Help save the last mercury arc rectifier substation

[tom66]

We had a radiation cabinet in our college. And the odd mercury thermometer and mercury tilt switches.

[GK]

I've got a dozen or so 866A's as spares for one of my radio transmitters. Not quite in the same league but still cool. Just never apply HT before the heaters have thoroughly heated the mercury.
 
 

[TerraHertz]

FWIW, I think it's now illegal for any school in the USA below college level to have any mercury whatsoever in their possession. Doesn't matter if its a chem lab or anything else. No mercury. IMO, the stuff isn't that bad if handled intelligently, but hot electrified mercury? No thanks.

Probably a good thing. Having a bottle of mercury in a school science lab is just inviting events like... what happened at my high school. Someone (not me!) thought it would be fun to sneak some outside at lunchtime. Where several mils inevitably ended up on the asphalt. Scene of maybe 50 teenagers having fun stomping on blobs of mercury, scattering them into tiny drops, over the entire surface. Asphalt of course being slightly porous, so that outside area is probably still mercury contaminated to this day.

[Fryguy]

I say they should be saved - working or not !

MARs are works of art - who had the idea to build them like this ? Is that large empty bulb on top necessary for them to work ?

I love big glass tubes . And sometimes they can be found in rather strange places .
In the movie "Captain Nemo and the underwater city" you can see a set of 4 or 6 ?!? (memory fail  ) brand new MARs installed right in the middle of the bridge on the second submarine without covers or cooling of any kind ! 

WTF are MARs doing on the bridge ? They belong in the engine compartment 

[N2IXK]

The big empty bulb up top provides a large, relatively cool surface for the mercury vapor to condense on.

When one of these is in operation, mercury is constantly dripping down the sides of the bulb, and running back into the pool at the bottom., It is then heated by the arc, turned into vapor, and rises up into the bulb to condense again. This cycle maintains a relatively constant vapor pressure in the tube, which is very important to avoid arcbacks and flashovers.

[Fryguy]

Thanks for the info N2IXK . I never bothered to look up how exactly these gigantic things work 

[G7PSK]

There used to be one in the science museum in London,

Any idea what date that was? I thought I'd seen a couple in the Science museum, one working, the other just for display. This would have been around 1970.

I am sorry to say that it was in the 1970's that I last visited the London science museum, that why I said they used to be one there as I have no idea if they still do. I would expect that they have one but not running. The one I recall working was somewhere near the large pendulum that was there.

[GreggD]

I could not start a new topic as this thread is a good one.
When I was young I never left the dentist without a drop of mercury in my hand. (Might explain a lot.)
I came across a terrific video of a working mercury-vapor-rectifier "diode" at the Fort Edmonton Museum Radial Railway. Now out of service.

https://www.youtube.com/embed/xNNnv3TOUbQ

[james_s]

About the only thing that seems to get people freaking out irrationally more so than mercury is "radiation". Mercury is just not all that dangerous. Yes it's toxic, no you don't want to take a bath in it or eat it, but with a little care it's just not a big deal. Most of the mercury the average person is exposed to comes from consuming seafood.

[rdl]

I remember "accidentally" breaking a thermometer once to get some mercury. I was only around 12 years old. I don't really recall why I wanted it, maybe just to say I had some in my basement lab.

[james_s]

I have a small bottle of it collected from old mercury tilt switches. They can often be found in the hood and trunk lights of older cars in junkyards. I figure I'm doing the environment a favor by collecting the mercury switches prior to the cars getting crushed. It's neat stuff.

[dmills]

Pure mercury is not too much of a problem, just don't try methylating it, dimethyl mercury is **NASTY**, in fact try to avoid making any organo-mercury compound, most of them are better avoided.

regards,, Dan.

[stj]

that's nothing, you should see an old "mercury arch" safety cutout device used on early electric railways!!

[Seekonk]

I used to work with ignitrons.   After a while they would short out due to mercury plating on the glass.  I would just turn them upside down and shake them.  Good for another six months.

[schmitt trigger]

The father of a friend of mine worked in a mining company where, among other metals, obtained mercury.
He once showed us a gallon-sized glass bottle full of mercury.

The bottle was so heavy we could not lift it.

Those were the days where mercury was heavily used (pun intended) in the electrical and electronics industries.

[mmagin]

Do you know what happens, if one of those break while in service?

Why would this ever happen?

[TimFox]

Did any power-station level equipment use xenon rectifiers instead of mercury vapor?  Transmitter power supplies often used xenon diodes such as the 3B25 and 3B28.
Note that gas rectifiers did not use gas discharge in normal operation, while "voltage-regulator tubes" such as the 0D3 did use glow discharge.  The rectifiers were operated at currents within the capability of a hot cathode.  The very heavy ions (mercury or xenon) could not move rapidly with the applied voltage, and loitered about the cathode, reducing the space charge dramatically, allowing high plate current at relatively low plate-cathode voltage.  The data sheet for a gas rectifier has an absolute maximum peak cathode current rating, which must be observed carefully to avoid producing a discharge that would damage the internal components (especially the cathode).

[calexanian]

As much as I love old stuff i gotta say, this no longer needs to be in service. Pull it our and stick it in a museum, but really for reliability sake get something more practical in there. 

[james_s]

Reliability? It's been operating for how many decades? Seems it's pretty reliable.

I agree though that a museum is the best place for it, or at least install it in a room with a big window so people can admire it.

[stj]

point a webcam at it!

[tautech]

AFAIKT our DC link and mercury rectifiers are still running after 50 years:
https://en.wikipedia.org/wiki/HVDC_Inter-Island

There are some mentions that the mercury arc rectifiers are to be replaced but I've not found mention that this has happened yet.
Further historical record:
http://ipenz.org.nz/heritage/itemdetail.cfm?itemid=53

[vinicius.jlantunes]

I always wanted to stick my finger in a little pool of mercury. Never did so of course. But something about it mesmerizes me and makes me wonder what the "texture" of it feels like.

Maybe I'm a bit weird 

[Macbeth]

I could not start a new topic as this thread is a good one.
When I was young I never left the dentist without a drop of mercury in my hand. (Might explain a lot.)
I came across a terrific video of a working mercury-vapor-rectifier "diode" at the Fort Edmonton Museum Radial Railway. Now out of service.

https://www.youtube.com/embed/xNNnv3TOUbQ

Thanks for this thread necro, it's been fascinating. The only tube rectifiers I've encountered are the likes of the Mullard EZ81 with a heater glow at most.

For the video I was amused at the presenters sudden "Fawlty Towers Nazi Goosestep" @ 1:37 in, however it becomes clear @ 2:10 that the reason was for Elfen Safety 

Looking at the old facebook posts from 2014 I would like to know what meter this Peter guy was using "to measure current" on the exciter fed from a live autotransformer. I think it's the rare Fluke 830B? 



It looks like a "current reading" should be done with the 830B upside down, because this is the Isle of Man, not Australia. 

[Macbeth]

I always wanted to stick my finger in a little pool of mercury. Never did so of course. But something about it mesmerizes me and makes me wonder what the "texture" of it feels like.

Maybe I'm a bit weird 

Not at all. Check out Cody's Lab.

[vinicius.jlantunes]

Oh now I want even more to do it! 
Thanks for the video Macbeth