Lateral Thoughts Physics World  August 2017
(iStock / akiyoko)

When colour sometimes matters

A few years after the middle of the last century, having recently graduated, I turned up at the premises of a famous aircraft manufacturer clutching my letter of appointment as an assistant engineer. A cursory medical examination, which consisted solely of a nurse examining my legs for varicose veins, along with a quick explanation of the dire provisions and consequences of the Official Secrets Act, and a signature and I was in.

My first task was to measure the temperature of the jet pipe of a Vampire fighter using a type of radiometer. I was to be lying on the floor of a following Mosquito combat plane, with the radiometer peering through a window in the nose. I began to have doubts about how long my new career would last. After all, the exhaust gas from a jet engine is very hot and the Mosquito is made largely of wood. As luck would have it, I was rescued just in time as I was preparing the equipment for this somewhat worrying job – there was a vacancy in the physics section that urgently needed to be filled. I had the necessary qualifications, so would I agree to a transfer? It sounded like more of an order than a request, and I did agree, although I have always regretted the loss of the opportunity to fly in such an historic aircraft.

I made my way to the physics lab. About half a dozen young men (no women at that time) stood around a panel covered with valves and pressure gauges, connected on one side to a large cylinder of nitrogen. The other side was attached to a short length of fine tubing, which ended with a small finned device inserted in the tight-fitting neck of a miniature, transparent Dewar flask. I joined the group and watched. All eyes were concentrated on the tiny reservoir at the end of the flask. It gradually filled with a swirling mist and then suddenly a jet of liquid nitrogen shot from the end of the device and rapidly filled the reservoir. There was jubilation all round. What we had just witnessed was our first successful operation of a prototype, miniature Joule–Thomson liquefier. It had been recently acquired from the Royal Radar Establishment where it was developed for use with infrared detectors that needed to be operated at very low temperatures.

Within a few months, I was responsible for seeking solutions to any problems arising from the integration of these intriguing devices and the associated detectors into the guidance system of infrared, air-to-air missiles. There were, of course, many such problems, but two in particular were rather serious. The jet of liquid leaving the nozzle (a somewhat grand term for what, at that time, was the end of a very fine tube squeezed nearly closed with a pair of pliers) impinged directly on the base of the reservoir, on the far side of which a photosensitive mat­erial was mounted. The consequent, inevitable vibration stressed the detector material, which caused electrical noise or “microphony” – much more than could be tolerated. In addition, there was an operational requirement that liquid nitrogen should remain in the reservoir for at least a specified minimum time (the dwell time) after the liquefier was shut down. It didn’t.

These two problems had to be solved. We thought long and hard, and came to the conclusion that we needed to fill the reservoir with some sponge-like material. This would diffuse the liquid jet and hold the liquid in the reservoir, solving both problems at the same time. Cotton wool looked like a good starting point. A laboratory supplier’s catalogue yielded the information that cotton wool came in a range of about a dozen colours. Apparently, bacteriologists like to plug their test tubes with colour-coded material. I wish I could say that I thought that the properties of the cotton wool might vary with colour, but actually I just couldn’t make my mind up so I ordered one pack of each colour. Secretly I thought that a colourful array on the bench might brighten the place up.

I wish I could say that I thought the properties of the cotton wool might vary with colour, but actually I just couldn’t make my mind up so I ordered one pack of each colour

We set about systematically testing each sample, measuring the microphony and dwell time in each case, while also varying the amount of cotton wool we put in the reservoir – a pinch, half a pinch, a pinch and a bit. There was no doubt about it, the green cotton wool was best – it produced the lowest microphony and the highest dwell time. We then got a bit more scientific and established over what range of weight the green cotton wool gave acceptable results. Finally, we wrote instructions for its use and the job was done. So that the right material remained available for as long as needed, we bought and tested several kilograms, which were eventually deposited in some central government store.

Some 40 years later on a visit to an Admiralty establishment, I was shown some equipment incorporating a cooled detector. I was told of the mysterious requirement to draw green cotton wool from a store and put a weighed amount in the detector before use. I was asked what I thought that was all about – a question I could and did answer.

There are two morals to be drawn from this tale. Decisions can have consequences long after their reasons have been forgotten, and colour sometimes matters in unexpected ways. If you have ever been puzzled by green cotton wool, you now have the explanation. And please, if there is any left in a store, could I have a pinch as a souvenir of an interesting and entertaining period of my life?