‘Derek, please, not so fast’ 

Ferdinand Mount

In all the history of second-guessing in warfare, the Window affair is one of the most extraordinary. As early as 1934, Post Office engineers reported that passing aircraft could interfere with radio reception. Less than a year later, Robert Watson-Watt demonstrated by a simple experiment in a field outside Daventry that aircraft could be detected by radio. Radar was born. Remarkably, it was only two years after this that Lindemann demonstrated to Churchill that tinfoil strips cut to a certain length and jettisoned from a height would simulate aircraft on the enemy’s radar screen and baffle anti-aircraft batteries. Churchill, always a sucker for gadgets, loved the idea, but the scientists in charge ‘looked down their noses at the suggestion’, according to Lindemann’s protégé R.V. Jones, who had first thought of it. Partly they didn’t care to see their amazing discovery so quickly outfoxed, but also they worried what would happen if the Germans got hold of this simple device. For the next five years, no research was done on Window – as the scheme came to be known. So in the first raids of the war British bombers flew over German defences like so many flights of sitting duck.

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