Do I cut the red wire or the green wire?

[Desmostylus]

Assuming a “perfect ammeter” capable of measuring picoamps without interrupting said flow of picoamps:

There’s no way of distinguishing that tiny flow in the booby trap wire from a tiny leakage flow in the detonating wire, or a tiny induced flow in the detonating wire.

[lucwarm]

*Originally posted by Chronos *
I think that the situation that lucwarm is thinking of is something like this: You have two trigger wires, which (unless the bomb is exploding) carry zero current. Then, you have two (or more) booby-trap wires which do carry current. Cut a boobie-trap wire, break the current, and trigger the explosion. But cut any wire that has zero current, it won’t be detected, and if you cut all of the zero-current wires, then you’ll get the trigger wires in the process.

That’s pretty much my thinking. I guess I’m thinking about the traditional “Lethal Weapon” type scenario where the hero has to decide which wire to cut – if he cuts the correct one, the bomb won’t go off. If he cuts the wrong wire, then BOOM.

**

Admittedly, this won’t work if the current in the boobie trap wires is too small, so it isn’t detected by your meter. But in principle, with a perfect ammeter, it’d work. **

Well, as long as you use an ammeter that is just as sensitive as whatever bomb component is set up to measure the current in the booby trap wire, you should be ok – no?

[lucwarm]

*Originally posted by Musicat *
**None at all, and I never said there was any. Your post orginally said:
Your attempt to determine which wire carries current has problems of its own, but I guess I was talking theoretically. **

Keep in mind that the goal is not necessarily to figure out which wire is the trigger wire – the goal is to disarm the bomb. Let me illustrate:

Let’s suppose there are 4 wires – A, B, C, and D --; ‘A’ is a trigger wire; B is a booby trap wire; and C and D are irrelevant. If you cut D, C, and then A, I would count that as a total victory for the bomb disarmer.

The fact that my proposed method cannot distinguish between ‘A’ and ‘C’ hardly seems to matter.

[ftg]

I don’t see why a booby trap wire would be carrying current. It could have a voltage applied to it, but with a capacitor/transistor at one end. Virtually no current flows after the initial setting. You cut a wire, the charge leaks, at some point “boom.”

Given the very low current needs of digital counters (sans 007 LEDs), there probably is no easily measured current anywhere in the circuit between setting it on to firing.

There’s some other points I could make about hiding/misleading stuff, but I think I’ve gone far enough on this topic.

[Ferris]

Since the OP has pretty much been answered, I’d just like to add a supplemental question:

Can liquid nitrogen/other freezing agent affect the ‘timing’ of a bomb to the extent that was portrayed in Lethal Weapon 2?

IIRC, Danny Glover had about three seconds to reach cover between triggering the bomb (getting up off the toilet seat) and the resultant explosion.

I know that electrical resistivity is affected (decreased) by temperature, so I’d presume that this was yet more Hollywood licence, but is there any basis in fact for this?

[Doc_Nickel]

I believe what they were trying to do there was supercool the actual explosive, which presumably would have the effect of reducing it’s power somewhat.

I have no idea if that’s actually what happens or not, though I did read an article that tested rifle cartridges in both hot and cold temps (though just “refrigerator” cold, not “liquid nitrogen” cold) and did note a definite, though slight, reduction in muzzle velocity.

Still, they were all of two feet from the device and protected only by a cast-iron bathtub (they seemed to pull the “ballistic blanket” on top of them, and not into the tub with 'em) which would have been little protection against even Dynamite, and none at all against C4.

I tend to doubt chilling it would reduce it’s power that much, and C4 is nasty stuff- Dynamite shoves stuff aside, but the plastic explosives tend to shatter things, since the blast wave is so much faster.