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what is electricity? part 2 – the mystery gets murkier

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Canto: So we were trying to comprehend early ideas about electricity as a fluid, which led Franklin to define two ‘states’ of the fluid, ‘negative’ for having a deficiency, and ‘positive’ for having an excess. He also called the negative state ‘resinous electricity’ and its opposite ‘vitreous electricity’. Presumably he thought the fluid was in a balanced state before these different elements started rubbing against each other.

Jacinta: And they were trying to regain this balanced state, which made the sparks fly?

Canto: Dunno, but let’s return to Britain, where Francis Hauksbee (1660-1713), a lab assistant to Isaac Newton, was being inventive with air pumps and pneumatic engines, decades before Franklin’s 1840s experiments.

Jacinta: I’d ask you what a pneumatic engine is, but I suppose that’d take us way off topic?

Canto: Probably. It apparently has something to do with compressed air, and some kind of energy derived from un-compressing it, or something. Anyway, air pumps were used to create vacuums, or relative vacuums. Apparently, Hauksbee, an ingenious instrument maker, noted that glass was a really good material for viewing experiments, and in 1705 he performed a remarkable experiment with one of his air pumps and that mercurial, and very dangerous element, mercury (though ‘elements’ in the modern sense, weren’t known or at least defined at the time).

Jacinta: I suppose elements wouldn’t have been defined until the atomic theory became a thing.

Canto: Anyway I’m betting that his experiments with mercury shortened Hauksbee’s poor life (he was accepted into the Royal Academy in 1703, just as Newton became its president with the aim of reinstating its grandeur, but he was given special ‘low class’ status). He’d created a version of Otto von Guericke’s electrical machine, made of glass, with air pumped out, and some mercury inside. He rubbed the sphere to create a charge, and the mercury glowed when he put his hand on it (the globe, not the mercury). Fantastical, but nobody knew what it meant, except that it could be used as a source of night-light, which actually happened, but much later.

Jacinta: But nobody had much idea about whys and wherefores at this time.

Canto: They presumably speculated. A similar phenomenon, in large, was St Elmo’s fire (he was the patron saint of sailors), a bluish glow around a sailing ship, or more recently, around an aircraft. We know now this is a form of plasma, the ionised state of matter. During thunderstorms the voltage differentials are greatest – it requires a particular differential for it to happen, and the shape of the body around which the light is seen is an important factor. Pointy objects create a more intense field (Franklin realized this). The violet-blue light is caused by the nitrogen and oxygen in the atmosphere.

Jacinta: Are you sure you know what you’re talking about?

Canto: I’m never certain about anything, that’s my vocation, or just my fate.

Jacinta: Pneumatic tyres are filled with compressed air, or gas. So that helps to understand what a pneumatic engine might be, maybe.

Canto: So Hauksbee had found a way to accumulate an electric charge, and in 1745, in Leyden, Holland, they found a way to store this charge – an instrument that came to be known as a Leyden jar. Let me quote from the scientific historian, Thomas Crump:

The so-called Leyden jar was simply a substantial glass chamber, with separate layers of metal foils on the inside and outside surfaces. The inside was charged by a metal chain connecting it to a charged body, which then lost its charge to the air.

And this was apparently the first capacitor. We’ve talked about capacitors and supercapacitors before, but of course we barely understand them. In any case this Leyden jar device allowed a lot of electrostatic potential to build up between the inner and outer surfaces – enough to kill small birds who came in contact. Nice.

Jacinta: Or were forced to come into contact. I know they tried it on monks too. Presumably they couldn’t find the nuns.

Canto: Anyway they now had some control over this electricity thing, even if they hadn’t a clue what it was. They had some idea as to how to create and release this electrical charge thingummy.

Jacinta: So now we come to Coulomb?

Canto: No, Alessandro Volta (1745-1827) first. I’m following Crump, for better or worse. But more importantly than people, it’s batteries we’re going to focus on now. And I’m not sure where to begin.

Jacinta: It was a term – battery I mean – first used by Franklin in 1749, but what he actually created were capacitors, devices that accumulated charge, until they were discharged. Batteries – I’m kind of guessing here – are devices that store charge more or less permanently, and can release charge in a controlled way, and be recharged in a controlled way.

Canto: And what is this thing called charge?

Jacinta: Well let’s continue to grope toward an understanding. So I’ll return to Franklin. He wrote a book, Experiments and observations on electricity, made at Philadelphia in America, published in 1751. His researches led him to believe that everything contained charge, positive and negative, but that they were almost always in equilibrium, a neutral state. Or the fluid, which could be ‘negativised’ or ‘positivised’ by friction, could be returned to balance by ‘discharging’ it.

Canto: And surely therein lay a mystery. How or why did this build-up of negativity or positivity get discharged? I just don’t understand it. Not just the discharge but the creation of the charge.

Jacinta: I suppose they – Franklin, Hauksbee and the rest – just made the observation and called it ‘charge’. From whence, ‘discharge’. Maybe you’re just overthinking it. They certainly didn’t know what was going on, they just noted this reliable cause-and-effect behaviour and sought to utilise it, and find out more about it. Anyway, keep on overthinking, it might be a good thing.

Canto: Okay, Franklin was exercised by the discharge side of things. He found that pointy objects – we now call them lightning conductors – were most effective at discharging this build-up of charge, and recreating neutrality, the safe, ‘natural’ condition. A great, practical solution for buildings. But he developed a theory of sorts, of zero-sum conservation of this thing called charge. Whatever was accumulated in, say, a Leyden jar, was restored on discharge, nothing gained and nothing lost. I think.

Jacinta: Well, here’s a quote from Crump’s book, which might unenlighten us further:

Franklin succeeded in giving Leyden jars both positive and negative charges, and showed that the force itself was stored in the glass of the jar with the charge being proportional to its surface area.

Canto: Yeah, that needs unpacking, if possible. The ‘force’ being stored, is that the charge? If so, why does he use different terms? Charge is either negative or positive, isn’t it? So he was able to give these jars either a negative or a positive charge/force, but not both at the same time, though it’s ambiguous in this quote.

Jacinta: What I think he’s saying is there’s this force, which we now call electricity, which can either be negatively or positively charged, and its strength will be proportional to the surface area of the glass jar. I don’t think he was giving the jar different charges at the same time, but how he knew that the charge was sometimes positive, sometimes negative, or what that even means, I’ve no idea.

Canto: Yes, I’m more confused than ever. Let’s try to understand Leyden jars a bit more. Apparently it was invented in 1745 by one Pieter van Musschenbroek as a ‘cheap and convenient source of electric sparks’. That’s from Britannica on electromagnetism. So, to be more precise about this first jar, it was a glass vial partially filled with water, which ‘contained a thick conducting wire capable of storing a substantial amount of charge’.

Jacinta: Presumably that ‘thick conducting wire’ corresponds to the ‘metal chain’ in Crump’s description. I don’t know what the water’s for.

Canto: And Britannica makes no mention of the ‘separate layers [how many???!!] on the inside and outside surfaces’.

Jacinta: Okay, here’s a simplified picture, which might help.

So, in this one there’s no water, but I’ve seen other pics that indicate a jar more than half-filled with water, so who fucking knows. Note that there’s one layer of tin foil on the outside and another on the inside. Note the metal rod passing through a cork into this evacuated jar, and then a wire, presumably of some kind of metal, connecting to the tin foil.

Canto: Is tin a good conductor?

Jacinta: Apparently so. Not as good as silver or copper, but better than lead. And please don’t ask me why some metals are better conductors than others. It’s so frustrating trying to learn from the internet, even when you know which sites to avoid. For example, take this statement on what I’d expect to be a reliable site:

Although Leyden Jars allowed the storage and dissipation of electricity, there were still issues present. One issue was the lack of energy from the charge. While it could only attract small objects like a bit of paper, that was all it could basically do. Also, it could only perform that function after the jar was charged, which also took lots of time.

And then this, from Britannica:

The Leyden jar revolutionized the study of electrostatics. Soon “electricians” were earning their living all over Europe demonstrating electricity with Leyden jars. Typically, they killed birds and animals with electric shock or sent charges through wires over rivers and lakes. In 1746 the abbé Jean-Antoine Nollet, a physicist who popularized science in France, discharged a Leyden jar in front of King Louis XV by sending current through a chain of 180 Royal Guards. In another demonstration, Nollet used wire made of iron to connect a row of Carthusian monks more than a kilometre long; when a Leyden jar was discharged, the white-robed monks reportedly leapt simultaneously into the air.

Canto: Hmmm. One of these descriptions is not like the other. Where’s Micky Faraday when you need him?

Jacinta: I can but do my best. Let’s get back to batteries, again. Franklin’s ‘battery’ was really a capacitor, as mentioned, a way of accumulating more electric charge, and temporarily storing it, until it was required for a sort of ‘big bang’ release, I think. You can do this with Leyden jars linked together:

The above ‘device’ was used for demonstration purposes back in the day. Franklin’s electrostatic machine, though, didn’t look anything like this. It was a mammoth device of cranks and pulleys, created with much help from his friends. The mechanisation was presumably for creating as great an accumulation of charge as possible. Crump writes that Franklin built a glass and lead battery consisting of eleven condensers connected in series – which is clearly not his electrostatic machine. And apparently it wasn’t a battery, either, at least not in the modern sense. And WTF is a condenser? Anyway, this confusion has gone on long enough. We’ll try to clear some of it up next time.

References

Thomas Crump, A brief history of science

https://en.wikipedia.org/wiki/Francis_Hauksbee

https://en.wikipedia.org/wiki/St._Elmo%27s_fire

https://www.britannica.com/science/electromagnetism/Invention-of-the-Leyden-jar

https://www.bluesea.com/resources/108/Electrical_Conductivity_of_Materials

https://en.wikipedia.org/wiki/Franklin%27s_electrostatic_machine

 

Written by stewart henderson

December 6, 2021 at 10:57 pm