Australian Nobel laureate’s wartime inventions honoured by US Army physicist
Adelaide’s William Lawrence Bragg recognised as acoustic pioneer. Phil Dooley reports.
Australian-born physicist William Lawrence Bragg, the youngest ever winner of the Nobel Prize for Physics, is set to be honoured for his work on sound waves at a meeting of the US Acoustical Society of America (ASA) in Kentucky, in May this year.
Bragg (1890 to 1971) also earned the Military Cross and an OBE for his work during World War 1.
His story will feature in a tribute to the pioneers of sound to be presented at the ASA conference by US Army acoustic physicist Daniel Costley.
“Bragg encouraged the innovation that solved a lot of the practical problems,” Costley says.
By the end of the war, Bragg had taken sound-ranging from a barely-functional idea to a technique that could pinpoint enemy guns to within 25 metres, and was implemented by battalions as far afield as Palestine.
In 1915, senior British officers learned of experiments by a pair of French physicists who were attempting to develop a method to locate the position of enemy guns based on the times the sound of an explosion arrived at different locations.
Bragg was seconded to investigate. He had moved from Adelaide in Australia to England when his father, William Henry Bragg, had taken a position at Leeds University. The son then went to study mathematics at Cambridge in 1909, joining a cavalry unit as a way to make friends in the unfamiliar town. When war broke out, he signed up and was commissioned as a second lieutenant.
However, the reality of serving was not as he hoped, and when, out of the blue he was asked to take on the task of investigating whether sound-ranging could become a practical reality, it made him, he later recounted, “walk on air”.
It was an inspired choice by the British. By 1818, Bragg’s sound-ranging system was a vital element in negating the German guns at Amiens, in, as one soldier put it, “the black day of the German army”.
Bragg met with the French pair, astronomer Charles Nordmann and Lucien Bull, a medical researcher who had been working on a method to record heart beats.
Although the idea was good, there were many challenges. Wind could play havoc with the sounds, and the microphones couldn’t pick up the boom of the guns; the frequency was so low that it was something one felt rather than heard.
Indeed, one day, while in the toilet, Bragg noticed that when a British gun fired several hundred metres away, his “bottom was elevated perceptibly off the seat, even though often he heard nothing at all”.
He assembled a team of physicists and one by one solved the problems. Approaches included wrapping a microphone in camouflage netting to cut wind noise, and turning an ammunition box into a microphone that was well-tuned to the low frequencies of artillery explosions.
The new Tucker microphone, named after its inventor William Tucker, a London University physicist and member of Bragg’s team, was a major advance for the system. A heated platinum wire over the mouth of the ammunition box was the active element. The resonance from a low frequency boom disturbed the air around the wire, cooling it, changing its resistance and creating the signal pulse.
Unlike the previous carbon microphones, it could distinguish between the launch explosion of the gun and the sonic boom it generated as it travelled overhead, and even distinguish between the types of artillery.
Another innovation was the “harp” galvanometer. Its strings were an array of copper wires between magnets, each connected to separate microphones hidden across a kilometre or more in either direction.
When an electrical signal came from the distant microphones, the current would cause the wire to move due to its interaction with the magnetic field. A continuous roll of film underneath the wires recorded the exact timing of the pulse, much more accurately than earlier methods based on human observations.
Mere minutes after an attack the film could be developed and the calculations completed to reveal the enemy location. The Germans' accuracy never rivalled the Allied system, despite having distinguished physicist Max Born (who went on to win a Nobel prize in 1954) working on the problem.
Costley says the films from the harp galvanometer are effectively recordings. “People have digitised the films and can play them back: you can hear the cannons.”.
Ultimately the success of the group was due to Bragg’s scientific leadership. “He was really good about giving credit to people on his team,” notes Costley.
Bragg’s focus on recognising individuals’ achievements perhaps came from an early slight delivered by his father. In his letters to the journal Nature reporting their work, William Henry had thoughtlessly not credited Lawrence by name for coming up with the equation describing X-ray scattering, mentioning him only as “my son”.
Lawrence also had wished to accompany his father to Second Solvay Conference in Brussels in 1913, to personally report his discovery, but was left behind. His consolation was a postcard of congratulations signed by Einstein and many other prominent scientists.
Although his father did his best to make it up to Lawrence, it pained the son for the rest of his life. The silver lining, though, as Harvard University professor William Van der Kloot wrote in 2005, was that “Bragg’s lifelong vigilance to see that credit was always given where due [was] a key element in his successes as a scientific administrator”.
William Lawrence Bragg and his father William Henry Bragg are perpetually honoured by the Royal Institution of Australia, publisher of Cosmos, which is based in Adelaide, Australia. Each year, the institution recognises distinguished scientists by awarding them Bragg Fellowships. You can see the list of Fellows here.