Lightning must surely have been a fascinating sight for our far distant ancestors. After all, they might have seen it start fires, turn sand into glass, or strike down their fellows. It is also generally accompanied by a loud, sometimes deafening noise. Is it any surprise that it has been worshipped as the manifestation of a god?
I think you are probably all familiar with the unwise experiments with lightning conducted, if you will, by people like Benjamin Franklin and other wealthy and famous people. I say they were "unwise" because a fair number of the people involved were electrocuted for their trouble. For example, the story is told of a gentleman who was in a tall tower, with a metal rod fixed to the roof; this was connected to a spark gap just inside a window, where he was standing, and the other side of the spark gap ran to the ground. He got too close to the gap, and then what do you know, there was a sound as of a gunshot and he fell over dead, his brain fried by a bolt of lightning at a distance of a few inches.
So, be careful.
But beyond getting themselves killed or shocked silly, people of that time were experimenting with various ways of storing electricity. It was known that you could make some of it by rubbing a glass rod with fur, for example, but not until a certain device was invented could this be made useful.
What was it? Well, in 1745, a certain fellow named Pieter van Musschenbroek, who lived in Leiden (a town in the Netherlands), invented the capacitor. It is a device for storing charge. A simple Leyden jar is... well, it's a jar, with the outside surface covered in foil, the inside surface covered in foil, and some sort of chain or wire connecting the inner layer of foil to a nice useful-shaped electrode sticking out of the top. The way it works is, once you apply a charge to it, you get a wire to connect the outside sheet and the center electrode, and you'll see a spark as the charge between the foil layers is equalized.
A capacitor, as I said, is a device for storing charge. The way a Leyden jar works, it has a strong charge on the plates (relative to each other, of course). Normally, charges tend to be neutralized in short order, but you've interfered with that by putting a layer of insulation, called a dielectric - here it's the glass of the jar - in between the plates. (You can make Leyden jars out of a lot of materials, by the way, even mineral oil. Any suitable insulator will do.)
Building these things is ridiculously easy. It is also relatively easy to build up a charge that will hurt like crap, knock you on your arse or possibly kill you if you let yourself become a wire for the charges to equalize. Do not allow this to happen under any circumstances. I'm really not kidding.
However, I think I would first like to share a sort of a prank that was devised in the mid-18th century, involving Leyden jars and wine. Basically, the host of a party would get some really strongly charged jars and impart some of their charge to the wine in glasses. Shortly thereafter, he would provide the wine glasses to guests, who would drink it and receive a shock to their lips (if they were well-shorn of facial hair, which would allow the charge to escape). This was called an electrified bumper. I understand that it fell out of fashion, probably because of the ease with which a host might accidentally - or deliberately - wound or kill his guests, if he used too large a capacitor in charging the drink.
Let us go beyond pranks, though. I think we all are familiar with electrochemical batteries, the kind in a car or a pocket calculator. Unlike capacitors, these use a chemical reaction that is manipulated to force electrons that would normally be free to jump from one particle in the reaction to another, to instead travel through a wire. Hence, in the absence of a wire, the reactions proceed very, very slowly. However, they don't quite stop, in the absence of wires, which is why batteries that are stored slowly lose their charge. Also, many of the chemical preparations designed to be used in batteries can only run the reaction one way without inconvenient side effects. Consider the AA battery, which is a self-contained reaction vessel. For a given temperature, the only real speed control on the reaction going on within it is the amount of current flowing from one end to the other. So if you put it in a penlight, well, it takes a while to run down because a penlight only lets through a small amount of current. But what happens if you try to drive the reaction backward instead of forward? Well... one of the many things that happens is that you start producing heat, and possibly also gas. Remember, the battery is a self-contained reaction vessel. It doesn't have any exhaust valves or anything - so what happens? The damn thing bursts open and spatters you with poisonous and caustic filling.
Even rechargeable batteries have limits to how often they can be cycled. Most battery technologies require ions, charged particles of appreciable size, to move from one place inside the battery to another. This can be reversed, but that's not so efficient, because it's slow (just as discharging it, even at the maximum rate, is slow). Also, the ions can get stuck - they don't necessarily end up in the same place they started, even if they end up with the same electrical charge. If you've ever compared an old and heavily used car battery with a new car battery (same brand and everything), the old battery will have dust on the bottom of the case and its electrodes will be pitted and worn; the new battery will have no dust, and the electrodes will be intact. That dust is what you get when particles of the electrodes that were driven off by the discharge get together in the wrong place, i.e., not on the electrode, during charging.
Sure, capacitors have their drawbacks. No questions about that - if you want to hold a lot of power in a capacitor, you're probably going to have to have a big, heavy capacitor. But the thing is, they charge up as easily as they discharge, almost. And, well, pretty much the only way to destroy a capacitor, other than physically smashing it to pieces or melting it to slag, is to force a current to cross the dielectric, causing undesired physical or chemical changes. If you use a charging circuit that prevents that, though, you have a method of storing energy that can be discharged and recharged as often as you like, and if your capacitors are well-ventilated, the heat caused by doing this won't affect them too badly. (Compared to batteries, anyway, which are notoriously heat-sensitive.)
Now, what does this mean?
Well, let's see. With the advent of a technology, appropriately named "supercapacitors", that allows capacitors using this technology to store three or four orders of magnitude more energy than more traditional capacitor types. And because of the speed with which all capacitors can be charged, well, this opens a niche for them in things like regenerative braking. If you're not familiar with the term, regenerative braking is a way of slowing down vehicles that, instead of using friction to turn speed (kinetic energy) into heat (which is mostly useless), uses generators to turn speed into electrical energy that can be stored and used to speed up the vehicle later. There is hope that they will be able at some time to completely replace batteries in many applications, such as powering electric or hybrid vehicles. You could have a car with the same range as a plug-in hybrid, but which would charge up in a few minutes. The convenience factor of that should not, I believe, be underestimated. Hey - if the price of a hybrid ever gets down to $16,000, I might buy one.
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Previous MSPW diaries can be found as follows (and don't read them if you're trying to preserve your unwarped mind):
MSPW 11: If you can control machines, they can control you
MSPW 10: Powered armor leads the way
MSPW 9: Noise-Marines, forward!
MSPW 8: Rapid prototyping brings engineering to the masses
MSPW 7: Putting Mentos and Diet Coke to good use
MSPW 6: Why Bjorn the Fel-Handed is probably unhappy
MSPW 5: Combining the latest concepts in farming and power generation
MSPW 4: Project Orcon, or why pigeons make good pilots
MSPW 3: Can cuttlefish drive?
MSPW 2: The hafnium bomb
MSPW 1: Building a better skunk
PS: if you have one of those disposable cameras, and somehow the case got damaged beyond repair, you can pull the thing apart to rescue the film cartridge (send it in to be developed, I promise your vacation photos are not ruined) and expose a capacitor of moderate size. If you short this out, it will create a fairly impressive bang and a shower of bright blue sparks. I heartily recommend this, but be careful not to let the current go anywhere near your skin; if it does, you will get a shock that feels like your hand is being shredded by a meat grinder, and has the potential to cause you to become somewhat deranged for a few minutes. So treat it with the same respect you would treat a firework - potentially deadly, but highly entertaining if used properly.