an autodidact meets a dilettante…

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Posts Tagged ‘Francis Hauksbee

Towards James Clerk Maxwell 2 – Francis Hauksbee’s experiments

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an electrostatic generator – one of Hauksbee’s many ingenious experimental devices

Canto: So we’ve witnessed electricity since we’ve had the wit to witness, in lightning. And through our attempts to understand and harness those scary bursts of energy we’ve transformed our world.

Jacinta: We’ve written about lightning before, but the info we presented there was accumulated over centuries. Now we’re going to travel back to the early years of the Royal Society in England, the early 1700s, a mere 300 years ago, to reflect on the first experiments with electricity – remembering that there was no electric power and light in those days, that gods were in the air and much was mysterious.

Canto: Electricity from the start was much sexier, and scarier, than magnetism – lightning very very frightning was the most obvious physical manifestation, and its power was easily recognised. It could kill at a stroke, while magnetism seemed all about metals getting stuck together, and needles pointing north. Interesting, but hardly earth-shattering.

Jacinta: Lightning was all about gigantic sparks shattering the sky, and the ancients, who spent so much of their time in darkness, must have seen other, less impressive and dangerous sparks, the sparks of static electricity, and wondered.

Canto: In the recent BBC documentary The story of electricity, narrator Jim Al-Khalili begins by describing Francis Hauksbee‘s experiments with static electricity and electroluminescence in the early 1700s, which dazzled visitors to the Royal Society. These were the first properly documented experiments with the mysterious force, and a collection of his papers describing these experiments was widely read by the 18th century cognoscenti – including Joe Priestley and Ben Franklin. He employed the newly-invented air pump (simply a pump for pushing out air, as in a common bike pump), popularised in England by Robert Hooke some decades before. Hauksbee made his own improvements, enabling the pump to create a vacuum.

Jacinta: Yes Hauksbee was a more interesting figure than The story of electricity presents. He didn’t ‘lose interest’ but worked on his experiments and reflected on them until his final illness in 1713 – and I’m thinking that illness, since he was only in his late forties – may have been due to mercury poisoning. Hauksbee was ‘lower class’ so few details of his life are documented. However, in these experiments he wasn’t thinking so much of electricity as of ‘attractive forces’. Also as an experimenter who must always have seen himself as an underling (in his book he mentions his ‘want of a learned education’), he doubtless felt obliged to follow the guidance of his Royal Society ‘master’, Newton, which took him into different fields of research….

Canto: The term ‘electricity’ was possibly not in common use then? You’re right, though, about Hauksbee, who rose from obscurity to become a member of the Royal Society, probably under the auspices of Newton. In late 1705, as a result of some spectacular effects displayed to the Society he became intrigued by ‘mercurial phosphorus’. The fact that mercury, in a vacuum, glowed when shaken, had already been noted by Jean Picard, a 17th century French astronomer, and the Swiss mathematician Johann Bernoulli.

Jacinta: And this has to do with electricity?

Canto: We shall see. Hauksbee wanted to work out the conditions under which this mercurial light was produced. He found that the more air in the container, the weaker the light. Also the light’s intensity depended on the movement of the mercury. He concluded that the friction of the mercury against the glass was the major cause. But was it only mercury that had this property, and was it only glass that brought it out? He experimented with other materials, finding a means of rubbing them together in a section of his air pump, Amber rubbed with wool produced a light, brightened in the absence of air. By contrast metal on flint only produced sparks when air was present. Remember, oxygen wasn’t known about at the time. In late 1705 Hauksbee presented one of his most spectacular experiments for the Society. Ingenious instrument-maker that he was, he created a glass globe, from which air could be pumped in and out, on a rotating spindle. The spinning globe came into contact with woollen cloth, and the contact created a weird purple light inside the evacuated globe, which reduced as air was let in. It was a fantastic mystery.

Jacinta: I’m hoping you can solve it.

Canto: Great expectations indeed. He experimented further, and found that when he pressed his own hands against a spinning evacuated globe, the same bright purple glow was produced, which again faded when air was let in to the globe.

Jacinta: Okay, what Hauksbee was exploring in these experiments are what we now call triboelectric effects. I remember playing around with this in schooldays by rubbing a plastic pen along the sleeve of my jersey and watching the fibres stand on end as the pen passed, and hearing the prickling sound of static electricity. The pen was then capable of lifting scraps of paper from the desk, for a time. But I didn’t see any purple lights and I’m not sure how the presence or absence of air relates to it all.

Canto: Yes, triboelectricity is about the exchange of electric charge between different materials – the build-up and discharge of electrical energy. It seems that some materials have a more or less positive charge and some have a more or less negative or opposite charge (because positive and negative are really arbitrary terms, the key point is their relation to each other), and we know that like charges repel and opposite charges attract.

Jacinta: You’ve brought up the word ‘charge’ here, and I’m wondering if that’s just an arbitrary word too – like degree of positive charge just means degree of being repulsed by its opposite, negative charge. In other words, different materials are attracted to or repulsed by each other to varying degrees under various conditions, and that degree or ‘amount’ of attraction or repulsion is referred to as ‘charge’. So ‘charge’ is a relational term…

Canto: Ummm. Maybe. In any case, if you take these different materials down to the atomic level, and I’m not sure how you take plastic and wool down to that level – I mean I know plastic is a petrochemical product, but wool, which I’ve just looked up, has a very complex chemistry – but the fact that the plastic pen, after some rubbing, pulls the fibres of your woollen sleeve towards it is because there’s an attractive force operating between opposite charges. In fact there’s a movement of electrons between the materials, from the wool to the plastic. This electron transfer leaves those woollen fibres with a net positive charge, which is attracted to the now negatively charged plastic due to the electron flow. I think.

Jacinta: Mmm. None of this was understood in the early eighteenth century, obviously. But before we go back there, we’ll stay with this concept of charge, which is nowadays calculated as a fundamental or base unit, based on the electron or its opposite, charge-wise, the proton. These elementary particles have the same but opposite charge, though they’re very different in mass (something which seems suspect to me). Anyway, taking things on trust, a unit of charge is ‘defined’ in standard macro terms as a coulomb, named for the 18th century French physicist Charles-Augustin de Coulomb. One coulomb equals approximately 6.24 x 1018 protons (or electrons). We’ll come back to this later, no doubt. Returning to Hauksbee’s experiments, he soon realised that it was the glass, not the mercury inside it, that was the agent of electrical effects. His experiments with glass globes were written down in great detail, a boon to later researchers.

Canto: Interestingly, I’ve discovered that, more or less exactly at the same time, one Pierre Polinière was conducting and presenting experiments on electroluminescence in Paris:

A closer examination of these experiments reveals not only that Polinière had personally presented them before the French Academy of Sciences, but that Polinière and Hauksbee, starting from a common interest in the ‘mercurial phosphor’, had conducted similar investigations and had in fact simultaneously announced their independent discoveries of the luminescence of evacuated glass containers.

Pierre Polinière, Francis Hauksbee and electroluminescence: a case of simultaneous discovery.
David Corson, 1968.

Jacinta: So we might finish by trying to explain our current understanding of electroluminescence (EL) and its applications. It’s a sort of combo of electricity and light, as you can imagine, or electrons and photons on the level of particles. For example, semiconductors emit light when subjected to a strong electric field or current….

Canto: Is that the basis of LED lighting?

Jacinta: Absolutely. Electrons in the semiconductor material recombine with electron holes, emitting energy in the form of photons. So it has taken us three centuries to really effectively harness the electroluminescent effects demonstrated by Hauksbee in the early days of the Royal Society.

Canto: What are electron holes? I’m thinking not ‘holes in electrons’ but holes left by electrons as they’re displaced in an electric current?

Jacinta: Yes, or almost. It’s like the lack of an electron where you might expect an electron to be. These holes where you might expect an electrically charged particle (an electron) to be, act like positively charged particles – a positron, say – and move through a lattice like an electron does. We could get into very complicated electronics here, if we had the wherewithal, but these holes are examples of quasiparticles, which mostly exist within solids. Fluid movement within solids (not apparently a contradiction in terms) is extremely complicated. Who would’ve thunk it? This movement of electrons and protons within solids is ‘regulated’ by Coulomb’s Law. Remember him? We’ll be getting to that law very soon, as it’s essential to the field of electromagnetism. And that’s our topic don’t forget.