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Old 31st Jan 2019, 1:14 pm   #1
ounce1966
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Default Mercury Arc Rectifier - How...

Hi,
Following on from my intro post, I would very much appreciate some advice.

Very briefly - I have a lovely bulb - see the pictures here...

https://photos.app.goo.gl/www9XAKdC8e7RTt47

But no other gear or casings etc....

I want to fire it up for purely cosmetic reasons; would it be possible to safely apply power to it - and if so what do I need and where do I attach the leads!

If
> this would be very difficult, expensive or just too dangerous. NB. One of the arms has some damage (see the pictures) and I am concerned it may be a weak spot that might fracture when in operation - is this genuine a concern or me over egging it??

> Then I'm considering opening it, disposing of the mercury (properly of course) and using some modern leds to light her up.

Thoughts and advice would be very welcome and of course I would show off what I eventually achieve.

Many thanks...
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Old 31st Jan 2019, 1:51 pm   #2
Lucien Nunes
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Default Re: Mercury Arc Rectifier - How...

Yes, a nice bulb indeed!

3-phase main circuit of around 60A by the look of it, and unusually it seems to have 3-phase auxiliary and mercury neck starting.

I have found Nevelin bulbs pretty solid and reliable, although that damage looks a bit naughty. If it's vertically above the anode, it's probably the result of overload. In extreme cases, the envelope would melt and pinhole. It will remain to be seen whether you have a good vacuum; I don't recommend the tilt-test these days as seals can be damaged, so you might have to run it up electrically before you find out whether it is operable. You will need to mount it by supporting the bottom of the bulb on a padded ring with the cathode leadout poking through, and a soft band around the top of the bulb to stabilise.

To operate a MAR for visual effect you need current-limited supplies of a few amps per anode capable of a fairly high O/C voltage, although the forward drop is only in the order of 14-16V. Historically, a very heavy series inductor was needed to avoid arc extinction, but there are workarounds. There will be no need to run the excitation anodes as they are hardly visible when the main anodes are under load. They were there to keep the arc alive when there was no load on the main circuit.

You also need starting gear. Your bulb used one of the two less-common starting methods, where a narrow neck of mercury lies between the starting electrode (angled downward) and the cathode pool. Tilt-start versions used a solenoid to tilt the bulb cradle, breaking the link between the two, interrupting the current through the solenoid and using its back-emf to ignite the arc. Other units had a starting transformer that sent a pulse of some hundreds of amps through it, causing the mercury to displace and break the link. You could easily arrange a tiltable cradle operated manually, along with something like a fluorescent ballast as a starting inductor.

A correspondent recently wrote to me to tell of his success with setting up compact gear for operating MARs at home. I will loop him in as he has practical experience of this. I normally only run them from their original gear.

Final warning for now: DO NOT TWIST THE TERMINAL CAPS! Nevelin seals seem to be strong, I have had much less trouble with them than say Hewittic, but assume they are all fragile. When making connections, gently put a strap around the cap, or even just clip the wire on. Never risk applying any force.
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Old 31st Jan 2019, 4:13 pm   #3
The Philpott
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Default Re: Mercury Arc Rectifier - How...

As mentioned the click test for ascertaining whether there is still a vacuum, is a risky strategy and not recommended because of the risk of a slug of Hg cracking the glass. If however your bulb is still in the loft in the summer it will have gone through some temperature cycles and there SHOULD be some small beads of Hg clinging randomly to the inside of the glass. These will be too small to cause damage, but a gentle tap will dislodge them and they will fall back into the Hg pool. If there is a vacuum they will make a 'tink' noise as they fall back down. If you hear this tink-tink-tink i would say the vacuum is still good.

Needless to say whatever is being powered by the rectifier will be something that demands quite a high DC current, such as an electric motor.

Personally i would not evacuate the Hg from the bulb, my reasons:

-You will be charged for disposal- if you can find a firm that agrees to do it.
-If it's inside the tube everyone knows what it is- and where it is.
-The value (to an enthusiast, at least) of the bulb will plummet by more than 75%.

It's all your choice of course!

I eventually chickened out of keeping mine in the loft, i had an overactive imagination regarding the contents escaping and gravitating through the whole house. There are a few animated discussions on this forum regarding safety (or otherwise)- worth a read. They are certainly a 'heritage' item and deserve to be saved if possible.


Dave
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Old 31st Jan 2019, 6:41 pm   #4
Lucien Nunes
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Default Re: Mercury Arc Rectifier - How...

I totally agree with Dave that the best place for mercury is inside a rectifier. If the bulb works, you know it's sealed and therefore not a source of vapour. A rectifier that is down to air may have such a tiny leak that it is of no consequence either, although if any cap is loose there could be a larger hidden breakage around the seal.

I suggest keeping an impervious tray underneath, getting spill control materials and learning how to use them, and taking all reasonable precautions to avoid seal damage. The bulb itself is strong, the arm joints less so, and the seals the weakest points.

Take care when carrying not to let mercury surge up the arms. This is how the click test was done, with the intention of hearing whether there was any gas to cushion the impact of the mercury (I've no idea why I called it the tilt-test earlier, that's for buses or heart patients!)
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Old 31st Jan 2019, 6:42 pm   #5
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Default Re: Mercury Arc Rectifier - How...

This won't help at all. But you have properly brought back a memory. When I was in my teens, we helped out by doing a charity collection for something at our local cinema. In return we were given a tour of the projection room and facilities.

Even though we are talking late seventies / early eighties, the projectors were all quite old, and used carbon arc. They had been installed when the town used DC. So when it changed to AC they installed one of these, and a bank of resistors in 'the resistor room'. Later the AC voltage changed, so a big transformer was added.

Seeing one of these, working, hidden in some kind of cupboard with it's eerie purple glow is a long lasting memory.

I just wish I'd taken a camera that day!
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Old 31st Jan 2019, 7:37 pm   #6
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Default Re: Mercury Arc Rectifier - How...

Quote:
Originally Posted by The Philpott View Post
As mentioned the click test for ascertaining whether there is still a vacuum, is a risky strategy and not recommended because of the risk of a slug of Hg cracking the glass. If however your bulb is still in the loft in the summer it will have gone through some temperature cycles and there SHOULD be some small beads of Hg clinging randomly to the inside of the glass. These will be too small to cause damage, but a gentle tap will dislodge them and they will fall back into the Hg pool. If there is a vacuum they will make a 'tink' noise as they fall back down. If you hear this tink-tink-tink i would say the vacuum is still good.

Needless to say whatever is being powered by the rectifier will be something that demands quite a high DC current, such as an electric motor.

Personally i would not evacuate the Hg from the bulb, my reasons:

-You will be charged for disposal- if you can find a firm that agrees to do it.
-If it's inside the tube everyone knows what it is- and where it is.
-The value (to an enthusiast, at least) of the bulb will plummet by more than 75%.

It's all your choice of course!

I eventually chickened out of keeping mine in the loft, i had an overactive imagination regarding the contents escaping and gravitating through the whole house. There are a few animated discussions on this forum regarding safety (or otherwise)- worth a read. They are certainly a 'heritage' item and deserve to be saved if possible.


Dave
Another reason for not removing the mercury is the glass bulb will be under a decent vacuum if intact and if unlucky in breaking it could implode sending mercury and glass everywhere in all directions!!

Christopher Capener
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Old 31st Jan 2019, 8:30 pm   #7
ounce1966
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Default Re: Mercury Arc Rectifier - How...

Thanks for all your replies. It's only been in the loft a few days, I was only given it on Tuesday. I haven't done a click test as such, but while I was driving home with it wrapped up in the foot well, occasionally I heard some clicking sounds, a bit like the glass cracking. Since I can't find any cracks I'm going to assume that the impromptu click test was ok!

Lucien - your first post was great, but I'm afraid you've wildly overestimated my knowledge and understanding of things electrical! What I really need is something like
a. Get this and that
b. Connect this to here
c. Connect that to here
d. Switch on
Maybe your correspondent will be able to help!

I really do appeaciate all responding to the newbie. Thanks, I'll be reading some more before deciding what to do.
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Old 31st Jan 2019, 9:13 pm   #8
dave walsh
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Default Re: Mercury Arc Rectifier - How...

Depending on how this thread is accessed, either yesterday's introduction is not combined or it is with just ounces opening remarks showing.The posts from yesterday expressing concern don't appear either way . Maybe it's a problem at my end? It wasn't just what I've read by the experts that prompted a response ounce but you were also very frank about your [lack of] confidence re electrical matters which added another layer for me! You've got all the best people on board now so perhaps it will all be ok? Does your user name relate to an ounce" of Mercury I wonder? I seem to recall that was the term used at one time!

Dave

Last edited by dave walsh; 31st Jan 2019 at 9:40 pm.
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Old 31st Jan 2019, 9:59 pm   #9
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Default Re: Mercury Arc Rectifier - How...

Apologies Mods. I thought both threads had been linked when the first part of yesterdays came up here but I see the Intro thread is still there... intact. Don't know how this happened but I suppose I've done a link myself now!

Dave
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Old 1st Feb 2019, 10:32 am   #10
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Default Re: Mercury Arc Rectifier - How...

If you power it up there could be UV light emitted so some sort of effective eye protection should be worn.

Mercury can be disposed of via the local waste authority at no cost. A few years ago I wished to dispose of a small stone stoppered container holding about 200 grams of mercury.

I consulted my local authority (Sefton Council ) and they organised the safe collection

Interestingly the container provided was about 10 times larger than my container.

I would not in any circumstances let the mercury escape from your bulb
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Old 1st Feb 2019, 11:26 am   #11
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Default Re: Mercury Arc Rectifier - How...

Quote:
Originally Posted by duncanlowe View Post
This won't help at all. But you have properly brought back a memory. When I was in my teens, we helped out by doing a charity collection for something at our local cinema. In return we were given a tour of the projection room and facilities.

Even though we are talking late seventies / early eighties, the projectors were all quite old, and used carbon arc. They had been installed when the town used DC. So when it changed to AC they installed one of these, and a bank of resistors in 'the resistor room'. Later the AC voltage changed, so a big transformer was added.

Seeing one of these, working, hidden in some kind of cupboard with it's eerie purple glow is a long lasting memory.

I just wish I'd taken a camera that day!
I once replaced a failed mercury rectifier with some silicon diodes in a cinema. The manager could not believe that such a complicated device could be replaced by what to him looked like a few large nuts & bolts. Every time I met him in the bar after, he would introduce me as some sort of magician!
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Old 1st Feb 2019, 12:10 pm   #12
Lucien Nunes
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Default Re: Mercury Arc Rectifier - How...

We should conduct a proper study on the UV emission but as far as I am aware, occasional direct viewing of a working MAR is harmless. My understanding is as follows:

MARs operate at very low pressure in order to minimise voltage drop. The sole function of the bulb is to maintain the low pressure by providing a condensing surface for vapour that is emitted from the cathode pool. At low pressure, UV emission from the discharge column is mainly UV-C at 184.45 and 253.7nm. These short wavelengths are harmful, but almost completely filtered out by the glass envelope.

By contrast, mercury lamps designed to emit UV are typically medium-pressure types in which significant radiation occurs at 365.4 nm (UV-A). They use quartz envelopes that are transparent to UV.

I am not aware of special precautions being taken in industry to prevent casual exposure to light emitted by MARs. Their cabinets usually surround the bulb itself, but grilles without louvres or light-traps are standard. Although most MAR installations date from the days of lower levels of worker safety than today, photokeratitis (arc-eye) was well understood and recognised in industry when MARs were commonplace.

It would be good to get real photometric data at all wavelengths of concern from a functioning bulb though.
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Old 1st Feb 2019, 1:27 pm   #13
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Default Re: Mercury Arc Rectifier - How...

Interesting to note that Sefton Council assisted with disposal (perhaps the same is true of other areas....and you don't find this out until you 'phone them.)

I can understand being cautious around UV emissions of any sort and limiting exposure. I should imagine increasing the distance between yourself and the source reduces the exposure by three-quarters. (correct me if i'm wrong)

Even the BLB T8 tubes i have are unpleasant to briefly glance at, at close range. If you have old glass bottles containing trace Manganese it is possible to turn them pink or purple if you bathe them in UV for long enough. (I hope this doesn't qualify as Steampunking..)

I commented elsewhere that a rectifier bulb with a failed seal will emit red spectra due to infiltration of atmospheric gases. (Visible through a red filter.)

Dave
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Old 1st Feb 2019, 1:45 pm   #14
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Default Re: Mercury Arc Rectifier - How...

Quote:
Originally Posted by ounce1966 View Post
What I really need is something like
a. Get this and that
b. Connect this to here
c. Connect that to here
d. Switch on
Maybe your correspondent will be able to help!

I really do appreciate all responding to the newbie. Thanks, I'll be reading some more before deciding what to do.
From memory, these are fascinating to watch, even mesmerising. However it is industrial heavy engineering kit and the chances of running it with no control gear, backup or knowledge are vanishingly small. The risks, both legal and financial, however, are all too real.

Our advice is to study the subject thoroughly and think very carefully before going any further.
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Old 1st Feb 2019, 2:26 pm   #15
kalee20
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Default Re: Mercury Arc Rectifier - How...

Just thinking, crudely, can't you get it to light with a decent DC arc welding kit, with a short-circuit current of 40A or so? Assuming the arc welding set has some output inductance so that current does not fall to zero twice each cycle. Lucien says it is tilt-start, so keep the cathode as common, connect the starting anode and another anode together by a temporary link, switch on with the bulb tilted, swivel back to break the mercury path, and then disconnect the starting electrode.

Obviously this would give a continuous discharge to one anode only, so heat generated would be concentrated and not spread around. So not a good idea to run for more than a minute or so, unless current is kept low. But you'd see it 'alive!'

However, I'd strongly suggest that Ounce1966 does a lot of homework first!
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Old 1st Feb 2019, 9:42 pm   #16
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Default Re: Mercury Arc Rectifier - How...

In the late 50' s my parents house backed on to the electrified Southport to Liverpool railway ( 650 DC supply to third rail ) About 200 yards away was a mercury arc rectifier sub station , the building dated about 1904 with tall glass windows. The whitish / blue glow at night was very impressive.
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Old 1st Feb 2019, 11:00 pm   #17
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Default Re: Mercury Arc Rectifier - How...

There's a M.A.R on display in Birminghams Think Tank. I don't know if it runs everyday but it was nicely lit up last time I was there (November )
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Old 1st Feb 2019, 11:26 pm   #18
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Default Re: Mercury Arc Rectifier - How...

They used to have one in the science museum in London with a button to start it up.
It had a striker that dipped into the pool of mercury to lift the arc to the anodes.
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Old 2nd Feb 2019, 10:47 am   #19
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Default Re: Mercury Arc Rectifier - How...

Yes, I remember enjoying that many moons (or rather decades) ago.
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Old 2nd Feb 2019, 12:05 pm   #20
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Default Re: Mercury Arc Rectifier - How...

Quote:
Originally Posted by The Philpott View Post
... I should imagine increasing the distance between yourself and the source reduces the exposure by three-quarters. (correct me if i'm wrong) ...
Once you're far enough away, in the so-called 'far field', doubling the distance between you and any conventional (not laser) light source does lower your exposure to a quarter of what it previously was.

But in the case of a distributed (i.e. laterally spread-out) source then things are more complex when you're relatively close to it, in the 'near field'. As a general rule doubling your distance doesn't then achieve as much of an exposure reduction as you might hope.

Cheers,

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