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Prindagelf t1_iy9nocp wrote

it's not entirely accurate, the assembly housing the Mercury and contacts sit on a metal spring, of a material I can't remember, the spring flexes as the temperature changes causing the Mercury to slide from one end of the glass to the other connecting the contacts


ksquires1988 t1_iy9ogky wrote

IIRC that metal spring is actually 2 metals. I think it's called a bi-metallic strip.


WFPBvegan2 t1_iy9pmy3 wrote

This is correct.


darrellbear t1_iybvsvj wrote

It's called a mercury switch, used to be pretty common.


Significant-Funny-14 t1_iybltvo wrote

That is correct. I believe one piece of metal expands more with heat than the other, causing a flex rather than just extending very slightly


Iain_MS OP t1_iy9oze8 wrote

Oh cool! I just assumed it would be expansion based like a thermometer. Thanks for insight!


JSONJSONJSON t1_iyauebs wrote

Yep, thermos are very small tubes, so very slight volume change is apparent.


SFXBTPD t1_iybd7fy wrote

Well its the way they taper down from a large bulb. The volume only changes by about 2% from 0 to 100C


JSONJSONJSON t1_iybftwk wrote

Oh! Is that because of decreased rate of change at higher temps? So the scale we read is consistent.


damaltor1 t1_iy9zufa wrote

exactly. setting the temperature higher tips the vial to one side, and actually rising temperature tips it back to the other side. it will find its equilibrium point when set temp and measured temp are equal.


lorddoa t1_iy9r071 wrote

Yep. Us po' rednecks used mercury in straws as the trigger to make our parachutes deploy in our homemade rockets. Like you said, it was about completing the circuit as opposed to thermal expansion

Just be sure to wait until the fucker is on the launchpad before hooking the wires up ;)


UnReAl0 t1_iyb0kp2 wrote

The really cool part is in the center dial with the numbers. Thats the heat anticipator that helps increase or decrease temperature overshoot in the room. Technology and stuff


GrecoAP t1_iycn5zp wrote

Okay, but how do I change the temperature?


gilareefer t1_iy9ujkz wrote

The mercury does not expand/contract to activate these. It only acts as a switch


D_daKid t1_iybf2m5 wrote

Just out of curiosity, why was mercury the metal of choice for these thermostats?


crispy1989 t1_iybk11u wrote

/u/gilareefer explained how the mechanism works, but to answer your question more generally, this type of switch mechanism is used because it's very resistant to oxidation/corrosion, and because a traditional mechanism used with a bimetallic strip would be prone to arcing.

The temperature-sensitive part is a bimetallic strip shaped in such a way so as to "tilt" one way or the other based on temperature due to the difference in the metals' thermal expansion coefficients. A simple switch mechanism based on this might be to just allow the strip (or a contact attached to the strip) to touch another contact in a certain position; but this type of switch is prone to corrosion over time due to being exposed to the elements. The complicated liquid-metal-drop-inside-sealed-vial mechanism prevents corrosion because there's no oxygen inside the vial.

Additionally, a simple switch contact mechanism used here would likely cause arcing and damage to the contacts since the switch would open and close extremely slowly. Things like normal light switches prevent this by integrating a "snap" mechanism to very quickly open and close the circuit; but a bimetallic strip supplies very little force, not enough to operate such a snap mechanism. The liquid-metal-inside-sealed-vial mechanism solves this problem by a) surface tension keeps the drop of mercury together so once the vial tilts even a little the whole drop will quickly slide over and complete the connection; and b) any damage due to arcing would be minimized in a low-oxygen environment inside the vial.

For both of these reasons, mercury metal is used because it's the only metal that's liquid across the usual range of human environmental temperatures, so is the only metal suitable for use in this type of switch mechanism.


D_daKid t1_iyblqnc wrote

Thank you for your descriptive response. If I had an award I'd give it to you! 😂


gilareefer t1_iybffpy wrote

Because it works well for this type of switch. Mercury is used as the conductor material in the switch... As the spring it's attached to heats/cools, the glass tube tilts & rolls the mercury into "on" position


D_daKid t1_iybi4b5 wrote

Interesting. I've seen these before in older buildings and it's always intrigued me. Thanks!


iwillneverreadthiscr t1_iyaxnw5 wrote

Nope... It has nothing to do with the temperature of the mercury; the actual sensor is a coiled piece of metal onto which the capsule containing the mercury is attached; when that coil expands, the capsule is tipped over enough to have the mercury close the circuit and power up the cooling system; as the room cools, the coil contracts, tipping the capsule in the opposite direction and breaking the circuit.


armerdan t1_iyb86y6 wrote

You are correct. The Mercury is just the switch, the thermometer is a coiled bimetallic strip.


Imbalancedone t1_iy9vdsp wrote

Its a bulb with mercury attached to a bimetal strip. The strip bends with temperature change and moves the bulb to make the switch action.


jayg76 t1_iy9wf78 wrote

they all used to.


BoringCrow3742 t1_iya5wuy wrote

any thermostat worth using still does.


aztech101 t1_iyat9j0 wrote

Digital thermostats are objectively better in the vast majority of cases though?


dscottj t1_iyax999 wrote

True story: my brother and I were rural latchkey kids, left alone for hours in an (for then) advanced house. I got bored one day, I think I was twelve, my brother was ten. We had this kind of switch for our thermostat, but it was in the cover. I'd pried that thing free months before my bright idea.

I thought the giant jiggling lump of mercury was fun to watch as it went back and forth. I didn't know what a bimetallic spring was, but I did know if I touched one side of the spring it made the whole thing twist in an interesting way and the mercury ball slid back and forth.

Here's where it gets fun: we were one of the first families on the block with a microwave oven. We were alone. My brother was bored and driving me bonkers.

"I know!" I said. "Let's put this spring thing with the mercury switch in the microwave!"

Oh yes I did. I didn't count on all the sparks. A half second after I turned it off I yanked it out and that spring was wound TIGHT. Physics probably saved me from mercury poisoning that day.

There's a reason why women live longer.


Iain_MS OP t1_iyaxloo wrote

Amazing story. Glad no mercury poisoning took place


bhops24 t1_iy9t031 wrote

All thermostats prior to going digital use the Mercury switch.


KruxAF t1_iya3359 wrote

As someone else said, “The mercury does not expand/contract to activate these. It only acts as a switch”


CyberNinja23 t1_iyaub2f wrote

BiMetallic strip. The thermal expansion properties of the different metals at different room temperatures allows the metal holding the mercury to flex back a forth, ideally allowing the mercury in the tube to break the circuit or comeback to complete the circuit.


RadBadTad t1_iy9rf7a wrote

This is how most/all thermostats worked before the recent digital ones. It's always neat to me, looking back and seeing purely mechanical solutions to problems that we solve digitally now.


Kurotan t1_iya833x wrote

What do the digital ones use then? I didn't know they stopped mercury.


RadBadTad t1_iya8s9i wrote

Modern thermometers use a variety of different methods and sensors to determine the temperature, but some of them include, thermocouples, Resistance Temperature Detectors (RTDs), thermistors, and Solid State sensors. These all use the ideas of voltage, resistance, and current to determine what the ambient temperature is.

The technology is a lot more complex than something like a bimetallic spring and some mercury, but if you're interested there is good reading about it!


nayhem_jr t1_iya9bjk wrote

Thermocouples measure voltage across a material that is sensitive to temperature, same thing that digital thermometers use. This allows for much more precision and faster response than that old bimetallic switch.


BoringCrow3742 t1_iya5zz1 wrote

analog is still better, digital fucks up all the time.


RadBadTad t1_iya6k1b wrote

Digital thermostats are WAY more accurate, and react a lot faster than analog thermostats. Old analog thermostats also need to be calibrated semi-regularly, as even bumping against it wrong can throw off the metal pieces inside that are used to react to the temperature in the room.

I don't know what you're referring to when you say that they "fuck up" all the time, but there isn't really any widespread issue with the accuracy or reliability of a digital thermostat, and certainly nothing related to the mechanics of actually reading the temperature of your home. Hell, they can even account for being in direct sunlight by subtracting the warmth they gain from the light. Analog thermostats can not.


Fartingonyoursocks t1_iyap6dh wrote

What do you mean when you say analog? Mine doesn’t look like this but looks like an analog clock. It’s connected to a gas furnace in the floor. Is this the same thing? I’m just curious.


leannerae t1_iyblwh0 wrote

I'm on your side, I've come home twice to a cold house because the batteries died. Another fancy programmable one just quit working and again, house was cold. I bought one of those old fashioned round analog ones and I love it.

Before anyone mentions it: I am capable of remembering when to turn it up or down to save energy and I don't want one that will email me when it's time for new batteries


BoringCrow3742 t1_iybvcl2 wrote

up and down? nah just needs to be 72 F all year all day. set it and forget it =D


flannelmaster9 t1_iya4362 wrote

Think it's called a Mercury switch. Spring expands and contracts and the Mercury shifts completing the circuit.

Haven't seen a mercury stat in the wild in a long time. And I've replaced hundreds of stats.


your_Assholiness t1_iya7a7y wrote

The spring is a BI- Metal strip that expands at different rates to tip the mercury switch and make contact.


your_Assholiness t1_iya7ht1 wrote

sorry I only read the first entry... I agree with ksquires1988 it's a bi-metal strip


skittlebog t1_iyaykiz wrote

That was a standard design. They would last for decades, because the contacts did not wear down. My grandfather's house had one of this design that kept working for 60 years.


LivingGhost371 t1_iyb1v5i wrote

House has been in the family for 50 years. The first 30 we had a mercury thermostat. It was still working when my parents ripped it out to put a digital one in, which needs new batteries every year and the unit itself seems to need replacing every five years.


seanmorris t1_iyba390 wrote

Na, its using a bimetallic strip to move the glass in the tube. The mercury slides back and forth, and makes contact with the conductors only one one side.


Gunch_Bandit t1_iybrfzu wrote

Tell us you've never removed the cover of the thermostat without telling us.


Iain_MS OP t1_iybrlhq wrote

Haha! You got me there. Definitely a first.


Gambit3le t1_iy9wwat wrote

Mine is the same. It works well with my oil fired steam boiler.


HTorv t1_iya5mmx wrote

Yep … old one for sure!!


ftminsc t1_iybcja0 wrote

Since it’s been covered, I’ll just add that based on the coefficient of thermal of expansion of mercury, a 10mm blob of mercury at 60F will become a 10.003mm blob of mercury at 70F. So while you could design a thermostat based on thermal expansion of a mercury blob, it would have to be seriously precise, and resistant to arcing.


RJFerret t1_iybes2y wrote

All of our thermostats work with bimetalic strips prior to digital battery ones.


dwarfgiant6143 t1_iybhyn7 wrote

I usually change those out when I can. They worked great for their time, but now people want them gone.


Iain_MS OP t1_iybr6km wrote

Any safety concerns? (Aside from if the mercury gets out somehow, which would obviously be bad).


dwarfgiant6143 t1_iychnfx wrote

just the mercury. The thermostats were basically bulletproof otherwise.


Iain_MS OP t1_iydc5pj wrote

Okay, good to know! Thank you


cipherd2 t1_iybod33 wrote

No, it doesn't. You're getting warmer, though. Pun intended.


Funny-Record-5785 t1_iybolp6 wrote

Silent switches were sick too sucks they took them off the market


kneecapped33 t1_iybv4xp wrote

Mercury is viscus enough of a liquid to not bounce back and forth on the relay


Dylsnick t1_iybw9gf wrote

HVAC tech here. You're all wrong. It's magic. Don't ask questions, just pay us to wave our magic wand and fix shit.


Iain_MS OP t1_iybxdvo wrote

To be fair mercury is pretty magical if you don’t think about how toxic it is.


snuffy_tentpeg t1_iycd87y wrote

When I upgraded my thermostat I took the ampule of mercury to the waste management personnel at my factory for disposal.


5_under t1_iy9yod8 wrote

So a thermostat?


Skoodge42 t1_iya5btz wrote

Anyone else get MacGyver nostalgia when seeing a mercury switch?


Professional-Eye8981 t1_iybvq1s wrote

The mercury doesn’t expand and contract. The mercury vial is mounted on a bi-metal coiled spring that causes the vial to tilt during a temperature change. When it does, the mercury blob rolls to the other end of the vial and opens (or closes) the circuit.


Ok-Drink-1328 t1_iyc2f0w wrote

it's a mercury switch, it works by leaning on one side or the other (mechanically), when the mercury touches both contacts inside the glass vial the contact is formed and stuff turns on


InterestedDudeFr t1_iyccs4f wrote

bro went to far to science lessons, that's actually impressive but also not that much accurate in my opinion


mr78rpm t1_iybzrke wrote

No, it for sure ridiculously does not.

The mysterious "bimetallic strip" consists of two pieces of metal bonded together. The metals do not expand or contract at the same rate as one another, so when the air in the room changes temperature, the bimetallic strip bends, shifting the position of the glass bulb and hence the mercury... when the mercury moves such that it connects together the two wires at the left side of the bulb, the heater comes on.


LogicallySound_ t1_iyc93fo wrote

There is something quintessentially “Reddit” about someone showcasing very old technology as if it were new and confidently describing it’s function incorrectly as if Google didn’t exist.


Buschwick66 t1_iyeehmw wrote

Um. That's called a mercury switch. And that's not how they work.

I took one years ago and tied it up at just the right angle in a buddy's engine bay. I had the switch completing the 12v circuit for the horn when the front end nose dived while braking.


_thankyoucomeagain_ t1_iyac13j wrote

All of them do. Mercury switched are very common.


slade797 t1_iybbfs7 wrote

Most are digital nowadays, no mercury.


_thankyoucomeagain_ t1_iybcyuz wrote

Seen very few digital but the do exist. I just repair them. What do I know.