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Science of war 2: spotting enemy guns using toilet-inspired technology

Towards the middle of World War 1, an Australian physicist at the front was lifted off the toilet seat one too many times. In part 2 of this series, Dr Karl explains how sound ranging technology changed the course of the war.

Last time, I started on how the military technology of sound ranging was invented. Sound ranging technology locates enemy guns, using only microphones.

The 'Eureka' moment for a 25-year-old Australian Nobel-Prize winner, who was serving at the front, came when he got lifted off the toilet seat, by the firing of the big guns, just once too often.

In September 1915 (the second year of World War I) William Lawrence Bragg was serving in France. He heard the terrible news that his brother had died fighting at Gallipoli. Soon after, he received a much happier letter from his father telling him that they had been jointly awarded the Nobel Prize in Physics. They are still the only father-son team to share a Nobel Prize, and William Lawrence Bragg is still the youngest Nobel Laureate in Physics — at the tender age of just 25 years.

Their prize was given for their work in understanding the nature of X-rays — which they did in the years before The Great War. This made it possible to blast a beam of X-rays into a crystal, measure how the beam is bent, and then work out the locations of the atoms inside that crystal.

The structure of DNA, one of the most profound insights into the nature of life we humans have ever made, was obtained using the knowledge they had uncovered.

But back in that war-torn year of 1915, even though Bragg was a Nobel Prize winner, he was not exempted from having to serve. He had started the war riding horses in the mounted infantry, but soon ended up in the maps section of General HG — trying to pinpoint the position of German artillery pieces using various technologies. The most promising one was to record the signal from an array of microphones.

But they had a problem. When a cannon fires, it generates mostly low-frequency sounds. A field gun booms at 25 hertz, but a larger artillery piece emits most of its sound energy as inaudible infrasound — way down at 10 hertz. Back then, nobody knew this.

The breakthrough needed to capture the low frequency infrasound came as two separate hints from nature.

Hint No. 1: Bragg was billeted in accommodation in Flanders. The toilet was a small room, with a door, but no window. When the door was shut, the only connection to the outside world was the pipe leading from under his toilet seat. There was a British six-inch artillery piece about 400 metres away. When it fired, his bare bottom was actually lifted off the toilet seat by the inaudible infrasound energy, even though he could often hear nothing at all. So now he knew there was enormous energy in the inaudible infrasound - but he didn't how to detect it.

Hint No. 2 came via Corporal W.S Tucker, also a physics graduate, who served in Bragg's team. Tucker slept in a shack that had walls, not of wood or brick, but tar-paper — literally stiff paper that had been covered with tar to make it waterproof. There was a hole in the tar-paper next to his pillow. Every now and then, even though there was no wind and he had not heard artillery fire, a puff of chilly air would annoyingly strike his cheek.

The two physicists realised that when a big gun fired, its high-energy low-frequency infrasound was causing the puff of air. To them, the solution was obvious. They got a small empty wooden ammunition box, ran a thin platinum wire along inside the box, and drilled a hole in the box near the wire. They ran electricity through the wire, which heated it up. When a big gun fired, the infrasound pressure wave forced a puff of air onto the hot wire and cooled it down. This changed the resistance of the wire, which they did have the technology to measure.

Using their method, the allied sound ranging could locate German artillery to within 25 - 50 metres.

It was brilliantly demonstrated on 20 November, 1917, near the German-occupied French city of Cambrai. Thanks to the new technology of sound ranging, the German artillery was quickly silenced. It was a great hour for the Allies — and church bells were rung in London. Sound ranging again was used to devastating effect in the Battle of Amiens, on 8 August, 1918. And shortly thereafter, the war was over.

I guess that proves, once and for all, that men might do their best thinking on the toilet seat, and should be allowed to sit there for as long as they like ...

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