5 ms·

Salt Water Dimmers

85 points by dcminter 2y ago

dcminter 2y ago
I thought this was mildly interesting; I'd never heard of them but they popped up in a Facebook post about a retired PDP-11 system from a theatre and the term caught my eye! Presumably the "strong smell" was chlorine!?
- thfuran 2y ago
  Presumably, yes.
idiotsecant 2y ago
Still used in 3rd world countries (quite effectively) to make dirt cheap DIY arc welders. Super dangerous but basically free, just need electrodes. Not as high-current as a modern machine, but good enough for fixing light tractors and bicycles.
- 0_____0 2y ago
  ???? I think you must be thinking of a different device.
  - aaron695 2y ago
    They are thinking of exactly the correct machine. A liquid rheostat is just a rheostat https://en.wikipedia.org/wiki/Liquid_rheostat https://en.wikipedia.org/wiki/Liquid_rheostat They were used in the past just like the light dimmer, but I challenge a present day photo in a developing countries workshop. Heaps of Youtube spam and 'lets try', I'd like to see one in a real workshop in the wild. (That would be cool to see)
  - idiotsecant 2y ago
    I think if you think it through carefully you'll understand how they are fundamentally the same.
    - 0_____0 2y ago
      This is essentially a big variable resistor - how does one apply that to arc welding?
      - idiotsecant 2y ago
        What is it that you suppose a modern arc welding system does? Stick welding needs about 20-30 volts during the weld (60v to strike the arc if we have a fancy machine). In a modern machine we turn high voltage low current mains power into low voltage high current arc welding power. This rheostat would turn it into low voltage slightly lower current but it would still work fine for light gauge.
        0_____0 2y ago
        I've done a fair bit of TIG, a little bit of SMAW. Are we talking about welding with basically reduced mains current? AC? SMAW and TIG both require current well in excess of what you can get out of a wall without tripping a breaker. I think for welding 14ga mild steel I was running at minimum 80A on the machine, but more like 130A with the foot pedal. Are you talking light gauge like as in spot welding territory?
mech975 2y ago
At a previous job we manufactured fairly large trailer-mounted generators. During design engineering testing, you would want to power a load with the generator to check performance. The test load we had was a giant resistor bank, in a pallet-sized enclosure about 4 feet tall, with fans for active cooling of the oodles of resistance heat being put off by it. I talked to one of the electrical engineers about how ludicrous this device was, and he said that we used to use something even weirder. A giant tank filled with salt water (can't remember for sure if they were using NaCl or a different electrolyte) with probes just dumping all that current right into the water. Wish I had seen it.
- XorNot 2y ago
  I'm not sure what's ludicrous about this setup though? For testing you want the simplest possible setup - which a giant resistor bank really is. Power is power when you run it through simple resistance - no AC shenanigans will hide effects.
  - thfuran 2y ago
    The electrolysis rig seems a bit ridiculous, since it generates a bunch of flammable gas you have to deal with. A big resistive load for testing stuff seems pretty normal though.
    - amluto 2y ago
      Not just flammable hydrogen gas — you are likely to generate chlorine gas, potentially in large amounts.
      - thfuran 2y ago
        True, if it's very salty.
    - typh0n 2y ago
      It's not electrolysis, it's more like electrode boiling.
CaliforniaKarl 2y ago
For more on resistance dimmers in theater, check out this video from Jonathan Bastow: https://youtu.be/lTHNYWw6yt0?si=ygt0HpSd_aPcm3tq https://youtu.be/lTHNYWw6yt0?si=ygt0HpSd_aPcm3tq
- deleted 2y ago
  [deleted]
JKCalhoun 2y ago
As a "mad scientist" when I was perhaps eleven or so, I created a carbon arc by cutting the end off an electrical zip-cord extension cord, stripping the insulation off the two wires (lol, yeah, no ground wire) and wrapping the bare copper ends around a pair of carbon rods pulled from D-cell batteries. I held down each carbon rods by attaching it to a brick. Move the bricks together to touch the rods together to get a spark and an orange growl, and then draw them apart into an intensely bright white-hit light. Insane. Yes, I tripped the circuit breaker (thankfully not fuses in that place) repeatedly. Decided to make a saltwater rheostat to control the current. This time a square glass casserole dish with salt water was the tank. One of the mains wires was cut and enough insulation stripped to wrap around two rocks placed in the brine. Move the rocks closer, more current, move the rocks apart, less current. Holy hell, that was something. When the arc got to lighting up the basement I looked around to the rheostat to see it arcing under water as it bubbled (as another mentioned, yeah, probably chlorine gas). When the insulation on the zip cord soon started to smoke I decided the experiment was over and jerked the cord to pull it out of the outlet. This quick yank also sent the glass casserole dish crashing to the ground.... Ahhh ... if only YouTube were around then. I might have tried to do it more safely. https://youtu.be/VTzKIs19eZE https://youtu.be/VTzKIs19eZE Instead I got the idea from the book "700 Science Experiments for Everyone" (p. 185): https://archive.org/details/isbn_9780385052757/page/184/mode/2up https://archive.org/details/isbn_9780385052757/page/184/mode...
- luckman212 2y ago
  Love hearing stories like this about unfiltered, unfettered (and slightly insane) childhood play. Parents today don't even let their kids walk 2 blocks to school unsupervised, lest something terrible happen.
  - JKCalhoun 2y ago
    I wouldn't let me do that now, ha ha. I could have burned the house down. (I did burn my friend's eyebrows off, but that's another story...)
  - kadoban 2y ago
    The danger in that story is pretty off the charts. That could have _easily_ led to death, like maybe 1/100? That's something parents _should_ stop. It could have been redirected to safer versions, you can have a lot of fun with lower voltage DC, or at the very least safer setups.
jfengel 2y ago
Oh, holy hell. I thought "old sparky" was bad. It was a board cobbled together from a bunch of residential rheostats. The entire power supply of the theater flowed through that board. But at least Old Sparky didn't produce toxic fumes. It was merely a fire hazard.
- mkwarman 2y ago
  Do you know of anywhere I could learn more about this? Googling "old sparky" just yields articles about electric chairs, even when including keywords such as "rheostat" and "theatre"
  - jfengel 2y ago
    That was just the theater's name for it. I imagine other theaters had similar contraptions, and because of the expression they probably came to similar names. It was at least 40 years old. I would hope they've all been retired.
locallost 2y ago
> the last dimmers in London theatres being replaced in 1959. I'm always amazed how long some things were actually used that you'd think stopped being used 100 years ago. Another one, steam locomotives, not completely phased out in some developed countries until the 70s.
- eru 2y ago
  Britain used steam locomotives until the 1980s. See https://en.wikipedia.org/wiki/Retirement_of_steam_locomotives_by_country https://en.wikipedia.org/wiki/Retirement_of_steam_locomotive... Germany apparently used them from time to time until 1994, but that was a function of East Germany being behind the Iron Curtain. My grandfather also used punch cards well into the 1980s in East Germany.
  - Xylakant 2y ago
    Funfactlet: There's a steam locomotive in active duty shunting work trains around at the moment in western Germany. The Riedbahn between Frankfurt and Mannheim is currently being redone and they were short on Diesel engines, so one Verein for historic railroad equipment offered their steam engine and was accepted. https://www.swr.de/swraktuell/radio/riedbahn-sanierung-dampflok-als-hingucker-100.html https://www.swr.de/swraktuell/radio/riedbahn-sanierung-dampf... (German only) Another use for steam engines is for shunting duties in power plants - the engine has no boiler and just gets “charged” from the main plant steam generator.
    - femto 2y ago
      A similar thing happened here in Australia (New South Wales) in the mid 80s. I was part of a rail preservation group and we were taking a stream locomotive with an empty train from West Ryde Station to Central, from were we were going to depart for an enthusiast's trip. One of the suburban trains was cancelled, so Jock, the station master, asked if we would like to run the regular suburban service, which we did. It was great fun pulling in and watching the passengers do a double take as their regular train turned out to be a steam train.
userbinator 2y ago
Water is used because of its extremely high heat capacity and wide availability: https://en.wikipedia.org/wiki/Table_of_specific_heat_capacities https://en.wikipedia.org/wiki/Table_of_specific_heat_capacit... The concentration of salt has a huge effect on the resistance --- there is an infamous YouTube video of this effect being demonstrated with an electrode boiler.
the_third_wave 2y ago
These things were used at fairground attractions - bumper cars, merry go-arounds etc. - until at least the 1980's. They tended to come in the shape of a wooden barrel with a hinged lid which had a number of shark fin shaped metal plates on the bottom. There was a rope attached to the lid which was used to raise and lower it. The water got quite hot in use which led to steaming barrels later in the evening.
enb 2y ago
There's a period between the "beginning" of the modern age and today where we had these strange interstitial technologies. It sometimes feels like we went from living in the dark to electricity to the digital age in big jumps, but there's been a lot of evolution in between. Today will seem quite quirky to people 10, 20 and 50 years from now.
- hnlmorg 2y ago
  Today already feels quirky to me now.
guenthert 2y ago
"Salt water dimmers were gradually replaced by semiconductor dimmers, the last dimmers in London theatres being replaced in 1959." I would have expected that they would have been replaced by adjustable transformers as soon as lights were powered using AC.
- beAbU 2y ago
  I suspect the power requirements would have required truly massive and very expensive transformers. Stage lighting requires massive amounts of power. My guess is salt water resistors remained the cheaper and simpler alternative until semiconductors took over, completely skipping variacs.
keepamovin 2y ago
So cool. Had no idea about this and I love theatre. Maybe someone could build a mechanical/electrical/chemical computer using these jars of salt water? Ie, Salt water dimmers are Turing complete...arxiv://doi...
deleted 2y ago
[deleted]

85 points by dcminter 2y ago

I thought this was mildly interesting; I'd never heard of them but they popped up in a Facebook post about a retired PDP-11 system from a theatre and the term caught my eye! Presumably the "strong smell" was chlorine!?

thfuran 2y ago
Presumably, yes.

idiotsecant 2y ago

Still used in 3rd world countries (quite effectively) to make dirt cheap DIY arc welders. Super dangerous but basically free, just need electrodes. Not as high-current as a modern machine, but good enough for fixing light tractors and bicycles.

0_____0 2y ago
???? I think you must be thinking of a different device.
- aaron695 2y ago
  They are thinking of exactly the correct machine. A liquid rheostat is just a rheostat https://en.wikipedia.org/wiki/Liquid_rheostat https://en.wikipedia.org/wiki/Liquid_rheostat They were used in the past just like the light dimmer, but I challenge a present day photo in a developing countries workshop. Heaps of Youtube spam and 'lets try', I'd like to see one in a real workshop in the wild. (That would be cool to see)
- idiotsecant 2y ago
  I think if you think it through carefully you'll understand how they are fundamentally the same.
  - 0_____0 2y ago
    This is essentially a big variable resistor - how does one apply that to arc welding?
    - idiotsecant 2y ago
      What is it that you suppose a modern arc welding system does? Stick welding needs about 20-30 volts during the weld (60v to strike the arc if we have a fancy machine). In a modern machine we turn high voltage low current mains power into low voltage high current arc welding power. This rheostat would turn it into low voltage slightly lower current but it would still work fine for light gauge.
      - 0_____0 2y ago
        I've done a fair bit of TIG, a little bit of SMAW. Are we talking about welding with basically reduced mains current? AC? SMAW and TIG both require current well in excess of what you can get out of a wall without tripping a breaker. I think for welding 14ga mild steel I was running at minimum 80A on the machine, but more like 130A with the foot pedal. Are you talking light gauge like as in spot welding territory?

mech975 2y ago

At a previous job we manufactured fairly large trailer-mounted generators. During design engineering testing, you would want to power a load with the generator to check performance. The test load we had was a giant resistor bank, in a pallet-sized enclosure about 4 feet tall, with fans for active cooling of the oodles of resistance heat being put off by it. I talked to one of the electrical engineers about how ludicrous this device was, and he said that we used to use something even weirder. A giant tank filled with salt water (can't remember for sure if they were using NaCl or a different electrolyte) with probes just dumping all that current right into the water. Wish I had seen it.

XorNot 2y ago
I'm not sure what's ludicrous about this setup though? For testing you want the simplest possible setup - which a giant resistor bank really is. Power is power when you run it through simple resistance - no AC shenanigans will hide effects.
- thfuran 2y ago
  The electrolysis rig seems a bit ridiculous, since it generates a bunch of flammable gas you have to deal with. A big resistive load for testing stuff seems pretty normal though.
  - amluto 2y ago
    Not just flammable hydrogen gas — you are likely to generate chlorine gas, potentially in large amounts.
    - thfuran 2y ago
      True, if it's very salty.
  - typh0n 2y ago
    It's not electrolysis, it's more like electrode boiling.

CaliforniaKarl 2y ago

For more on resistance dimmers in theater, check out this video from Jonathan Bastow: https://youtu.be/lTHNYWw6yt0?si=ygt0HpSd_aPcm3tq https://youtu.be/lTHNYWw6yt0?si=ygt0HpSd_aPcm3tq

deleted 2y ago
[deleted]

JKCalhoun 2y ago

As a "mad scientist" when I was perhaps eleven or so, I created a carbon arc by cutting the end off an electrical zip-cord extension cord, stripping the insulation off the two wires (lol, yeah, no ground wire) and wrapping the bare copper ends around a pair of carbon rods pulled from D-cell batteries. I held down each carbon rods by attaching it to a brick. Move the bricks together to touch the rods together to get a spark and an orange growl, and then draw them apart into an intensely bright white-hit light. Insane. Yes, I tripped the circuit breaker (thankfully not fuses in that place) repeatedly. Decided to make a saltwater rheostat to control the current. This time a square glass casserole dish with salt water was the tank. One of the mains wires was cut and enough insulation stripped to wrap around two rocks placed in the brine. Move the rocks closer, more current, move the rocks apart, less current. Holy hell, that was something. When the arc got to lighting up the basement I looked around to the rheostat to see it arcing under water as it bubbled (as another mentioned, yeah, probably chlorine gas). When the insulation on the zip cord soon started to smoke I decided the experiment was over and jerked the cord to pull it out of the outlet. This quick yank also sent the glass casserole dish crashing to the ground.... Ahhh ... if only YouTube were around then. I might have tried to do it more safely. https://youtu.be/VTzKIs19eZE https://youtu.be/VTzKIs19eZE Instead I got the idea from the book "700 Science Experiments for Everyone" (p. 185): https://archive.org/details/isbn_9780385052757/page/184/mode/2up https://archive.org/details/isbn_9780385052757/page/184/mode...

luckman212 2y ago
Love hearing stories like this about unfiltered, unfettered (and slightly insane) childhood play. Parents today don't even let their kids walk 2 blocks to school unsupervised, lest something terrible happen.
- JKCalhoun 2y ago
  I wouldn't let me do that now, ha ha. I could have burned the house down. (I did burn my friend's eyebrows off, but that's another story...)
- kadoban 2y ago
  The danger in that story is pretty off the charts. That could have _easily_ led to death, like maybe 1/100? That's something parents _should_ stop. It could have been redirected to safer versions, you can have a lot of fun with lower voltage DC, or at the very least safer setups.

jfengel 2y ago

Oh, holy hell. I thought "old sparky" was bad. It was a board cobbled together from a bunch of residential rheostats. The entire power supply of the theater flowed through that board. But at least Old Sparky didn't produce toxic fumes. It was merely a fire hazard.

mkwarman 2y ago
Do you know of anywhere I could learn more about this? Googling "old sparky" just yields articles about electric chairs, even when including keywords such as "rheostat" and "theatre"
- jfengel 2y ago
  That was just the theater's name for it. I imagine other theaters had similar contraptions, and because of the expression they probably came to similar names. It was at least 40 years old. I would hope they've all been retired.

locallost 2y ago

> the last dimmers in London theatres being replaced in 1959. I'm always amazed how long some things were actually used that you'd think stopped being used 100 years ago. Another one, steam locomotives, not completely phased out in some developed countries until the 70s.

eru 2y ago
Britain used steam locomotives until the 1980s. See https://en.wikipedia.org/wiki/Retirement_of_steam_locomotives_by_country https://en.wikipedia.org/wiki/Retirement_of_steam_locomotive... Germany apparently used them from time to time until 1994, but that was a function of East Germany being behind the Iron Curtain. My grandfather also used punch cards well into the 1980s in East Germany.
- Xylakant 2y ago
  Funfactlet: There's a steam locomotive in active duty shunting work trains around at the moment in western Germany. The Riedbahn between Frankfurt and Mannheim is currently being redone and they were short on Diesel engines, so one Verein for historic railroad equipment offered their steam engine and was accepted. https://www.swr.de/swraktuell/radio/riedbahn-sanierung-dampflok-als-hingucker-100.html https://www.swr.de/swraktuell/radio/riedbahn-sanierung-dampf... (German only) Another use for steam engines is for shunting duties in power plants - the engine has no boiler and just gets “charged” from the main plant steam generator.
  - femto 2y ago
    A similar thing happened here in Australia (New South Wales) in the mid 80s. I was part of a rail preservation group and we were taking a stream locomotive with an empty train from West Ryde Station to Central, from were we were going to depart for an enthusiast's trip. One of the suburban trains was cancelled, so Jock, the station master, asked if we would like to run the regular suburban service, which we did. It was great fun pulling in and watching the passengers do a double take as their regular train turned out to be a steam train.

userbinator 2y ago

Water is used because of its extremely high heat capacity and wide availability: https://en.wikipedia.org/wiki/Table_of_specific_heat_capacities https://en.wikipedia.org/wiki/Table_of_specific_heat_capacit... The concentration of salt has a huge effect on the resistance --- there is an infamous YouTube video of this effect being demonstrated with an electrode boiler.

the_third_wave 2y ago

These things were used at fairground attractions - bumper cars, merry go-arounds etc. - until at least the 1980's. They tended to come in the shape of a wooden barrel with a hinged lid which had a number of shark fin shaped metal plates on the bottom. There was a rope attached to the lid which was used to raise and lower it. The water got quite hot in use which led to steaming barrels later in the evening.

enb 2y ago

There's a period between the "beginning" of the modern age and today where we had these strange interstitial technologies. It sometimes feels like we went from living in the dark to electricity to the digital age in big jumps, but there's been a lot of evolution in between. Today will seem quite quirky to people 10, 20 and 50 years from now.

hnlmorg 2y ago
Today already feels quirky to me now.

guenthert 2y ago

"Salt water dimmers were gradually replaced by semiconductor dimmers, the last dimmers in London theatres being replaced in 1959." I would have expected that they would have been replaced by adjustable transformers as soon as lights were powered using AC.

beAbU 2y ago
I suspect the power requirements would have required truly massive and very expensive transformers. Stage lighting requires massive amounts of power. My guess is salt water resistors remained the cheaper and simpler alternative until semiconductors took over, completely skipping variacs.

keepamovin 2y ago

So cool. Had no idea about this and I love theatre. Maybe someone could build a mechanical/electrical/chemical computer using these jars of salt water? Ie, Salt water dimmers are Turing complete...arxiv://doi...

deleted 2y ago

[deleted]