Clocktime: John Harrison Wooden Regulator Longcase 1726, 04 Gridiron Pendulum

John Harrison - The John Harrison precision wooden regulator ebonised longcase gridiron pendulum.
Join Dr John C Taylor OBE from the Clocktime digital museum as he discusses the gridiron pendulum on the John Harrison Wooden Regulator, dated 1726.
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So, I'll open the trunk and show you the temperature compensated pendulum. It's got on each side, three steel rods which expand down and two brass rods which expand up and this means that the rods themselves move up and down but the pendulum bob keeps the same level. An ordinary longcase clock in the early 1700s has a metal rod pendulum which loses its accuracy as the ambient temperature changes. When it's warm, the metal on the rod expands causing the pendulum length to increase and to slow in period and when it's cold the metal contracts causing the pendulum to shorten and speed up. Harrison wanted to design a pendulum that could compensate for changes in temperature and still keep accurate time. In the early 1700s, the Royal Society in London was having learned discourses on the expansions of metals and yet Harrison has already found information about this. He discovered that brass expands around twice the rate of steel and so using this logic, a rod made out of brass would increase an equal distance to a rod twice the size of that made of steel. By connecting these two rods at a critical point, the expansion could be opposed and the lengthening compensated the one by the other. But for this system to work accurately, the pendulum must be positioned at the bottom of the rod. So, Harrison designed the gridiron system that linked the rods of brass and steel folded in a series that allowed the effective length of the pendulum to remain constant and oscillate evenly, despite changes in temperature. The bob remained at the same length, whereas the parts of the pendulum expanded up and down. But being a perfectionist, he wasn't happy there. And the centre steel rod, you come to his, what he called his tin whistle and the bottom of the whistle there you can see is fastened onto the steel rod. But the other piece of steel is just peeping through on the right-hand side and can you see the holes and you can move the pin up and down to get exactly 100% compensation of the actual steel rods against the actual brass rods. The pendulum bob itself sits on a calibrated nut to give a micrometre adjustment and you can see the figures engraved on the nut, so you know exactly how far you're moving it. It became, when he had finished calibrating it, the most accurate clock in the world for 150 years.