Interesting discussion on this subject, thanks for all the input guys! 
weapons and armors is a major focus, when arms and armors is a minor... I don't know the difference you make in English between weapons and arms...
I thought we were almost all saying it should be a Minor Focus (as in MoH) unless used for enchanting weapons and armor with any effects (not done in MoH), in which case the comparison is to Swords, meaning we'd then favor it being a Major Focus.
Chris
The way I read the rules on ArM5 page 98 that isn't possible. The triggering action by default must be performed by the person holding the device unless you set up an environmental trigger.
LoH and MoH give examples where the triggering is magical and not by the hielder. See Persephona in MoH (? not sure about the name) and Hermanus in LoH.
Err.... being struck by metal is a perfectly valid activation mechanism. Hell, winking is a perfectly valid mechanism of activation The only thing is that it is a specific action, not an enviromental one, but that does nto prohibit it at all
RAW (p. 98) "A trigger can involve... anything physical that you can imagine". So "being hit" is valid, as is "being hit by metal weapons", or "being hit by metal swords", or "being hit by Duke Tybol's sword" (assuming it was a distinct piece of weaponry), or "being hit by any sword wielded by Duke Tybol".
When it gets tricky is what the limits of "physical" are... "When Duke Tybol straps on his sword in the morning..." (Anywhere?) or "When Duke Tybol's sword hits something" (no matter how far away?) Physical, yet - how does the item sense this? Does it then have to be "Whenever any armour w/in Sight Range is hit by a metal weapon..." How does an item sense anything?, or does it even have to? "Whenever X mage is struck by a weapon, anywhere, anytime"... How does it know this? How does it work? Just fine, apparently...
I don't have the answer for your saga. "Mental" commands are invalid (without InMe effects in the item), but it's not clear that the item has to "sense" the physical effect, nor how it would if it had to.
Anyway...
We miscommunicate. I was thinkking of spell effects (altho' I guess I didn't make that clear) - I see nothing in the OP to limit this focus to enchantments.
So you both are perfectly correct, there is nothing wrong w/ that effect in an enchantment, but only because it is that enchantment that then does the casting as needed. The same end effect (to respond to a trigger) cannot be duplicated with a spell alone, not by RAW (or not w/out some very creative use and liberal interpretations of ReVi).
In this case, you are absolutely correct good sir, and we are appearantly in complete agreement.
Perhaps using a MuTe base (with Pe requisite) to give the armour an unnatural quality of destroying other metal that touches it - effectively a spell to make the armour act as if it were so enchanted for the duration.
I would ask for it to be made more specific. Like "Enchanting weapons and armour", which seems to fit the desired results decently. Broader than that i would certainly want it to be Major. I would really prefer the above being Major as well.
[quote="noliar"]
Perhaps using a MuTe base (with Pe requisite) to give the armour an unnatural quality of destroying other metal that touches it - effectively a spell to make the armour act as if it were so enchanted for the duration.
[/quote]
That would indeed be an "unnatural quality" - to be able to create a magical effect on other things. I'm not sure I'd want to open that can of worms for [u]my[/u] Saga, but you go right ahead (and [u]do[/u] let us know how it works out!) 8)
(For nightmare scenarios, see: Wards and "Pseudo-Wards", skip the long rules philosophizing at the start (if you want), and skip down to the post from Wed Feb 18, starting "More threadomancy 
"... read the next 2 very short responses as well. Enchant a rat, destroy the world... 
)
Let me just warn all of you that you are treading on very dangerous ground. How does it know the thing striking it is metal? Being struck is at least less risky. Why do I say risky? Because once you let something identify metal versus non-metal for free, you have laid the groundwork for free Intellego magic in items. I'm not saying it's wrong, I'm just saying you really, really don't want to go that way unless you want to examine every trigger individually very, very carefully.
Chris
I think it comes down to this: if you were to speak to a stone floor, could it tell you that a group walked on it before the Sun warmed it? Could it make the difference between leather soles, wooden legs and metal staffs? Could it say it was hit or it was touched?
Only risky if your players wants to play the system instead of playing along with the game flow. Been playing Ars for 15 years now, and we never had a "item activates when there is vis nearby and casts a fireball" trigger. it could be done, but that breaks the suspension of disbelief for us. being stuck otoh is the stuff of common items in stories of classic fantasy, so nobody argues against this.
same as per the samples of the count of CH's example.
Xavi
Here's an interesting and important other thing for the debate:
To know if an item is touching an animal, not touching a non-animal, canonically requires an Intellego spell. (HoH:MC p.24) Apparently that can't be done normally with a trigger, which is why this is "the solution" to the detection problem.
If an item can tell if it's touching metal instead of something else without Intellego, why can't it tell if it's touching an animal without Intellego? Yet at the same time ArM5 p.98 is pretty broad in what it allows. My solution is that I like to think of it as something being done to or with the item. You can speak to it, wave it, hold it in a certain way (stance), etc. I wouldn't allow something like winking, myself, unless maybe it's attached to your eyelid. I know there is no clear answer because of how broad p.98 is, but I do like to find ways to be consistent with canon, and I can't find a reason with which I'm happy why an item could identify metal versus non-metal but not be able to identify animal versus non-animal. That's why I like the general being struck trigger, just having the effect go off and fail when struck by non-metal.
Chris
Generally I would agree with you both on what has happened for us and for what is likely to happen. How would you rule this?
CrIm to make an invisible thing glow when it passes through the enchanted doorway, but we don't want it to make non-invisible things glow.
I don't think that's a very abusive idea. But would you allow it?
Chris
I reason thusly: the item is made of metal, so it "knows" metal when it feels it, which is why it goes "clang" when you hit it with metal and "thud" when you hit it with wood (for example).
In addition, I would say that as the effect in question specifically targets metal then it can't be activated by non-valid targets. This is subtly different to saying that the effect is activated by but fails to affect non-metal targets.
Now, this I think I would disallow. A doorway has no business "knowing" whether something visible or invisible has passed through it. The doorway can however "know" when the its threshold has been crossed, and what direction it was crossed in, and how many times its been crossed, and, say, the speed of the things crossing the threshold.
On the other hand, I would allow the trigger to activate "when invisible things crossed the threshold" if the item effect itself was designed so that it could only affect invisible things.
Nice design. And tricky as you point out
I think IMS we would rule that the door needs an InIm effect to "know" this, and act as a linked trigger. If it make everything glow, regardless of it being visible or not, it would not require a trigger, however.
Reasoning: total gut feeling. As said, "no free lunch" (read: intellego spells) here.
But it could be argued the other way around as well for sure.
Cheers
Xavi
Are you sure?
Serf's Parma; what precisely are you referring to on HoH:MC, page 24.
Canonically an invested rabbit's foot can't tell a rabbit touching it from a human touching it, nor can an invested wolf pelt tell a wolf touching it from a human touching it. Why does the metal item know metal while these don't know their own kind? So this still leaves me with a bad feeling.
I got lost in the "it's." Which goes "clang" vs. "thud"? I thought you meant the invested item, but as we all know, cymbals go clang when struck by wooden rods. And as I recently experienced (great drum player), cymbals can make much less clang when struck by metal. (I don't know what he was using. The best I can describe is that it was like a really short, metal, cat-o'-nine-tails.)
Chris