Thermometers

emnmnme

I think this is the right place to post this since its technology. How are mercury thermometers made? I was puzzling over this for the past few days. Mercury is a metal that is liquid above room temperature but can turn gaseous very easily. I guess it's placed inside glass tubing under pressure. Okay, so how do you seal off the tube under pressure with technology from the early 1900s?

LewieP

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captaink

I guess it's placed in there at a certain temperature. It doesn't have the room to expand to a gas, it's under pressure like you said. Sealing it off is probably not hard, we've known glasswork for a long time. I couldn't tell you exactly how it's done though.

Rook

You don't need the mercury to be under pressure. It's really not going to go anywhere.

Mystery Solved?

quovadis13

Wikipedia potentially knows all.

jimenex

emnmnme wrote: »

I think this is the right place to post this since its technology. How are mercury thermometers made? I was puzzling over this for the past few days. Mercury is a metal that is liquid above room temperature but can turn gaseous very easily. I guess it's placed inside glass tubing under pressure. Okay, so how do you seal off the tube under pressure with technology from the early 1900s?

As far as I know it's vacuum sealed. There is a bulb where most of the mercury is and then a tube comes out of that bulb so that when the mercury expands it has somewhere to go. Since at the temperature range in which they work, the thermal expansion can be considered linear, they just added marks to that tube comming out of the bulb.

Very simple principle really, suppose you place mercury in the bulb and it comes out say 2 cm into the tube when you are at room temperature. Well, you would just place a mark where the mercury level is indicating the temperature. After you do that for several temperatures you have yourself a thermometer.

emnmnme

Ah, so there's no pressurized air in the 'empty' part of a thermometer - it's in a vacuum.

jimenex

emnmnme wrote: »

Ah, so there's no pressurized air in the 'empty' part of a thermometer - it's in a vacuum.

yup, other wise the "movement" of the mercury through the pipe would be constrained and the relation between temperature and movement would not be linear, i.e. at higher temperatures the nitrogen would expand less due to the high pressure. I think there are thermometers like that actually, with a non-linear scale:

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emnmnme

OK, but I'm still not clear on how they seal off glass tubing in a vacuum in the early 1900's.

jimenex

emnmnme wrote: »

OK, but I'm still not clear on how they seal off glass tubing in a vacuum in the early 1900's.

I would think they work on the glass thing, put a vacuum on it, and then they add the mercury bulb later on, since ideally you want a highly conductive material in the bulb (at least higher than glass).

Rook

emnmnme wrote: »

OK, but I'm still not clear on how they seal off glass tubing in a vacuum in the early 1900's.

I hear glass melts.

Hevach

Probably the same way they did light bulbs. You form it longer than is needed, and pump the air out from the open end (with the mercury already inside). You heat the point that'll become the end of the thermometer, and twist it off. Doing that properly with hot glass will seal both sides when it breaks, so the vacuum inside the thermometer is maintained.

Nickle

Hevach wrote: »

Probably the same way they did light bulbs. You form it longer than is needed, and pump the air out from the open end (with the mercury already inside). You heat the point that'll become the end of the thermometer, and twist it off. Doing that properly with hot glass will seal both sides when it breaks, so the vacuum inside the thermometer is maintained.

What he said. For further reading:

http://www.zytemp.com/tutorial/History_Of_Thermometry.htm