One of the most useful and game-changing weapons of World War II was radar, a technology that allowed Allied pilots to know when and where to fly in order to intercept incoming German bombers, but Britain was actually hunting for a super weapon: A death ray.
In 1935, World War II had essentially not started yet. Japan was conducting limited, intermittent fighting with China, but Europe was technically at peace. Except war was clearly bubbling up. Germany was re-building its military in violation of the Peace Treaty of Versailles, and Italy launched a successful invasion of Ethiopia.
Britain knew, sooner or later, it would get dragged into a fight. Either Italy would attack colonial possessions in Africa that belonged to it or its allies, or Germany would attempt to conquer Europe. And there was a rumor that Germany had developed a weapon that could wipe out entire towns.
(This may have been a result of early nuclear research. German scientists made some of the critical first breakthroughs in what would later result in nuclear bombs.)
So British leaders asked Robert Watson-Watt if his research, using electromagnetic radiation to detect clouds, could be used to kill enemy pilots.
Yes, they wanted a death ray. But Watson-Watts quickly realized that he couldn’t get that much energy into the clouds. His work, which would lead to modern day weather radar, used a magnetron to send microwave radiation into the sky. But it wasn’t a focused beam of energy, and there simply wasn’t enough juice to kill or even seriously distract an enemy pilot.
To get an idea of how the death ray would’ve had to work, imagine a microwave that could cook a human in less than a minute while they were still miles away. That would be a huge, power-sucking microwave and essentially technically impossible to build.
But Watson-Watts came back with an alternative proposal. The death ray was dead in the water, but the magnetrons could be used to detect planes just like clouds, but even more effectively. And the early math around the idea revealed that the device could see enemy planes for miles and miles, eventually 100 miles out.
This was game-changing for British pilots when war did break out and reach British shores. Germany quickly conquered France and then began attacking England in the Battle of Britain, using the Luftwaffe to bomb British targets and take on British fighters. The British were outnumbered, and so they needed to make each flight hour of each pilot count for as much as possible.
Radar made this possible. If Britain could only spot incoming German forces with human eyeballs, it would need a large number of spotters on the ground and pilots in the air at all times. But with radar looking out a hundred miles, the Royal Air Force could fly fewer patrols and keep most pilots resting on the ground until needed, instead.
When radar detected incoming planes, the in-air patrols could fly to intercept as additional forces scrambled into the sky as necessary. The network of radar stations would become the “Chain Home” system, and it watched Britain to the north, east, and south.
Germany developed its own radar and deployed it operationally in 1940.
Britain never got its death ray, but Japan did experiment with making a death ray like Watson-Watts considered. They used magnetrons to create microwave radiation in an experimental design that did kill at least one rabbit targeted during tests. But killing the rabbit required that it stay still for 10 minutes, not exactly useful in combat. A groundhog took 20 minutes.