You’re probably not going to be able to get some automatic cutting system to cut wires at “exactly” the same time, either. Are there differences in the starting points of the cutting edges, the insulation, or the wires? If you’re off by more than a nanosecond, as shown above, you’re probably toast.
Umm… YOU go do it and test it. I’ll stand… way over there.
I’m surprised no one has ref’d the episode in Hogan’s Heroes where Hogan and Klink have to dis-arm a bomb. Hogan can’t decide which one (red or green?) to cut. He asks Klink, and then cuts the opposite one, and the bomb is disarmed.
So, there’s the OP’s answer - ask some dork and do the opposite.
Do you really think this is beyond our technology? Electricity isn’t magic; even if there is a nanosecond between cuts it takes nanoseconds for the circuit to be tripped and then send an impulse to the detonator.
And I’d probably test it on a few duds before I started selling them to the bomb squad. 
In some cases it might be unwise to risk accidentally trigger the bomb.
Nanoseconds probably won’t make a difference, depending on what makes the bomb go off. Most circuits would have some type inductance, like in relay coils. The magnetic field has to build up. This takes time. So, a matter of a few nanoseconds between cuts may not matter.
Someone who’s EE can remind us of the formulae for calculating rise times for caps and inductors, I hope.
I doubt that the formulae will help you much.
t = RC, t = L/R.
i = V/R (1-e[sup]-(R/L)t[/sup])
i = V/R e[sup]-t/(RC)[/sup]
There are other objections to this approach mentioned in this thread, but it sounds like a “Going to St. Ives” question to me – in a simple bomb, there is NO current in the trigger wire until the fuse begins the detonation. So if you detect >0 current, you have a nanosecond to get outa there.
I don’t really understand your response . . . what’s the harm in cutting the “trigger wire”?
As a bomb non-builder, perhaps I am using the wrong term. I assume that the trigger wire is the one that, when an electrical current is passed thru it, will initiate the explosion sequence due to a completed circuit. If that is true, then there will be no current flowing thru that wire until the desired time.
But if you are referring to a wire that might be going IN to a timing device, that carries the current blocked by the timing device until the desired moment, then that wire should be safe to sever.
Of course, we are talking about very simple, non-booby-trapped, elementary constructions.
All hollywood drama aside, there’s going to be two wires going into the detonator. Cut either one and the detonator won’t fire. There’s no need to ever cut two wires simultaneously. Electricity needs a circuit. If you break the circuit in one place, the electricity won’t flow.
Don’t fiddle with the timer. Just disable the part that actually makes it go boom. Fiddling with the timer wiring is Hollywood B.S.
I’ve never built a bomb much less tried to diffuse one, so I don’t know how common it is for someone to play games with the detonator wiring, like putting dummy wires on top and the real wires underneath. Cut the top wires and the bomb blows up. I imagine that sort of deception would make a bomb disposal technician’s job a bit challenging at times.
Back when I last used an ammeter (which was admittedly 20 years ago), they worked in series, not in parallel. So you would have to cut the wire to make the ammeter work.
Regards,
Agback
ABC’s Nightline! profiled the NYC bomb squad, Fraday night. They noted that even the amteurs tend to make some attempt to make their bombs tamper proof. These can be as simple as rigging the container to explode if opened/moved/compromised. One does not have to play games with the circuits to make the bomb squad’s life difficult (or short).
The notion of the booby-trapped bomb dates to WWII, when the Germans began deliberately creating complex fuze systems to thwart the UneXploded Bomb unit, thus sharing a little terror among the inhabitants of Britain. The fuzes were mass-produced according to continually developing specs, so the origins of the “red wire” has some basis in reality. ftg has noted that the IRA picked up the practice of deliberately making bombs hostile toward the de-fuzers, as well, although I doubt that the IRA bombs would have been mass-produced to easily identified specifications.
Ok, so, again my question: What’s the harm in cutting such a wire?
For what it’s worth, ammeters with a clamp-on design apparently do exist, even ones that can measure direct current.
Agback, even then there must have been inductive ammeters. You know, throw a transformer (or hall effect sensors [thanks engineer_comp_geek) around a wire and let the wire induce current into the transformer. Read the transformer voltage and you know what the induced current it.
No cutting needed.
In some movies we might see bombs disabled by something that disrupts the mechanism (other than cutting wires). Maybe immersion in thick oil would gum up a mechanical timer, or maybe liquid freon would damage a battery or degrade TNT etc. so that it wouldn’t explode. Just movie magic, or are there real-world equivalents? Anything that would have helped the pizza man in Erie?
A transformer only works on AC current. I assume most bombs, running off of batteries, use DC current. The only way to use a coil to determine if a current is flowing thru a wire would require moving the coil around and around. And then it might miss small blips of current. E.g., a connector to check if something is still wired might send out a small pulse every x seconds. Really hard to detect while waving a coil around.
For simple DC, you need to detect a weak magnetic field from a wire working off a low voltage battery. Pre-solid state, would you trust your life to such a device?
And post solid state, it still wouldn’t work. Input impedances of CMOS devices (like a clock/counter chip) are extremely high, and so the current flowing in any wires connected to them could well be in the picoamp range. I wouldn’t count on measuring that with a Hall-effect sensor.
Thanks, Tom! That’s exactly what I was looking for. It would make sense that the defusers (sorry, they were British) defuzers would need to know the model (oh, yeah–Brits) mark they were working on–I know how specs can shift from mark to mark.
I know the Czechs were a big source for explosives in the 70s and 80s–did they do timers, too?
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. The trigger wire would have zero current until just before the explosion, when the current would then measure non-zero. Of course, if you were measuring the current and the bomb went off, it would render this discussion academic.
For that matter, in a simple, non-booby-trapped bomb, severing EACH wire would be a sure way to disarm it as long as the timer hadn’t wound down first.
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.
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.