Help save the last mercury arc rectifier substation - Page 1
Forum section: electronics / general

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 03, 2013, 04:52:02 pm
content: https://www.facebook.com/SaveOurSubstationIf you don't know what a mercury arc rectifier is, check out my page:http://www.electricstuff.co.uk/mercarc.html

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: August 03, 2013, 06:43:38 pm
content: Do you know what happens, if one of those break while in service?

author: c4757p
author_rank: Super Contributor
timestamp: August 03, 2013, 06:46:49 pm
content: Superpowers for those within 50m?

author: ddavidebor
author_rank: Super Contributor
timestamp: August 03, 2013, 06:51:21 pm
content: Heavy metal intoxycation, police, court, 300.000+$ out of your pocket, lawyer, court again, prison for a few year

author: Dago
author_rank: Frequent Contributor
timestamp: August 03, 2013, 07:11:54 pm
content: Quote from: ElectroIrradiator on August 03, 2013, 06:43:38 pm?

author: plesa
author_rank: Frequent Contributor
timestamp: August 03, 2013, 08:05:25 pm
content: in actionIt's a shame not to save such a historic station.

author: IanB
author_rank: Super Contributor
timestamp: August 03, 2013, 10:16:44 pm
content: That stuff is practically steampunk. It should get listed for preservation as a historical monument. Do people not realize how wonderful it is?It's a sad thing that historic technology is always struggling to be preserved, and it always falls to the general public to do it.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: August 04, 2013, 12:02:37 am
content: Quote from: IanB on August 03, 2013, 10:16:44 pmDoyouknow what happens, when a wonderful, hot, operational, energized, multi-phase, multi-KW Mercury vapor rectifier, a historical piece of technology, breaks?Mike can do whatever he pleases, as far as I am concerned. I just wish to ensure he knows what he is up against. He is advertizing for these on his homepage, and apparently somebody in the UK is planning on putting one of these in a public museum. Operational, that is.Just for the record: I don't really believe in the Nanny state, nor in putting warning stickers on everything. Because they dilute the actual warning signs. Those, which you really, really shouldn't ignore. As a result these days it is hard to have a cordial exchange involving potential safety issues due to all the muppet replies you will attract.ProTip: MV rectifiers were dumped as a technology at the earliest convenient moment by most of the world, as soon as semiconductors were a viable alternative for the various power levels. They weren't dumped due to looking dull, or because they didn't work or were unreliable.

author: ddavidebor
author_rank: Super Contributor
timestamp: August 04, 2013, 12:45:34 am
content: Mmh if this thing break, mercury vapor will arch and short and maybe became plasma... Funky!

author: Andy Watson
author_rank: Super Contributor
timestamp: August 04, 2013, 12:56:25 pm
content: Quote from: ElectroIrradiator on August 04, 2013, 12:02:37 amNo. But I'm now curious. Please tell.

author: madires
author_rank: Super Contributor
timestamp: August 04, 2013, 02:09:02 pm
content: Quote from: ElectroIrradiator on August 04, 2013, 12:02:37 amThey could place it in a secured chamber as museums do with old artefacts :-)

author: cybergibbons
author_rank: Frequent Contributor
timestamp: August 04, 2013, 02:17:32 pm
content: Quote from: ElectroIrradiator on August 04, 2013, 12:02:37 amKew Bridge Steam Museum is planning on having one of these on display.It's normally far more productive to say what you mean rather than ask silly questions and say why they weren't dumped.So what happens?

author: SeanB
author_rank: Super Contributor
timestamp: August 04, 2013, 02:34:43 pm
content: Only reason they were discontinues is because the replacement units required less cooling, and were essentially maintenance free and could be operated in any orientation. The mercury pool needs periodic adjustment and cleaning of the glass outer envelope, and needs a lot of cooling. Thus the lower losses and reduced control gear requirement of the solid state rectifiers ( no keep alive exciter and associated resistors and transformers) made them attractive for upgrades that still used the same transformers ( most expensive part of the station) and which occupied a smaller footprint in the plant floor allowing either another feeder bank or extra output circuits to be connected in the freed up area. As well the lower losses made a smaller fan usable, and as well smaller cheaper and longer lasting filters and less frequent filter cleaning, a savings on cost of maintenance. Mercury arc recifiers have a closer tolerance of operating temperature than silicon diodes, which can run happily at sub zero temperatures and survive thermal shock much better than a large glass bowl.

author: MacAttak
author_rank: Unknown
timestamp: August 04, 2013, 03:47:33 pm
content: There are plenty of details on the wikipedia entry. But basically, these have a fair amount of liquid mercury in them, stored in a brittle glass bulb. If they shatter then the result is a very nasty environmental hazard / cleanup expense. And I suppose in an explosive shatter there is probably quite a bit of vapor ejected too. Think about how freaked out people get over a single cracked mercury thermometer or tilt switch, and multiply that by a million.Also, the larger ones (steel, not glass) required constant pumping to maintain sufficient vacuum. This essentially means that they were constantly emitting mercury vapors into the environment.Lastly, the beautiful purple glow they emit while operating is very heavy in a wide range of UV wavelengths. I'm not sure I would want to stare at one or even stand near it for too long.

author: TerraHertz
author_rank: Super Contributor
timestamp: August 04, 2013, 05:16:28 pm
content: FWIW, the old Glebe Island swing bridge in Sydney, has a big mercury arc rectifier onsite, that was used to provide the DC for the motors that swung the bridge.It's in a small shed at the Eastern end of the bridge approaches. Or at least it used to be, um... around 2001 or 2 when I saw it. At that time the cabinet it was in was still powered up, so I suppose it was still operational.Unfortunately, I don't have photos.

author: G7PSK
author_rank: Super Contributor
timestamp: August 04, 2013, 08:45:36 pm
content: There used to be one in the science museum in London, as I remember it was under the stairs and was used to power the lift, at certain times of the day it was opened to public viewing as well, I would expect that if you watched it too long you would get a good tan.

author: Andy Watson
author_rank: Super Contributor
timestamp: August 04, 2013, 09:26:28 pm
content: Quote from: G7PSK on August 04, 2013, 08:45:36 pmAny 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.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: August 04, 2013, 09:51:38 pm
content: Quote from: cybergibbons on August 04, 2013, 02:17:32 pmIn general I'd agree with that, and I had my doubts about how to approach this. In Mike's case in particular I found it would feel pretentious to give a long and potentially unwanted explanation out of the blue. I would much prefer if he had just said he was on top of things.Quote from: SeanB on August 04, 2013, 02:34:43 pmAs far as I can tell, then all of this is accurate. That is however not what I was getting at with the 'dump' comment. I am not old enough to have worked with MV rectifiers, back when they were widely used, but my mentors certainly are. According to them many industrial MV installations were actively decommissioned ASAP, as alternatives became available. This makes it even more amazing that some are still in active service.Quote from: MacAttak on August 04, 2013, 03:47:33 pmOh, but it is much better than that.OK, here is the (overly long) explanation: Why you (probably) don't want an active MV arc rectifier in your home or museum.The background here is that metallic Mercury at room temperature is pretty benign. It wouldn't be a good idea to ingest some, but having it sitting in for instance an open bowl would not be a major problem. The reason is that the vapor pressure of Mercury at normal room temperature and air pressure is quite low, so the rate of evaporation would be negligible. Even accidentally dropping some Mercury on the floor of your lab isn't that much of a problem. There are even standard lab tools for dealing with this situation, so you are probably fine if you pick it up in a reasonable time frame. The rate of evaporation will be miniscule due to the relatively small surface area exposed to the air.Enter the industrial, glass encased MV arc rectifier, and its smaller sibling, the 'vacuum tube' edition with a heated cathode.The first problem is that to make a decent live display of a large MV arc rectifier, you need to pass some *serious* power through them, along with the cooling to match. There is a video on YouTube showing Mike's substation in action, and here it powers a train, or rather a tram. To put this in perspective, then a KW level radio transmitter would just need a single pair of beer bottle sized MV rectifiers for its PSU. So you need serious power/current for your live display (or show it in complete darkness, and hope people won't be too disappointed).The second problem is that you need flexible leads for most of the electrical connections to the rectifier. Otherwise the mechanical stresses due to thermal expansion would cause cracks in or downright failure of the vacuum vessel.So even if gravity helps out, then in case the glass vessel breaks there is a very good chance you'd - briefly - have very conductive Mercury, lots of electricity and wildly dangling wires all over the place. This is in addition to the Mercury dropping down on the cooling fan and being splattered all over the place. Those of you, who may have seen an accident involving a car battery, may be able to guess what comes next."Mercury smoke. Don't breathe this."OK, so maybe we predicted what was about to happen, and teleported out of there just before the Mercury hit the fan. We'll just wait for the Mercury to condense out of the air and go clean up. Right?Erm, no, this is where it gets ugly.We just created a sizable amount of microscopic pellets of liquid Mercury, small enough to be carried short distances by the normal air currents. Genuine Mercury smoke. That stuff will goeverywherein the building in question, snuggling into every nook and crevice in fabric, untreated wood, clay/brick, etc. Even better, the tiny droplets will create an amalgamate with many metallic surfaces, making the Mercury impossible to clean off.And for fun, the total surface area of the combined mass of micro droplets is no longer trivial, compared to the vapor pressure. Additionally the Mercury will also stick to hot surfaces, like stove plates, room heaters, incandescent light bulbs, electronics and more. As a result you now have noticeable amounts of Mercury vapor in the local atmosphere, at least until you removeallthe Mercury smoke particles from, well,everysurface in the building. Which is physically impossible.You now have two options:*) Clean away any visible traces and spills from the accident, and pretend it didn't happen. This is likely to cause children spending much time in the building to grow up and be stupid. Literally. Adults are just likely to get sick, and may potentially die in due time. Mercury is like Lead, just a lot worse.*) Condemn the building, tear it down, and store its remains and the items within as hazardous waste. This will be a popular option for a museum.Quote from: MacAttak on August 04, 2013, 03:47:33 pmShhh! Some people may use this as a selling point, as the museum visitors may actually want a nice tan...

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 04, 2013, 10:20:05 pm
content: ..or you just put it in a clear perspex case, with a tray big enough to catch everything, probably with some sand to reduce splashing. You can avoid the fan issue by running at low power and/or intermittently...and of course avoid breaking the glass in the first place...Siemens Museum in Munich has a MAR on display btw.

author: Andy Watson
author_rank: Super Contributor
timestamp: August 04, 2013, 11:10:10 pm
content: Quote from: ElectroIrradiator on August 04, 2013, 09:51:38 pmIs this really a problem? I think you are seriously underestimating the competence and expertise of the people* that are dealing with these devices. You live in a very scary world - I'd better not trouble you with the experiment that we were asked to perform as part of the school chemistry (reduction of mercuric oxide, ... with a Bunsen burner and blow pipe).*Actually, considering the multitude of pratts on youtube, you may have a point.

author: lewis
author_rank: Frequent Contributor
timestamp: August 04, 2013, 11:35:25 pm
content: Mercury's not that bad, I used to crack open tilt switches as a kid and play with the stuff in my hands. Never did me any h 3g 874t 394ty 9ghghrifdf hdifg heruighierh giaeuger'[ergberiuvbiaf erubvhv[b avuig ivi

author: AlfBaz
author_rank: Super Contributor
timestamp: August 05, 2013, 12:34:41 am
content: On a work experience school excursion we visited our local technical college in Wollongong and they had one of these in their lab (early 80's). It sat in a big enclosure with Perspex viewing windows. They didn't energise it for us but I asked what it was and when they said it was a diode we all broke out laughing. I wonder if they kept it

author: Uup
author_rank: Regular Contributor
timestamp: August 05, 2013, 03:20:48 pm
content: The Fun Museum at Cowra (NSW) has three mercury arc rectifiers. One of them is set up with the transformers for demonstration. There was no one there when I was there so I turned in on and had a play with it!It's a very interesting place that has so much historical stuff in there. It is apparently a private collector's personal museum open to the general public.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 05, 2013, 05:58:40 pm
content: 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.

author: c4757p
author_rank: Super Contributor
timestamp: August 05, 2013, 06:03:39 pm
content: Quote from: Conrad Hoffman on August 05, 2013, 05:58:40 pmYou could probably fill in the blank with anything that anybody has ever been scared of.

author: tom66
author_rank: Super Contributor
timestamp: August 05, 2013, 06:11:46 pm
content: We had a radiation cabinet in our college. And the odd mercury thermometer and mercury tilt switches.

author: GK
author_rank: Super Contributor
timestamp: August 06, 2013, 09:18:15 am
content: 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.

author: TerraHertz
author_rank: Super Contributor
timestamp: August 06, 2013, 10:22:39 am
content: Quote from: Conrad Hoffman on August 05, 2013, 05:58:40 pmProbably 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.

author: Fryguy
author_rank: Regular Contributor
timestamp: August 06, 2013, 11:52:19 am
content: 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

author: N2IXK
author_rank: Frequent Contributor
timestamp: August 06, 2013, 12:32:07 pm
content: 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.

author: Fryguy
author_rank: Regular Contributor
timestamp: August 06, 2013, 01:49:18 pm
content: Thanks for the info N2IXK . I never bothered to look up how exactly these gigantic things work

author: G7PSK
author_rank: Super Contributor
timestamp: August 06, 2013, 03:27:01 pm
content: Quote from: Andy Watson on August 04, 2013, 09:26:28 pmI 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.

author: GreggD
author_rank: Regular Contributor
timestamp: May 22, 2017, 06:12:53 pm
content: 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

author: james_s
author_rank: Super Contributor
timestamp: May 22, 2017, 06:25:33 pm
content: 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.

author: rdl
author_rank: Super Contributor
timestamp: May 22, 2017, 06:35:54 pm
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: May 22, 2017, 07:17:56 pm
content: 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.

author: dmills
author_rank: Super Contributor
timestamp: May 22, 2017, 07:29:54 pm
content: 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.

author: stj
author_rank: Super Contributor
timestamp: May 22, 2017, 07:40:56 pm
content: that's nothing, you should see an old "mercury arch" safety cutout device used on early electric railways!!

author: Seekonk
author_rank: Super Contributor
timestamp: May 22, 2017, 08:20:25 pm
content: 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.

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 23, 2017, 04:16:25 pm
content: 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.

author: mmagin
author_rank: Frequent Contributor
timestamp: May 23, 2017, 04:39:35 pm
content: Quote from: ElectroIrradiator on August 03, 2013, 06:43:38 pmWhy would this ever happen?

author: TimFox
author_rank: Super Contributor
timestamp: May 23, 2017, 06:57:16 pm
content: 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).

author: calexanian
author_rank: Super Contributor
timestamp: May 23, 2017, 07:16:34 pm
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: May 23, 2017, 08:03:33 pm
content: 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.

author: stj
author_rank: Super Contributor
timestamp: May 23, 2017, 08:06:31 pm
content: point a webcam at it!

author: tautech
author_rank: Super Contributor
timestamp: May 23, 2017, 08:20:26 pm
content: AFAIKT our DC link and mercury rectifiers are still running after 50 years:https://en.wikipedia.org/wiki/HVDC_Inter-IslandThere 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

author: vinicius.jlantunes
author_rank: Regular Contributor
timestamp: May 23, 2017, 10:12:48 pm
content: 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

author: Macbeth
author_rank: Super Contributor
timestamp: May 23, 2017, 10:32:25 pm
content: Quote from: GreggD on May 22, 2017, 06:12:53 pmThanks 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 SafetyLooking 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.

author: Macbeth
author_rank: Super Contributor
timestamp: May 23, 2017, 10:45:07 pm
content: Quote from: vinicius.jlantunes on May 23, 2017, 10:12:48 pmNot at all. Check out Cody's Lab.

author: vinicius.jlantunes
author_rank: Regular Contributor
timestamp: May 23, 2017, 11:00:51 pm
content: Oh now I want even more to do it!Thanks for the video Macbeth

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 23, 2017, 11:20:10 pm
content: When I was at school, we made barometers by inverting a big tube full of mercury with our finger over the end into a bowl of mercury. This was in proper physics class, not messing about!

author: Brumby
author_rank: Unknown
timestamp: May 23, 2017, 11:28:39 pm
content: When I was 12, in my first high school science class we had a new teacher. She set up an experiment to show heat vs temperature for ice-water-steam.After entertaining us with lighting the gas tap instead of the bunsen burner, she fitted one of those 12" mercury thermometers in a cork which was placed in the neck of the flask containing the ice/water mixture. Having addressed the flame malfunction, she then proceeded to heat the flask.All went well until the water started to boil - and then we had a ballistic event that would have inspired any budding rocket scientist. The mercury thermometer was launched into the ceiling with such force that it completely disintegrated. Yes, she did not provide any means for pressure relief.We in the class were just amused by all this - and nobody started freaking out about the mercury.Somewhere along the line, I know I've played with a bit of mercury. I've even had some roll around in the palm of my hand - but that was just the once since I didn't want to risk losing any of such a cool material. The thing I noticed, though, was how heavy it felt. You just don't expect liquids to be that dense.

author: Brumby
author_rank: Unknown
timestamp: May 23, 2017, 11:32:28 pm
content: Yes, I washed my hands.Quote from: mikeselectricstuff on May 23, 2017, 11:20:10 pmCome to think of it, I have a vague recollection of that happening at my school - but by the teacher. Don't remember if they made contact with the mercury. We just weren't aware of such things.

author: cdev
author_rank: Super Contributor
timestamp: May 23, 2017, 11:55:53 pm
content: Mercury is particularly problematic in old factory buildings.Back in the day, energy was super cheap and buildings were drafty. So problematic mercury contamination of old warehouses and schools may not have caused the kinds of health issues it does now when air is confined indoors, until a successful closing up of all the air leaks had been done to save money on heating/cooling. This is likely one of the reasons why autism, etc. rates are rising, the rising costs of energy and reduced ventilation of buildings. Dilution is a solution for air contaminants. Poorer people cant afford as much ventilation in cold winter areas.The classic example of this here in the US was a real tragedy. A site-720 and 722-732 Grand Street in Hoboken, New Jersey. The area includes a former industrial building converted into 16 residential artist studio spaces at much effort and expense by an artists collective.But at some point during the building's renovation, somebody discovered a space under the flooring that contained liquid mercury and this led to the discovery that the entire building and area was heavily contaminated and many of the buildings residents were being poisoned by it. In the past it had housed a factory that made mercury lighting products, including a popular sunlamp used for indoor tanning.

author: The Soulman
author_rank: Super Contributor
timestamp: May 27, 2017, 09:35:52 pm
content: Photonicinduction had one: (from 3:20)

author: Zero999
author_rank: Super Contributor
timestamp: May 28, 2017, 10:15:06 am
content: Has anyone heard of one of a mercury rectifier exploding? What's the risk of it actually happening?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 28, 2017, 12:27:32 pm
content: Quote from: Hero999 on May 28, 2017, 10:15:06 amPretty negligible I think, other than from physical damage. I'm not sure what the internal pressure is normally, or whetehr it's under vacuum when cold, but all the ones I've seen don't seem to have particularly heavy glass to resist pressure.

author: amyk
author_rank: Super Contributor
timestamp: May 28, 2017, 04:07:16 pm
content: They operate in a vacuum. The metal-cased ones weren't completely sealed and so needed vacuum pumps to maintain that vacuum, which also continuously emitted mercury vapour into the air.(Attached image is fromthis book, which is also very interesting to read.)