Ideas for mechanics of an Arcanum-esque magic/technology conflict?

[igotsmeakabob11]

I'm starting a Pathfinder campaign next week including the Pure Steam rules that're releasing soon; part of the setting is that magic and (steampunk) technology don't coexist, they conflict and interfere with one-another, a la http://en.wikipedia.org/wiki/Arcanum:_Of_Steamworks_and_Magick_Obscura.

In Arcanum, the presence of strong technology (a steam engine, a load of guns, a skilled guns/explosives/alchemy specialist) would interfere with or weaken magic spells/equipment and vice-versa. Mages of significant aptitude had to ride in the caboose of a train for fear of their sheer presence interfering with the train's engine.
How far you leaned in the direction of magic or technology helped you overcome the negative effects of the opposite (or you could just ignore magic and technology and beatstick your way through things with a plain old sword/bow).

I've been looking for some advice/ideas for some rules to reflect this conflict, but so far all I could think of would be to have spells fail have a percent chance to fail in, say, a factory... or guns have an increased chance of misfire in a magical ruin etc. Any thoughts or hard rules on how this could work out?
I figure character level (wizard/sorc or alchemist/gunslinger for example) and/or certain skills (such as craft:alchemy or spellcraft) could factor in a player's ability to resist the interference.

[SolEiji]

Make it a concentration check.  Proximity to the disruptive element is like being in a hazardous shaking situation, with low easy DCs for a simple gadget, and high DCs with far reaching effects for complex mechanical monsters.

^{*Generates 2 cheese wheels in the background*}

[linklord231]

"Proximity" is based on Caster Level (or Tech Level, I suppose); something simple like 5' per CL.  You may also want to include a chance for reduced effect instead of outright failure, though this will be much more difficult. 

Tying those two ideas together somewhat, you could make it so that trying to use an opposed effect in an opposed area reduces the CL of that effect, scaling as you approach the source. 
For example, a CL 20 Wizard is standing 25 feet away from you, a TL 20 Gunslinger.  Everything is fine.  You move 5' closer, and suddenly both of you are in each other's aura.  Both of you function at -1 TL/CL, with an additional -1 for each 5' closer you get (you'd have to define what having a -1 TL means though).  In order for him to cast spells, he has to make a Concentration check, and you would probably have to make one in order to use your Grit abilities.  Since his CL is reduced, even the spells he successfully casts are at reduced power.  Presumably your bullets would also have some ill effect to mirror that, such as -1 to hit and/or damage. 

[Unbeliever]

FATE is a lot less crunchy compared to D&D and Pathfinder.  But, this sounds like the Dresden Files wizardry to a T, though I guess that's only magic interferes with tech and not the other way around. 

Concentration checks sounds nice b/c it's not just based on caster level.  You could, in theory, be a very powerful magic-user who has poor focus, or vice-versa.  So, that's cool.  Although I think PF deals with it differently, so ymmv. 

Then you'd want something analogous for tech users.  You also have to figure out how to deal with aggregate things.  Suppose a 20th level Mage is surrounded by 40 1st level soldiers with rifles, what happens then?  One possibility, a kind of standard D&D one, is that the Mage is going to have it much easier b/c he's higher level.  But, you have to decide if a pile of smaller machines count as one big one for tech purposes, etc.

As a side note, you should make sure your players want to explore this system and fiddle with it.  I  have found myself, when faced with rules that muck with the basics of how magic works, tempted to either (a) avoid them, or (b) circumvent them.  There's a big difference between the ambient considerations mentioned in the OP and a hard system that can come up nearly all the time.   You might also want to think about the party composition, as I'd imagine you'd have gunslingers and spellslingers in the same party.

[veekie]

On the Arcanum thing, fluff wise:
Low magic and simple technology - Reduced effectiveness at worst.
Low magic and advanced technology - Magic fails to work.
High magic and simple technology - Technology works at reduced effectiveness.
High magic and advanced technology - A tossup, usually including catastrophic explosion regardless of who fails.

The justification there is that technology exploits and reinforces natural laws, while magic distorts natural laws to achieve it's effects.

Mechanically, this can be worked with the following:
-Products of technology have a Technology level, this is equivalent to spell level.
-Whenever technology interacts with magic in a scene, make an opposed Tech level vs Spell level check. Winner functions normally, loser consults appropriate failure table.
-Failure table is a d10 modified by the sum of technology level and spell level. Three tables for each side, minor, moderate and major, as appropriate to the loser of the opposed check. Minor results in no effect at worst, with other consequences being partial functionality or even full functionality. Moderate results in destructive effects affecting the user or the device at worst, with other consequences being unintended side effects or increased critical failure odds. Major results are all catastrophic and spectacular.

[igotsmeakabob11]

I'm considering just making it an area-based effect, ancient mechanical/magical ruins imposing a small penalty, active factories/magic sites a larger one. That should keep it simpler, if not exactly what I wanted (which you basically nailed veekie).