Holding a thermometer upside down: Why no effect on the mercury?

I have a question about thermometers (the kind you use to determine room temperature). Why is it that if you hold it upside down, the mercury isn’t affected by gravity? What keeps the mercury in place?

That is a good question and I do not know the factual answer to this, however I do have a guess - the cohesive forces inside the mercury are stronger than gravity and the capilary action of the part of the tube with the mercury already in it keeps it there .

At first I considered that there’s some gas (like air) in the tube, but then I realized that this would make the thermometer non-linear due to gas compression and you’d have to compensate by either having non-linear scale or a varying the thickness of the tube - sounds like it’s too complicated for something that simple.

I do not know but I’ll take a WAG at this.

My guess is the answer is surface tension. From what I have seen mercury has a higher surface tension than water (e.g. makes a bigger blob without spreading out then the same amount of water does).

The hollow tube in the thermometer has a very small diameter. I am thinking that the surface tension combined with the very small opening from the reservoir at the bottom makes it such that gravity isn’t strong enough to pull it down the tube.

This is exactly right.

Not so much. Alcohol is used in place of mercury in most consumer-grade thermometers, yet alcohol has a much lower surface tension than water. Special-effects people make use of that fact when filming scaled-down flood scenes–the lower surface tension of alcohol makes smaller drops, making the illusion of greater size more compelling.

Now you lost me.

Why is groman right and I am wrong? Seemed to me we said the same thing albeit he said it better and got the post in sooner.

Because surface tension is not the governing factor here. Instead, it’s the attractive intermolecular forces between the liquid and the glass wall of the thermometer tube–what groman refers to, correctly, as capillary action. Unless I’m misreading groman’s post, you two seem to be talkking about different things.