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Basic physics….

01/07/2017

…. of all topics, given that it was one of my worst subjects at school. But one thing in physics that always fascinated me was a device known as the ‘bimetal strip’; and today, for some reason, I found myself wondering just when it was invented.

The physical principles involved are older than humanity itself – though it took us quite some time to discover them and put them to practical use. First we had to learn how to extract metals from the earth’s crust and turn them into things we needed – rather more than 10,000 years ago, when the ‘Stone Age’ made way for the ‘Bronze Age’ (named for an alloy of copper and tin which, among other things, allowed us to make stronger and more deadly weapons).

At some point it must eventually have become clear that different metals have different ‘coefficients of expansion’ – in other words, they expand and contract by different amounts when heated up or allowed to cool down again. And almost 300 years ago an English clockmaker called John Harrison worked out a way to use these differing properties for his own professional purposes.

Pendulum clocks had the disadvantage that the pendulum got longer or shorter as the surrounding air temperature changed; this in turn increased or reduced the swing of the pendulum, on which accurate timekeeping depended. But in 1726 Harrison discovered that he could use the different coefficients of expansion of iron and brass (an alloy of copper and zinc) to design a complicated framework of parallel iron and brass rods that kept the swing identical regardless of temperature; I’ll spare you the details, but it was ingenious.

Later such ‘bimetal’ systems were perfected and extended to other fields. The discovery of gas as a source of public lighting and heating in the late 18th century was an immense step forward in comfort; but gas had the unfortunate drawback that it could ignite with devastating force at the wrong moment (and the tiniest spark was enough, as coal miners still know to their cost). So gas-fired heaters and water boilers were eventually fitted with small ‘pilot lights’ which ensured that the appliances did not have to be turned on and off again each time they were used; any incoming gas was instantly consumed by the flame, so that it could not accumulate unseen in the chimney, room or building. For additional safety, piped ‘town gas’ – which was normally odourless – was given a characteristic strong smell that warned of a leak; but by then it was often too late. Simply switching on an electric light, let alone striking a match, to see where you were in a dark gas-filled room could provide the fatal spark that would blow the whole building (or street) apart.

To avoid wasting fuel, pilot lights were kept very small; but this made them liable to be extinguished by draughts, or if the supply was temporarily cut off or the intake got blocked by impurities, allowing the explosive gas to leak unnoticed. And it was here that the ‘bimetal strip’ came into its own.

A small length of iron, firmly bonded (riveted or welded) to a similar length of copper, was installed in a position where it was exposed to the pilot light. As long as the flame was alight, the heat made the two metals expand at different rates. Being bonded together, they could not simply expand in a straight line; instead, the whole component curved away to one side, keeping the gas intake open. But if for any reason the pilot light went out, the metals instantly cooled down again, allowing the curved component to straighten back into a position that sealed off the intake and prevented a leak.

This basic principle of physics operated with unvarying accuracy; and until quite recently the bimetal strip was the safety device of choice in gas-fired appliances. But eventually it was realised that a constantly burning pilot light consumes almost as much gas on its own as the apparatus does when it is actually being used; and modern appliances are fitted with more environment-friendly systems. However, some older ones still have bimetal strips, for after three centuries the device has not outlived its technical usefulness. It isn’t vulnerable to electricity failures, or computer viruses, or other such hazards of modern life, for it simply depends on what has happened to metals since time immemorial when they heat up or cool down.

In my previous flat I had an old cylindrical cast-iron gas heater that was protected by a bimetal strip. If the pilot light had gone out and I needed to relight it, that meant keeping the intake button pressed down for as long as it took the heat of the flame to act on the strip until it had curved well out of the way. If I was impatient, the flame simply went out again, sealing off the intake – no gas, no sweat, no big bang.

Good old basic physics – and thanks, Mr Harrison.

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From → Science, Technology

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