise, placing the thermometer on the floor, upright, leaning against the window sill beside my desk. This is fine until one day when a strong gust of wind blew and cause the thermometer to fall flat on the floor, smashing the glass casing and causing the mercury to leak out resulting in the evacuation of the whole class.

After that incident, I learnt my lesson and place the new thermometer lying flat instead of upright. But, the lesson that I didn't learn is how gravity affects the behaviour of such thermometer. Hope to learn something here.

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The influence of gravity is pretty much negligible. Gravity is only there to guarantee that the mercury gets concentrated on the right (lower) side of the glass cavity and there are no bubbles in it. But in the thin glass tube, this is guaranteed by the surface tension, anyway: liquids don't want to create holes in them even in the absence of gravity. As long as the gravity is strong enough to keep the mercury down and keep the air up, it doesn't matter how strong it is. At least the impact isn't detectable for changes of gravity we may achieve on Earth. At most, a stronger gravitational field could compress the mercury a little bit but because the liquid is nearly incompressible anyway, you won't be able to see much of a decrease.

The thermometer is a thermo-meter, not a gravity-meter, so it shouldn't be shocking that gravity's influence on its reading is negligible. It has to be so for the thermometer to fulfill its role. If your thermometer is placed horizontally, it is not a real problem because the surface tension is usually stronger than gravity and it prevents the air from penetrating "into" the mercury. The surface tension also keeps the shape of the mercury surface in the thin tube, relatively to the glass, more or less independent of the direction of the gravitational field.

If you shake with a medical mercury thermometer intensely enough, you can dislocate the mercury at the end.

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What if you put it upside down? –  Bernhard Apr 13 '13 at 8:07
    
Can you confirm whether there is air inside the thermometer or is it vacuum sealed? And how does it prevent dislocation at the bulb where mercury has a higher gravitational potential compared to the stem when the thermometer is laid flat on the ground. –  Question Overflow Apr 13 '13 at 14:14
    
Hi Bernhard, you may put it upside down but the mercury will tend to stay in the same part of the vessel, the bulb - which is now up - because in order to get to the bottom, one would have to create bubbles in the thin tube which increases the surface of the mercury and it costs energy to do it (surface tension). So if the thin glass tube is thin enough and/or the gravity/acceleration is small enough, you may neglect it, the thermometer will stay in the shape we expect it to, ignoring which directions are down and which directions are up. I feel that I am repeating myself. –  Luboš Motl Apr 13 '13 at 14:25
    
@QuestionOverflow, one usually puts nitrogen above the mercury or air at a much lower pressure. But it doesn't really matter. Even if you put regular air over there, the thermometer will work in some way because the air density is negligible and it's the mercury that is heavy, nearly incompressible, and that dictates where the boundary between the mercury and the gas will be. –  Luboš Motl Apr 13 '13 at 14:28
    
Thanks for the clarification :) –  Question Overflow Apr 14 '13 at 7:48

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