Something most Brits won’t have come across.

[Cabinetman]

So I was helping renovate an old house in the US recently and came across this old light switch, totally silent in use and something to be careful of, it’s a Mercury switch it uses the Mercury as a conductor across the contacts, a lot were made of glass which even by my standards is not too clever a material to put mercury in. Yes I know about mercury and this one will be disposed of in the correct manner.
Note the large unprotected contacts, still a feature on new American switches were are you to take the cover off, they only use 110 V so standards are a bit different over there.

And this is it with the cover off, this one has the mercury contained in a tin drum with a white rubber strip around it, which is the friction between the drum and the actual user switch, flick the switch and the drum revolves slightly and the mercury flows to the contacts or away from them. The contacts are to either side of the drum, just simple springs held against the ends of the drum.
Now we know about mercury (there is still a fair chunk in my mouth though!) But in those days it was a pretty good way of building a switch, no chance of it shorting out, and very little to wear out.
Mercury tilt switches were often used in box and freezer lids to turn the light on. Ian

 

[Argus]

Mercury switches were used all over the world mainly in small installations. No longer legal here, but there may still be some about.

I was a rerfrigeration engineer and worked for an (American) manufacturer of Air Conditioning units for many years up to the mid 1980s and the 12 and 24 volt controls that came with American-made units were all based on mercury - it didn't arc. We didn't use use them for any great electrical loads or voltages.

My first job leaving school was as an assistant projectionist (this was exactly when Kennedy was shot, and the Beatles were just starting out). The 45 Volt DC electrode feeds to the arc-rods in the projectors at that cinema had just then been converted to a transformer from a mercury-arc rectifier - a huge thing that held about a gallon of the stuff and glowed bright blue in use...... it was parked in a spare room for ages.


.

 

[PerryGunn]

I'm still using a doorbell chime that has a glass tube mercury switch - I think it was manufactured in the early 1960s so getting on for 60 years old.

It's one of the ding-dong type chimes with two metal glockenspiel bars and a sprung striker rod.

In the striker rod's rest position the mercury switch is closed. When the doorbell button is pushed it completes the circuit and energises the electromagnet which throws the sprung striker rod against the 'ding' bar. With the rod in this position, the mercury switch is tilted thus breaking the circuit, the electromagnet de-energises and the spring recoil throws the striker rod back against the 'dong' bar. This action tilts the mercury switch back to the closed position and the cycle repeats for as long as the doorbell button is held down.

 

[Spectric]

they only use 110 V

But still capable of killing you, site 110 is centre grounded so you have only 55 volts which is much safer and cannot push enough current through you to kill, unless you are wet and naked which would increase the chances.

I can just remember mercury vapour rectifiers, used in high voltage transmitters but now well replaced by solid state. Mercury tilt switches were also common in many places just like mercury thermometers. Those of the right age will also remember playing with mercury in the school science labs, along with Lithium and phosphorus, exciting days!

 

[Phil Pascoe]

I used to clean ice machines. These were primitive, really, they worked by water running down over a freezing plate, freezing solid until it triggered a mercury switch at a pre set depth switching the water supply off and a heater element under the plate on. When the slab of ice started to thaw it slid off the plate and onto a grid of wires which heated when another mercury switch was tipped. The ice then cut into cubes and fell through the wires. returning the mercury switch to its previous position which started the supply of water and the process all over again. I never once saw an EHO check a machine - I looked after ours, but I dread to think what the insides of some local ones were like.
I was the arch bodger. One of the machines was condemned by an "engineer". Two years later the same chap came to look at the other machine and saw this one still working. How on earth did you do that? he asked, there were no parts left available. The wire grid had failed. I wondered where I would obtain some reasonably HT wire, so I went to the music shop, got guitar strings and made a new one.

 

[danst96]

I used to clean ice machines. These were primitive, really, they worked by water running down over a freezing plate, freezing solid until it triggered a mercury switch at a pre set depth switching the water supply off and a heater element under the plate on. When the slab of ice started to thaw it slid off the plate and onto a grid of wires which heated when another mercury switch was tipped. The ice then cut into cubes and fell through the wires. returning the mercury switch to its previous position which started the supply of water and the process all over again. I never once saw an EHO check a machine - I looked after ours, but I dread to think what the insides of some local ones were like.
I was the arch bodger. One of the machines was condemned by an "engineer". Two years later the same chap came to look at the other machine and saw this one still working. How on earth did you do that? he asked, there were no parts left available. The wire grid had failed. I wondered where I would obtain some reasonably HT wire, so I went to the music shop, got guitar strings and made a new one.

Many US style home ice machines still work that way but without the mercury I would assume

 

[Cabinetman]

I used to clean ice machines. These were primitive, really, they worked by water running down over a freezing plate, freezing solid until it triggered a mercury switch at a pre set depth switching the water supply off and a heater element under the plate on. When the slab of ice started to thaw it slid off the plate and onto a grid of wires which heated when another mercury switch was tipped. The ice then cut into cubes and fell through the wires. returning the mercury switch to its previous position which started the supply of water and the process all over again. I never once saw an EHO check a machine - I looked after ours, but I dread to think what the insides of some local ones were like.
I was the arch bodger. One of the machines was condemned by an "engineer". Two years later the same chap came to look at the other machine and saw this one still working. How on earth did you do that? he asked, there were no parts left available. The wire grid had failed. I wondered where I would obtain some reasonably HT wire, so I went to the music shop, got guitar strings and made a new one.

You can’t beat a bit of innovation, saved me a lot over the years.
You don’t think of bacteria in cold places, but I remember well seeing an ice machine pretty well caked with pink gunge, something to think about when next you ask for ice in your drink in a pub!

 

[Phil Pascoe]

People go abroad and avoid drinking the water, then go to bars and ask for loads of ice in their drinks.

 

[Rorschach]

Still used in flammable gas areas I think.

 

[Argus]

One advantage of enclosed mercury switches is that, at lower voltages where a higher current-draw can give destructive arcing; a slow break action can result in a great deal of this destructive arcing which will reduce the life of a mechanical contact and add unwanted and hard-to-suppress RF interference..... especially where DC power was used. Prior to universal mains AC, much industrial power was based on DC.

Enclosed mercury switch-vapours eliminated this problem.

As time went on better machining and design of switch gear plus increased solid-state switching eliminated their need.
Perhaps they are still in use............ but mercury in any of its forms is hard to justify these days from a safety point of view.

 

[Spectric]

Mercury was not a safe material, vapours can send you mad (As mad as a hatter) but they now use Sulphur hexafluoride (SF6) to quench arching in breakers which is not toxic but is a greenhouse gas much worse than C02, levels in our atmosphere apparently reached new highs in 2019 due to loses from electrical grids across the globe. When you look at most things you overcome one issue and gain another and the cycle continues.

 

[flying haggis]

those american switches and sockets with exposed terminals scare me. how can they still make such a stupid design

 

[Sandyn]

in 70's electronics, mercury wetted relays were very common.

 

[Inspector]

I've never seen a light switch like that one but thermostats for the central heat had them sitting on top of the bimetal coil strip to shut the furnace on/off.
As for the exposed screws one isn't supposed to take the covers off the boxes with the power on and poke around. Those that do answer to Darwin.

Pete

 

[sirocosm]

I remember those, the little glass bulb would move as the bimetal coil changed temperature, and then the ball of mercury would roll to the other end and make/break the contact. They lasted forever, I had never seen one go wrong.

Haven't heard the word "furnace" in relation to home heating in quite some time, LOL!

 

[Anthraquinone]

Many years ago I used mercury diffusion pumps to create very high vaccums. One day a spill resulted in several Kg (or lbs as it was then) spilt on the floor. Not the easiest to gather until we poured liquid nitrogen over it and then picked up the now solid mercury.. Ah the good old days.

 

[Geoff_S]

When I was a kid in the chemistry lab, we played "mercury football". The idea was to spread a number of mercury blobs on the bench and try and score a goal avoiding creating bigger mercury blobs by bumping into them.