I am thinking of a magic sword that the Covenant's champion wields. The enchantments allow the sword to slice through an enemy turb. But it has zero penetration to avoid cutting down a magus.
But every now and then, the sword weilder may need to cut down a foe with significant magic resistance.
Is it possible to add an enchantment to the sword so that its Penetration can be increased on command (say for a Diameter)?
Not in the way you are thinking. You would have to work backwards to reach that goal.
Your normal turb killing effect would have your normal effect and 0 penetration. In addition to your normal setup you want to add a Linked Trigger that turns off the effect when the second one is activated.
To make this happen you need Concentration Duration, which allows the effect to be turned off, item maintains concentration, environmental trigger (sunrise/sunset), and more uses per day.
So math looks like this for effect one (note I am using Mag = 5, since I assume the base effect is at least high enough to reach level 5). I have balanced them and the total cost is added to the base effect. You would have to enchant both of them.
Standard Constant Effect: D-Sun (+10), Environmental Trigger: Sunrise/Sunset (+3), Linked Trigger (+3), Uses per day: 2 (+1) = 17. Note that this would not work, since the effect last till Sunrise/Sunset but I included the added cost of the Linked Trigger to give a basis for comparison.
Concentration Effect: D-Concentration (+5), Item Maintains Concentration (+5), Environmental Trigger: Sunrise/Sunset (+3), Linked Trigger (+3), Uses per day: 2 (+1) = 17. Increasing uses per day to 12 would raise the total cost increase to 20. You want at least 2 more uses per day then the uses on the second effect.
Now you make your second effect which has D-Diameter which frees up some points. It also does not need the two triggers since you are going to use the items normal activation trigger (some words or such). That means you are paying 5 for the duration, with 1 use/day (+0) and penetration 30 (+15) for a total cost increase of 20.
Continuing the example, say your effect was an "Edge of the Razor" (I know this is very low powered, but it is only for example purposes). It would be level 5, since it is base 3, +2 metal. Its range and target would be Personal/Individual since it is effecting itself. Normally to enchant this into a sword so that it is constant you are looking at a total level of 19 (Base 3, +2 Sun, +2 Metal, +1 2 @ day, +3 Environmental Trigger). To get the ability to turn off the base and short term activate the penetration version you are looking at two level 25 enchantments.
How it would work is that normally the sword activates the first effect (non-penetrating) at sunrise/sunset and maintains it. This is effectively a constant effect, since in normal play situations where it is not constant would rarely if ever come up. When you give the command and activate the second effect, it automatically turns off the first (non-penetrating) effect so that it will not bounce off of Magic Resistance from that effect. After the second effect ends, you would need to give the command to reactivate the first effect (though it would also reactivate automatically at the next sunrise/sunset).
Possible improvements?
Addition of a second linked trigger to the first effect, which turns it back on when the second effect ends. You just have to remember to make sure it has at least 2 uses per day more than the second effect.
Make the levels lopsided and add more penetration to the second effect. This would be very useful if your Lab Total was way higher than what you needed to enchant the first effect. You could also knock a couple of levels off the first effect since you do not need as many uses per day as I included.
Penetration is a property of an enchantment in an enchanted device, not of an enchanted device itself. So two different enchantments in a device can have different Penetration Effect Modifications (ArM5 p.99).
Hugh of Flambeau's bearded axe (MoH.p.51) Talisman: Troll's Wife has the MuVi enchantment Sharpening the Wizard's Blade of Vim to add Penetration to other enchantments of this axe with D: Mom - and "adds an extra round for the activation of these effects".
Indeed. And that effect is still debated. Or at least, I don't believe it should have been printed.
It never got errataed, and to me it looks plausible. D: Mom is somewhat longer than 6 seconds.
To be fair, spells in MoH may have been the results of experimentation, and have effects which do not fit the regular formulas because of discovery or other effects. Nothing indicates that these spells are canon standard spells, but rather personal spells developed by those particular magi.