From kettle controls to a clock which tells the time in a new way

Paul Boughton

Dr John C Taylor is the inventor whose kettle controls are used one billion times every day. He has also invented a new type of clock with concentric rings of vernier slits that expose lights to indicate the time. Jon Severn met Dr Taylor to discuss the process of invention and the Corpus Clock.

It was inevitable that John Taylor would become an inventor, having grown up watching his father inventing things. In the 1930s Eric Taylor created what today would be referred to as a technical fabric, namely the first windproof and waterproof cotton cloth. However, the fabric was so revolutionary that there was no mass market for it, hence it was used instead to make clothing for expeditions to Mount Everest, Greenland and other extreme environments, as well as flying suits for Amelia Earhart's endurance record attempts.

Eric Taylor was asked to investigate high-voltage heated suits for the Royal Air Force. He realised that a snap-action thermostat was required, so he developed a dished bimetallic thermostat. After the war, Eric Taylor formed a company called Otter Controls and sought new applications for thermostatic controls. The first order he received was for a choke control on Rover cars, despite the fact that only the prototype control existed and Otter Controls had no manufacturing facilities!

Other inventions followed, with the young John Taylor watching the process as he grew up. In 1956 John Taylor won a place at Corpus Christi College, Cambridge, to read Natural Sciences, for which he was awarded a BA three years later (he received an honorary doctorate at UMIST in 2001 and was made a visiting Professor of Innovation; he is also now an Honorary Fellow of Corpus Christi College, Cambridge). After graduating, John Taylor had planned to obtain a PhD, studying the geology of Antarctica. Disappointingly, an offer of sponsorship was withdrawn, so the PhD was cancelled. "Not having an alternative plan, I drifted back to Otter Controls as a graduate trainee," explains Dr Taylor.

In the 1960s products were commonly over-engineered and did not necessarily incorporate safety devices. But automotive manufacturers were starting to reduce vehicle weights, which meant that over-engineering fell out of favour. Electric motors for windscreen wipers, for example, needed to have lighter magnets and thinner wire for the windings. Previous designs of electric motor had relied on conventional thermostatic switches comprising a bimetallic disc, electrically insulating pusher and a leaf spring on which the contacts were mounted; if the motor stalled, heat would cause the thermostat to trip before the windings burnt out. With thinner windings, however, there was insufficient time for the heat soak to trip the thermostatic switch before the windings failed. John Taylor therefore redesigned the switch such that the current passed directly through the bimetallic component. Because heating is proportional to the square of the current (i2R), a motor experiencing even a small rise in current would benefit from fast-acting protection.

While at Otter Controls and, later, at Strix on the Isle of Man, John Taylor mainly designed thermostatic controls. With the trend towards miniaturisation, 12.5mm diameter became a common size for bimetallic discs, giving a typical snap-action movement of 0.3mm. However, because the bimetallic disc also has to be assembled with other components - usually an insulated pusher, leaf spring and contacts - the tolerance stacks mean that often the only way to achieve a complete assembly that will function correctly and give adequate life is to selectively assemble components. Clearly this is expensive and not conducive to automated manufacturing. With labour being in limited supply on the Isle of Man, Strix was an early adopter of automated assembly machinery, taking advantage of local engineers' knowledge of cams and Bowden cables that resulted from the island's motorcycling heritage.

Mechanical amplifier

Dr Taylor explains how he overcame the problem associated with tight tolerances: "What I came up with was a compact bimetallic disc with an integral built-in amplifier that gives ten times more movement. This extra movement comes at the expense of the force the disc can exert: whereas a plain disc exerts about one kilogram of force, the amplifier disc gives 400grammes - but this is still more than enough to operate the switch" (Fig.2). So successful was this invention that more than 5 billion bimetallic amplifying discs have been manufactured. Strix controls are incorporated in electric kettles from many of the world's leading manufacturers.

Design is not always a simple process, according to Dr Taylor: "Ideas are easy, but making the practical embodiment is not always as straightforward."

This point is illustrated well by an example Dr Taylor gives from the 1980s: "Plastic kettles became popular but they brought with them a problem that did not exist with copper or stainless steel kettles: plastic kettles melt if they overheat. At this time there was an increasing interest in safety standards and BSI (the British Standards Institution) decided that kettles must fail safely even in the event of the switch and thermal cutout failing to operate correctly. Initially manufacturers tried making the element out of thinner wire so that this would burn out in a safe manner, but these 'rupturable elements' were failing in the field; some types suffered 30 per cent failures within the guarantee period.

"I therefore set out to design a three-level kettle control: a switch to turn off the kettle when it boiled, a switch to turn off the kettle if it was switched on empty, and a switch to provide protection in the event of neither of the other two operating. Naturally, having always worked with bimetallic thermostats, this was the route I took. I designed a suitable three-level control, produced a prototype and it worked perfectly. Strix received orders for the control but, when it was installed in kettle, we were horrified to find that it did not work. It turned out that the time required for the heat soak to affect the third switch was also long enough for the surrounding plastic components to soften, relieving the spring pressure and preventing the ultimate protection from operating. A complete redesign of this third switch was required.

"In this case the problem had been that the plastic was softening, so I decided to turn this to my advantage. Instead of using a bimetal component, the new design featured a small plastic component that was in contact with the hottest part of the element. In fact this single-use thermal fuse was much simpler and cheaper to manufacture than the bimetallic thermostat, which was also a benefit. After all, you do not want to have a relatively expensive component if in 999999 kettles out of a million it will never be needed. On the other hand, for the one in a million kettles in which the thermal fuse does operate, you must be absolutely certain that it will - even if all the other components in the kettle are starting to age and wear." Of everything that Dr Taylor has invented, he is most proud of this thermal fuse.

To keep his kettle-manufacturing customers loyal, Dr Taylor has invented numerous features, including the lime scale filter, the 360-degree cordless connector and a control that switches off the kettle automatically either when it boils or at a lower temperature - typically for making coffee. In total, Dr Taylor has been granted over 150 patents.

Dr Taylor is also a respected horologist. His interest in early clocks stems from the 1970s when he was helping a Japanese manufacturer to improve the quality of its electric motors for automotive applications. He soon got bored with long commercial flights to Japan so, being a keen pilot, he decided to fly himself. A growing interest in navigation blossomed further when he joined the Royal Yachting Association and became a yachtmaster ocean instructor, which entailed learning to navigate by means of a sextant, stars and time. This, in turn, led to an interest in John Harrison (1693-1776), an English clockmaker who invented the marine chronometer for establishing longitude while at sea. In the 18th century, the problem of determining longitude had seemed so intractable that the English Parliament offered a prize of £20000 (equivalent to about £2.8 million or EUR3.2 million today) for a solution.

Two of Harrison's early inventions were the virtually frictionless grasshopper escapement and the gridiron pendulum that consists of lengths of brass and iron arranged in such a way that the length of the pendulum from pivot to bob is always constant, regardless of the temperature. The grasshopper escapement, in common with other features such as lignum vitae (a self-lubricating wood) rollers mounted on non-corroding brass spindles, helped to virtually eliminate friction.

The Corpus Clock

When he retired in 1999 Dr Taylor decided to do something he had always wanted to: design and build a clock. He chose to create something that was both a homage to John Harrison and completely different to anything that had been made before. The result was the Corpus Clock, a gift to his old college at Cambridge University to adorn the new Taylor library for which he is the principal sponsor.

"I decided to turn the clock inside out and put the escapement on the outside, hence the large, grasshopper-based monster at the top. With the escapement made into a visible feature, I wanted a new alternative to a set of hands for telling the time. Throughout my life I have been a hoarder of ideas; not writing them down but storing them in my memory. Years ago I played with circular verniers and, when I was thinking about the clock, I had the idea of using concentric rings of vernier slits to expose lights that would indicate the time, with an inner ring showing hours, a middle ring showing minutes and an outer ring for seconds. I started out with 60 slits for the minutes and a disc behind with 59 slits to give the vernier effect. The trouble was that when the clock advanced from one minute to the next the lights appeared to dart round anticlockwise. After a while I realised that I could break with convention and use 61 slots on the rear disc to make the lights run clockwise" (Fig.3).

However, building a clock this size posed a problem, as Dr Taylor explains: "The clock - and therefore the escape wheel - is about 1.5metres in diameter. We found that essentially it had too much inertia. But to overcome the inertia, the remontoire spring had to be so strong that the impulse on the pendulum increased its amplitude.

"If you watch the finished clock you will see that it sometimes seems to pause, run backwards or run faster, and the pendulum might only swing for half a cycle. It is the first clock ever to tell relative time. But whatever tricks the clock plays, it will always be accurate to within one one-hundredth of a second every five minutes, as it receives the MSF time signal from the NPL (National Physical Laboratory) transmitter at Anthorn."

The Corpus Clock is a mesmerising blend of art and engineering, incorporating six patented inventions. It is also laden with symbology, such as the centre drop and concentric ripples that allude to the Big Bang at the beginning of time and the pulsating waves that are still radiating outwards today. At the top of the clock, the sculpted chronophage has eyelids that close randomly, a mouth that slowly opens and snaps shut every minute (devouring time), and a sting in the tail that clicks up every fifteen minutes. Every hour, on the hour, the chronophage snaps its mouth and shudders, shooting its sting the number of times of the hour itself.

Dr Taylor says that the Corpus Clock will last for at least 200 years, which would certainly be a fitting tribute to the man who gave us something as elegant yet simple as the bimetallic strip, as well as something of such importance to world trade as the marine chronometer.

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