Would it be possible to invent a ReVi spell to interrupt Arcane Connections? Imwould like to craft a jar to keep ACs in but not have to worry about them being used against me. Ideally the jar would sit in my lab and when I need an AC I will just reach in and pull one out. The jar would be an invested item that would block ACs like lead blocks xrays. Is this possible?
Breakthrough idea!
I'm pretty sure that arcane connections (with a few special pleadings) are unidirectional. So lesser part to greater part. If you've stolen someones talisman or familiar (The 2 exceptions I am aware off) then you're in a whole heap of trouble
Pg 84 states that something is mystically still PART of the target. So I would suggest that an arcane connection would need to be a part of something so my blood is a part of me, I am not a part of my blood. So I can't hunt down the connections to me but they could be used to hunt me down.
A spell that blocks arcane connections however is a good original research project.
A
In our saga, we interpret that to mean that Arcane Connections work equally well either way, and that while the magus who takes the Arcane Connection is aware of it, other magi can also become aware of it. My character has a habit of picking up acorns and clumps of dirt wherever he goes. If another magus spots me doing that he can use the tree as an Arcane Connection to my acorn.
It seems like Circular Ward Against Demons ReVi Gen might be a good place to start my research. PeVi can be used to reduce the duration of an Arcane Connection, but I just want to block the effect temporarily, not cancel it. Maybe Pe should be a requisite?
If you are keeping the jar in your lab, the jar is superflous, as you are already protected by your covenants Aegis. And I doubt your're getting better than that on your own.
If you need to carry the jar around, just use the rules to create an item that gives a parma-like effect. No original research needed. This was one of the basic ReVi-Effects in 4th ed. Given how parma works in 5th, this should be even easier.
Besides that: As a Storyguide, I'd allow a magus to use the finger that has been cut of his hand as an AC. He knows which finger.
I wouldn't allow a tree as an AC to a random acorn, that a magus knows nothing about. The magus doesn't know the acorn, and there are hundreds of them, if not thousands...
As written, Arcane Connections are "one way" - if X is a part of and thus connection to Y, then Y is not a part of or connection to X. The problem is when a thing, for instance a stone, is "divided exactly in half" - what then? (I houserule "no AC either way", because magic is tricksy like that and I don't want to see 2-way AC's - but ysmv, natch.)
The PeVi rules describe "reducing the duration of an AC", and that implies a "permanent" effect, but I'd think that it's within the spirit of the guidelines to allow a sufficiently large PeVi with duration to "block" the AC. It would have to be houseruled in, but the mechanics are right there.
I've often wondered how best to create a "scry proof" container, such as is easily possible in some systems where scrying is blocked by "a thin sheet of lead" (which is not the case in standard Mythic Europe!).
Rego Vim can create a ward against all magical might (and by definition that seems to include hermetic wizards) - but that's very expensive, and useful against any and all magic - this would only be narrowly focussed. It can also "suppress a spell cast by another" - but btb that needs an existing spell as a target, it's not a ward.
PeVi can do something similar, but even more clearly (as written) needs an existing spell to target. However, as written it can target a "specific type" of magic, so maybe "incoming scrying spells" (as opposed to any scrying spell) would work... very subject to Troupe interpretation and houserule.
Is this the section you're refering to?
While this says the connection is often a former part of the target it does not make that a necessary condition. It also does not preclude the possibility of an object being both target and connection. My computer is both a client and a server, often at the same time, and often to the same remote computer. There is no incongruity there.
Out of curiosity, why not? A two-way AC seems to be a balancing device in game. If I can acquire an AC to another magus his counter is to become aware of it. My counter to his counter is then to keep it where he can't us it against me, or can't become aware of it. I'm quite new to Ars Magica but this seems likely to lead to richer sagas..?
I would have said:
- "My brother says I'm his brother."
- "My brother says I'm his sister."
I can be his brother even if he's my brother, but I cannot be his brother if I am his sister.
If a magus is an AC to his ACs, then he can track them down and destroy them all. He can know who stole an AC. If it is asymmetric, then there's no way to know if others hide trumps in their sleeves. That lack of knowledge has the richer stories.
I'm not sure which side this analogy is meant to support. I assume you mean that the AC only works one way. You may be right but argument by analogy is inherently flawed. Picking a different analogy leads to a different conclusion, for example, you and your brother and your sister are all siblings and therefore each shares the same relationship to the other. Therefore ACs work both ways.
According to RAW, a magus can only use an AC that he is aware of. If my enemy magus sees me pluck one of his hairs, or finds out that I keep one in a vial or counts his hairs one day and notices one is missing then he may learn of the AC. Otherwise the AC is unknown to him and therefore unusable.
That's pretty much the only section on AC's, so yes, that'd be it.
While you're right, that "it does not preclude it" specifically, neither does it certainly state that it's kosher - which is why I put so many qualifying comments in my post to that general effect. (3 in 2 1/2 lines, by my count.) It's not made clear at all. The only thing that is clear is that a part can be an AC to a whole - the opposite relation is not even suggested (and that would only have been a few words extra - "and vice versa" - but we don't have that).
Which is I don't believe it was intended, much less suggested or invited.
Completely false.
As with many situations where the desired Target cannot be directly sensed, all it takes is one additional spell, in your example an InCo effect w/ Range:AC, to find that other hair - after all, the caster would be an AC to that hair by your houserule. Once a mage has "sensed" it, then it's fair game to be used as desired, including being a Target of a PeCo Group effect, but hardly limited to that.
And that last is what I'm not crazy about.
The problem is that a part relates to one (and usually only one) larger whole - but a whole can relate to thousands of smaller parts (or more - how many hairs does one lose in a day?). How does a spell then know which "target" that part and effect are targeted at? In short order it's like having a spell with "Range:pretty much wherever I've ever been" - which is completely out of balance.
In my sagas.
That still means targeting something you have NOT sensed(as the spell is meant to allow you to sense it, so basically you cant cast this spell until after you have cast this spell).
What he means is that, if you're an AC to that lost hair, as it is to you...
An ennemy magus could surely use the hair to cast a "detect the magus" spell, without knowing where you are, or if you still exist (in which case it could fail).
Likewise, you could cast use yourself to cast an arcane connection range "detect the lost hair", without knowing where it is, or even if it still exists.
You could also use your connection to your lost hair to cast an AC range PeCo group spell "destroy the hair left behind".
So long as you have an AC to something (in this case, you to your lost hair), that's all you need to cast AC range spells to it. You don't need to "sense it" first, as you don't cast InCo spells before casting that AC range PeCo killing spell, or InIm spells before casting that AC range "read thoughts" InMe spell.
Highly questionable as you´re randomly targeting something you dont know if it even exists or not.
You need to know that it exists, otherwise you cant target it and might as well end up removing all hair ON your body as much as destroying whatever you left behind.
Let me start be reiterating that I'm quite new to Ars Magica. My comment on balance was the one I was least confident about as I've had little time to think through, or play through, all the scenarios relating to ACs. Certainly, having an AC to "pretty much everywhere I've been" is open to abuse
On the other hand, if you've dropped that many ACs, that alone seems likely to preclude you finding the specific hair my magus took. What will you do, complete each day by eating supper, saying your prayers, then scanning through every possible AC you've dropped throughout the day to see if any are in the hands of another magus? This seems impractical, especially if I craft a jar to hide the AC... 
I'm trying to follow your thinking here, and am failing. In your Saga, is the idea that, if I want to cast a spell at Arcane range on "that hair that magus took", that is sufficient data for a targeting? If so, why is "that hair I dropped back in that grove" insufficient targeting? I feel like I'm missing something in my attempt to understand the way you run ACs in your saga.
I don't have any problem with a spell detecting "all Arcane Connections to me" within the spell's Target - in the targeted Room, for example, or Structure or even Boundary if you want to spend the vis. But I can see the point made by Direwolf and others that using Range: Arcane Connection in an attempt to actually detect the Arcane Connection in the first place is a bit problematic.
Exactly - and yet it should be possible btr, since (by the "all AC relationships are symmetric" arguement) if you have an AC to an object, you should be able to find it.
Or - a quarry is not an AC to every stone* that came from it, a bird is not an AC to every feather* it's ever dropped, a warrior is not an AC to every drop of blood* they've ever shed, and so forth. An AC is to one thing, or at most a few, not many, certainly not "countless".
(* book examples w/ durations in "years")
That solves it too. 
The logic, right or wrong, is this: if there is an Arcane Connection and you are aware of it, then you can use it.
Two things need to be defined here. 1) What constitutes an Arcane Connection? 2) What does it mean to be aware of the Arcane Connection. IMS we say that an Arcane Connection works both ways, so a hair from my head is an AC to the rest of me, and the rest of me is an AC to the hair. I can use myself to cast spells at range AC on the hair. YSMV
The second point is crucial. If a magus is aware of the AC and has one end in his possession he can use it. Awareness is the key and it's hard to be sure exactly how aware you need to be so we require that you be specifcally and accurately aware of the other end of the AC. "Maybe a hair fell off my head somewhere along the path..." is not good enough. "My enemy might have one of my hairs..." is also not good enough. If you suspect another magus has one of your hairs (or some other item that constitutes an AC to you) you can investigate, talk to people who were at the party you both attended. Once you find the waiter who saw some of your hair disconnect itself from your head and float under some tables to a tall dark stranger in the corner, then and only then can you be reasonably aware of the AC.
The difference between one end of the AC and the other, to us, is awareness. I know that I took some of your hair and you don't. Advantage: me. There is also the fact that even when you become aware of the AC, I have an AC to you and you just have an AC to the hair, which may or may not be close to me or something I don't want discovered/destroyed. You still need to penetrate my parma with no sympathetic magic bonus because the Arcane Connection is not to me, but to an object I possess. Plus if I keep your hair in my aegis or in a magic jar it becomes all the more secure from your attempts to use it against me or destroy it at a distance.
The 2-way vs 1-way argument is interesting. I see the reasons (in game as well as the out-of-game-balance-issues) for it to work either way.
I feel inclined to use a 'significance' rule.
To a hair, the person/animal it came from is significant - arcane connection.
To the person/animal, that particular hair (out of the hundreds/thousands shed every day) is not significant - no AC.
If someone chopped my hand off, I'd regard that as signficant, so I could use myself to link to the hand, as well as vice-versa.
2nd example: bit of stone chipped off a statue: one way AC. Chopping off the whole head: 2 way AC.
Now, what are the problems with this approach? (I must have missed something, it can't be this easy to 'fudge' rules!)
To address the original question: In my saga, Hermetic Magic doesn't work across regio boundaries from a distance, so I'd just put the non-magical jar into a regio for later use. As for a spell, you could insist on some original research, or allow it to use a ReVi 'keep spells off' type of ward effect. This would need to be double the level of an incoming spell to be effective, unless you specified that it only works on one form, as is standard for dispelling effects. Your saga may vary.
IMO this is exactly what protects ACs from detection: sheer quantity. If every tear I shed, and every object I touch, and every hair I lose is an Arcane Connection to me and I to it, then to keep track of them all would be an impossible task. The average human (if there is such a thing) sheds between 50 and 100 hairs a day. Add to that all the things a person touches and every other conceivable Arcane Connection forged however briefly during the day, and consider how long it would take for a magus to check the status of each at the end of a day.
Let's say that walking around, I shed 50 hairs a day and touch 50 objects and that nothing else I interact with could be used as an Arcane Connection to me (this seems about as conservative as possible). That's 100 Arcane Connections. IMS you need to be specifically aware of an Arcane Connection in order to use it. We're pretty loose about what exactly constitutes full "awareness", but it has to be done individually (i.e. no group spells). So 100 ACs tested individually at say 10 seconds per spell is 1,000 seconds = 16.7 hours. That's way too much time for anyone, let alone a magus, and that's what keeps us in balance. So far at least...