You’ve all seen the movies where the dynamite is in one place, and there’s a wire leading to a big box a few hundred feet away. There’s a T-shaped plunger coming out the top of this box, and someone pushes the plunger downward to detonate the dynamite. Like this (only with TNT at the other end).
Is this how it’s done today? Or is it all done by remote control? And what was the purpose of the plunger? Wouldn’t a simple electric switch have sufficed?
The plunger powers a small dynamo, which provides the energy necessary to set off a blasting cap. To anyone with a more convenient source of electricity, a switch is an obvious improvement, and is, AFAIK the usual method. I think some folks still use the dynamo, though.
The plunger has the added benefit of being somewhat more safe; it’s very difficult to operate the plunger accidentally (you have to push quite hard) whereas any shmo can knock a switch. Modern switchbox blower-uppers have safety circuits and whatnot, and there’s the time-tested system of not hooking up the wires until you’re ready.
The dynamo is more than a convenient power source, it’s a safety device. Unless the dynamo is spinning, there is no source of energy to trigger the blast and it is impossible to create an accidental short leading to premature detonation. Batteries existed long before dynamos did, but they weren’t used because of the possibility of switch malfunction. Today, some detonators use piezoelectric elements instead of dynamos for the same reason.
In World War II movies, you often see them use a similar device, except it is a handheld gadget with a handle that is inserted in one end and given a vigorous twist when the explosives are detonated.
Dang, all the good images on google returned “not found” (Hmmm, HSA?).
Anyway, I looked at an old one at Pioneer Village* in Bakersfield, CA. You could see the toothed rack on the side of the shaft that went inside the box. These teeth meshed with the wheel on the magneto the made the current to ignite the explosives.
*Interesting place, if you’re interested in very old oilfield equipment and such. Old houses and stuff, too.
Peace,
mangeorge
Thanks, Q.E.D.. That look’s just like what I saw.
Boy, they built just about everything with quality back then, didn’t they.
They don’t do it like they used to anymore.
Mine Safety and Health Administration has approved two new hand held devices with testing and other features to insure safe and efficient detonation of explosives.
They don’t use dynamite anymore, do they? At least not on big jobs.
Not so much. These days they generally use safer, more stable plastic explosives, like PETN or RDX.
My experience on SDMB is that although we’re fighting ignorance, there are some topics where one would be well advised going elsewhere for further inquiries. 
The best example of this is in the movie: Bridge on the River Kwai, where Alex Gueiness gets shot and falls on one.
In the movie they describe the plunger merely makes contact at the end of the stroke.
But that’s Hollywood balderdash,: like navigating the Fla Keys in a boat with a gas station map.
My uncle used to use dynamite and ANFO, but the dynamite was only there to initiate the ANFO. He said that ANFO was the most economical explosive, at least for his needs on his farm.
Not quite. What they do is insert the handle on one side and wind up a spring against a ratchet. They then insert the handle in the other side and, at the right moment, give it a quarter turn, releasing the pawl and letting the spring whirl the magneto. Just a quick twist like that by itself probably wouldn’t generate enough to detonate the blasting cap.
[hijack] As long as we’re talking about blowing things up: A long time ago in an episode of Kung Fu a group of Chinese was shown preparing for some, um, direct action agains their evil oppressors. One fellow was shown preparing fuses for dynamite charges, cutting about a foot length of fuse with a knife, lighting one end with a candle flame, letting it burn a moment, then dousing it in a can of (presumably) water. Anybody know what that was all about? [/hijack]
DD
Checking fuse speed? I read something once that stressed the importance of actually testing fuse speed, since it was too easy to accidentally use a fast burning fuse instead of a slow burning fuse.
I was helping out at a class that was teaching “dynamic safe opening” Lots of different devices most was RDX based (c4) and lots of different means of setting it off. IMO, the wireless remote is the best. Hard wired is limited by the wire and it there are lots of shots ya better bring tons of wire.
That old fashioned box still looks cooler
We’ve had a thread on the box-with-plunger thingy before.
Detonating explosives are set off with a cap or detonator (same thing, different names) which is a little metal tube (typically contains a heat/flame sensitive compound e.g. lead styphnate, a small quantity of heat-sensitive detonating explosive e.g. lead azide, and a booster charge e.g. PETN) that injects a shockwave into the explosive’s body.
One kind of cap uses a burning fuse as the delay device - you crimp the cap onto the fuse cord, embed the cap into the explosive, light the fuse and retire. The other kind of cap is electronic - it has a couple of leg wires sticking out of it. You pass current through the leg wires and they heat an internal bridge wire, much like a lightbulb filament, which triggers the cap.
You can have a lot of caps wired in series, e.g. if you’re demolishing a building and you’ve placed seperate charges in all the load-bearing areas. Since you want to have a fair distance between your source of current and the charges, the total resistance can be quite high. That’s partly why you have these mechanical stored energy devices like the plunger box and the spring box - they can momentarily produce a good bit of power. Modern devices store the energy in capacitors, not too unlike a camera flash unit.
An alternative to laying long cables to the charges and cap is to use det cord, which is a line of plasic cord containing a thread of PETN in its core. One end of the det cord is embedded in the body of the explosive, the other end is triggered with a cap. Det cord has some nice safety advantages - you can branch it or run it in series like a cable, but you can leave attaching a cap to the last minute before shooting the charges.
The term “dynamite” originally referred to nitroglycerine absorbed into diatomaceous earth, which greatly reduced its shock sensitivity. Later a whole host of “dynamites” were produced with different compositions. Sawdust, apricot pits and other cellucose was substituted as an absorbing medium. Nitrates such as saltpetre, sodium nitrate or ammonium nitrate were added to react with the cellusose and allow the percentage of nitroglycerine to be reduced. The weaker dynamites would basically consist of a mixture of waste cellulose, sodium nitrate and just enough nitroglycerine to propagate a detonation.
There are modern products calling themselves “dynamite”, but I’m fairly sure none of them contain nitroglycerine any more - it’s too sensitive compared with more benevolent explosives, and it can become increasing dangerous if it’s stored for too long. Instead, modern “dynamites” will typically be water-gels - fancy derivatives of ANFO, or RDX / PETN powders cast into a polymer binder.
ANFO is popular for many civilian uses because it is extremely cheap and also extremely safe - it is so insensitive that you can’t even set it off with a cap - it is not “cap-sensitive”. Instead you have to use the detonator to fire a booster charge as mks57 describes, and that fires the ANFO. Booster charges tend to be cast TNT with just enough PETN or RDX in them to make them cap-sensitive (cast TNT on its own is not cap sensitive either.)
I don’t know why, but this just cracks me up. “Here’s how we slaughter hogs around these parts, son. It’s faster than the more common methods, but it takes longer to gather the meat afterwards.”
Amen on the insensetivity of ANFO. When I was a kid I wound up unearthing a 30 gallon barrel of that stuff that had misfired. The guy had tried to use an old cap and fuse arrangement to detonate the ANFO. I believe he was using mercury fulminate caps. (This was in the early '70s.)
The next day we used dynamite as a booster and got a very satisfactory explosion. 
Regards
Testy
