In the early nineteen thirties the Air Ministry had a problem it could not solve, and it was this. If bombers are coming, you find out when you can see them, and by then it is far too late to get fighters up to the right height in the right place.
The best available answer was a row of enormous concrete dishes along the coast, built to focus the sound of approaching engines onto a man with a stethoscope. They still stand in Kent. They are magnificent and they were nearly useless, because they could not tell you much beyond the fact that something was out there, and because a passing lorry ruined everything.
So in nineteen thirty five, somebody at the Ministry wrote to a scientist named Robert Watson-Watt with a rather desperate question. The newspapers were full of talk about death rays. Could radio waves be used to destroy an aircraft, or kill its crew?
Watson-Watt handed the sums to a young assistant, Arnold Wilkins, who worked out that the answer was plainly no, and not by a small margin. The energy required was absurd.
But Wilkins added a second thought at the bottom, almost as an afterthought. He said that although you could not destroy an aircraft with radio waves, you could almost certainly detect one, because radio waves bounce off metal, and an aircraft is a large piece of metal in an otherwise empty sky.
They tested it in February nineteen thirty five, in a field near the village of Litchborough, and the way they did it is my favourite thing in this whole story.
They did not build a transmitter. Building one would have taken months and money nobody had yet agreed to spend. Instead they borrowed one, in the sense that they used the enormous shortwave transmitter that the British Broadcasting Corporation was operating at Daventry, which was already flooding the sky with radio waves all day long for entirely peaceful reasons.
They parked a van in the field, strung up a receiving aerial, and arranged for a bomber to fly back and forth through the beam.
On a small cathode ray screen inside the van, a line of light twitched as the aircraft passed. It kept twitching for eight miles.
There were three men watching. One of them was from the Air Ministry, and he authorised funding almost on the spot. Within two years there were stations going up along the coast.
The system was called Chain Home, and by the summer of nineteen forty there were about twenty stations covering the approaches to Britain from the Shetlands round to the Isle of Wight.
You could not miss them. Each station had transmitter masts of steel, over a hundred metres tall, and receiver towers of wood, because wood does not interfere with the signal the way metal does. They stood on cliffs in plain view of anybody in France with a pair of binoculars.
By later standards the equipment was crude. It used a long wavelength, which meant it could not see aircraft flying low, so a second system had to be bolted on to cover that. Reading it was an art rather than a science. An operator watched a wobbling green line on a screen and, from the size and shape of a blip, estimated how many aircraft there were and how high they were flying, and the estimate could be badly wrong.
The British did not call it radar. That word was invented later, by the Americans. They called it Radio Direction Finding, and they chose that name deliberately, because direction finding was an old and boring technology that everybody already had, and the name invited nobody to look closer.
Here is the part that gets missed, and it is the important part.
Germany also had radar in nineteen forty. In several respects it was technically better than the British equipment. The masts on the English coast were photographed, identified and understood.
What Germany did not have was the rest of it.
A radar station produces a number. On its own a number is worthless, and worse than worthless if it contradicts the number from the station next door, which it constantly did, because two stations watching the same formation would report different heights, different counts, and sometimes what looked like two separate raids.
So the British built something underneath the masts that nobody photographed, because it was in a basement. Every station reported by dedicated telephone line into one room, called the Filter Room, where the conflicting reports were reconciled into a single agreed track for each raid. Those tracks were then passed down to the fighter groups and to individual sector airfields, each with its own map table, so that a controller could look at a board and see where every raid was and which of his squadrons were fuelled and ready.
From a blip on a screen to a squadron in the air took, at best, about four minutes. That was the invention. Not the radio waves. The plumbing.
In August nineteen forty the German air force did attack the radar stations, and on the twelfth they hit several at once, including the station at Ventnor on the Isle of Wight, which was knocked out completely.
The British response was a small masterpiece of nerve. A mobile transmitter was driven to the site and told to broadcast the usual signals, saying nothing useful, simply making the noise a working station makes, so that German listeners would conclude the station was still operating.
It worked. And after a few more attempts, the attacks on the radar stations were largely called off, on the grounds that they were not producing results worth the cost.
This was a serious mistake, and it came from a reasonable misunderstanding. From the outside, a radar station looks like a tall mast. Knock down the mast and you have blinded them. But the masts were latticework steel and the bombs mostly went straight through without bringing them down, and the part that mattered was not the mast anyway.
You cannot bomb a system if you have not worked out that there is one.
There is a sequel to this, and it happened that same autumn.
Bombing a city at night is mostly a navigation problem. The German air force solved it with a clever system of radio beams: one narrow beam laid across the target from a transmitter in occupied Europe, which the aircraft flew along, and a second beam crossing it at the point where the bombs should be released. Fly the first, listen for the second, let go.
A young British scientist called Reginald Jones worked out that this was happening, largely from fragments — a scrap of a prisoner's conversation, a phrase in a captured logbook, a note about equipment nobody could account for. Senior people did not believe him. He was twenty-eight and he was telling the Prime Minister that the enemy had a radio system nobody had detected.
An aircraft was sent up to look, and found the beam exactly where he said it would be.
What followed was better than jamming. Jamming tells the enemy you know. Instead, the British transmitted their own signal to widen and shift the beams very slightly, so that the crews would follow them faithfully, believe they were exactly on track, and release their bombs into an empty field several miles from anything.
For a while it worked, and there are parts of the English countryside that were bombed with great precision for no reason whatsoever.
Watson-Watt is usually given the credit, and he did the political work of getting it built, which mattered enormously. But the calculation that started it was Arnold Wilkins's, and the two men's reputations have never quite balanced out fairly.
Radar was not a British secret in the sense of nobody else having it. Britain, Germany, the United States, France, the Soviet Union, Japan and the Netherlands were all working on the same idea at the same time, more or less independently. What Britain did first was decide that detection was a plumbing problem rather than a physics problem.
There is a lovely footnote. Long after the war, Robert Watson-Watt was driving in Canada and was stopped for speeding by a policeman using a radar gun. He is said to have written a rueful little poem about it afterwards, on the general theme that a man ought to be careful what he invents.
Here is something to think about tonight. Two countries had the same invention, and only one of them built the boring part that made it useful. Can you think of something you own that would be much more useful if you built the boring part around it?