That looks like a mercury rectifier.
Sadly, I had not yet the opportunity to see one in real life.
However, It looks like a lightbulb designed by Benoit Mandelbrot :bigsmile:
I gotcha! 
That heatsink does sorta look like a pill for an LED! When I came across that the other day, that is EXACTLY what crossed my mind!
The rules are, before someone can post another pic, the previous pic has to be identified.
I’m 67 years now and have been working with electronics since the mid 1960s.
And the next picture does look like a mercury rectifier. We had many of them in motor speed controls for Navy surplus Monarch lathes.
Creepy looking thing, isn’t it? I can’t imagine that they’re terribly durable. It says these were used in electric railroad locomotives… seems like a harzardous cleanup in the event of a collision.
:weary: For some reason I thought you were saying the heatsink was used with the bulb. As odd as that would be.
Of course I was reading things you never said.
Embedding disabled on the Mercury Arc Rectifier video
http://www.youtube.com/watch?v=yjMZ5qtyCU
I believe we have some of those lamps at my company's other office. They were still being used up until late last year! I was trying to push them toward a more modern cap lamp so as to drop the weight of that darn big battery on their side! Here is a pic of the lights and the charging station at our other office (pic taken 1 1/2 to 2 years ago):
-Garry
It amazes me how sometimes “old tech” has an ability to hang in there when you would think something else would replace it.
Well this is typical in the mining industry (at least this is what I see and hear). Many things are still done the old fashioned way with old technology. Even when you try to push the new technology as being better, more productive, less downtime, etc . . . they are still reluctant to change! My co-workers now use Li-Ion powered LED cap lamps. I haven't seen them yet, but from what I hear the battery is built into the cap lamp assembly and is charged inside (i.e. battery is not removeable). I think they only put out about 150 lumens (I couldn't find LED cap lamps certified for u-gnd use which had higher output), but they are rated for over 8 hours continuous use which is the most important consideration. I would love to see these new ones and see what the beam is like and what the tint is like. I'm willing to bet it's a very throwy beam with nasty cool white tint! One of the issues is that sometimes (not all the time) the cap lamps need to be "certified" for use u-gnd and therefore your stuck with what is available!
-Garry
I would think the biggest concern in a mine would be sparks. With all that coal dust and perhaps flammable vapors. Lead-acid batteries just don’t sound that safe to me. I would think that what you would want is an integrated light and battery, perhaps hermetically sealed so that there would be no sparks. Charging would be done above ground.
Well our use is in stone mines, not coal and technically for stone mines we didn't need "certified" lamps but bought them just in case we did end up in a coal mine. Oh, and it's amazing how bright 30 lumens is underground in complete darkness!
(Sorry if I've taken this thread too far off-topic.)
-Garry
This should be real easy for some, and a history lesson for others.
It used to be that getting a part like this was very hard to do and expensive. Sure you could go to Radio Shack and buy a consumer version of this part, but even then they were incredibly expensive and no where near the quality or value of these. Almost every electronics project needed them. I was always deprived of this particular type of part………until I found out how to get them - cheap. When I finally got a hold of some, I bought a whole case 24 for $5.00. I just checked eBay, they sell for upwards of $30 each today. It is little wonder that the day I got them I actually went to sleep that night with one of them under my pillow! I absolutely Cherished these. The color, size and shape and most importantly, what they could do.
Here are 2 of them next to a C8 for size. Before the company that manufactured this particular part was sold in 1978, they also manufactured DuraCell batteries!
The questions are:
What is it?
What was it called?
Where was this particular item to be used?
Bonus: Why was I able to get so many of them so cheaply? Hint: 1971
Hmm . . . looks like a capacitor. And it looks like ones I used to see in older HiFi stereo receivers.
-Garry
Creepy and beautiful at the same time. 
For some reason, old tech stuff like that always attracted me.
@dchomak
They look like capacitors, also known as condensers to me.
With some nice screwing terminals. Maybe they were used as start/rum capacitors for some kind of motor (or generator)?
They look like Mallory electrolytic capacitors.
Filter caps for power supplies in vacuum tube amps?
Cheap because they were not needed for solid state devices?
I'm thinking the 1971 date is tied to computers somehow.
-Garry
If I was in an underground cave I would rather have 30 lumens of warm high CRI light. I can understand the resistance to upgrading if you have gear that is well known (and well known flaws) vs. new stuff that needs testing and familiarising, and training others on use (health and safety).
The suspense is killing me. Someone guess this correctly now darn it.
I would guess some type of capacitor, but the sleeping with it under your pillow the first night thing is throwing me off. I can't imaging wanting to have a capacitor anywhere near my rock filled head. I wasn't playing with electronics much back then. So I have no appreciation for the shortage of this type of part.
I wasn't even born yet in 1971!
-Garry
I remember my much older sister telling me that until one goes deep down into the earth, they have NEVER seen darkness. Only then can you experience the total absence of light.
Another coincidence! Only last night I read this article. Huge Dark Matter Experiment Finds Nothing but More Mysteries | WIRED
How’s that for a LUX meter………and it’s buried in a cave.
It looks like a chassis mount capacitor, the kind used in output stage of power amps. Maybe you could get them cheaply because of transistor amps becoming more popular?