How do they work exactly?

What?

Do you put grand gems in them or are they one use abilities.

Your talking about those random rods that you can stick soul gems into and have lightning strike them? I only used one of those once but you just insert soul gem (i think it might have to be grand but not sure) and voila, you should have yourself a black soul gem. Again, I only used it once so if i'm completely wrong, then i'm sorry.

Yep thats what I am talking about, I'm standing at one with an empty grand gem and nothing. Guess I'll just chalk it up as a glitch and move on. Thanks for the info though

Like I said, I only used one once so maybe i'm not remembering it right or something. Maybe they take greater? Sorry its not working for you and sorry i'm not more help

It's all good you answered my question

where are this rods?

I chose the black star 'cause I like to enchant stuff and thusly usually have soul gems on me. I believe it is a grand soul gem you need or greater and I believe they need to be filled and once you activate the lightning rod it turns it into a black soul gem, which are great for enchanting and much easier to fill then grand. And if you have moral issues with black soul gems I look at it as I only fill them with those that attack me. If you don't start swingin' I move on with my day.

And in answer to http://www.gamesas.com/user/803407-marcus84mtl/: in the soul carin (yes I spelled it wrong, should be soul carron anyway since it feeds on the living) there are a few lightning rods on pedestals that convert soul gems to black soul gems. Can't tell you where they are 'cause I only know where the bone yard and the dragon wall are there. Well and the exit.

You can only use them once, on revisits they do not reset. But I thought the first time most worked. There was at least one with a strange collection of bones that did nothing I could see though.

You do not need to take soul gems down there either, there are plenty of them to loot of all the enemies you killed if you looked around.

You put a grand/greater soul gem, then wait and lightning will strike the gem, creating a black soul gem.

I walked out of the soul gem with 20 filled black soul gems, it's a fantastic place for them.

The soul fissures fill up your largest soul gem with the largest soul available, so always use them.

Actually it's Cairn (as per Battlespire) not carron or carin. Now on topic, cool in the Cairn there's a soul gem charging station? Can't wait to get this on pc

There are actually cracks in the ground where souls leak out. Clicking on the fissure captures a soul. Similar to picking up an ingredient.

Yup. They're called something like soul fissures. I kept "harvesting" them and couldn't find what it added to my inventory (like the husks). It clicked after the 10th one or so.

Is it a one by One basis or is it a chance of failure?

It hasn't failed for me. Though they don't reset so it only works once per. I also assume you need a filled grand or greater soul gem on you.