THE HISTORY OF GEOPHYSICAL PROSPECTING
PART EIGHT - SOUND RANGING IN WORLD WAR ONE

Chapter 19  -  Sir Lawrence Bragg

The most vigorous individual of my acquaintance is Sir Lawrence Bragg. In 1965 he celebrated the 50th anniversary of his award of the 1915 Nobel Prize in Physics. At the age of seventy five, he heads the Royal Institution of Great Britain, founded on March 9, 1799. The present house in Albemarle Street, London, was purchased in June, 1799. The library was commenced in 1803 but its volumes go back to the dawn of printing. Sir Lawrence flits around over the earth's surface on enterprises of the Royal Institution as though he had received the Nobel award in 1965. Distinguished Directors of the Institution include Sir Humphry Davy, Michael Farraday, Lord Rayleigh and James Dewar. Sir Lawrence Bragg's father, William Henry Bragg, became the Director of the Royal Institution in 1923, upon the death of Sir James Dewar. The Bragg story is the story of a man following in the footsteps of his father and his every footstep a distinguished one.

William Henry Bragg and William Lawrence Bragg both received First Clas Honors in Physics at Trinity College, Cambridge. William Henry Bragg was made Cavendish Professor of Physics at the University of Leeds in 1908, while William Lawrence Bragg was appointed Cavendish Professor of Physics at Cambridge in 1938. In 1915, father and son collaborated in publishing "X-Rays and Crystal Structure." Soon thereafter, William Lawrence Bragg was commissioned a Captain in the Royal Engineers and placed in charge of all British Sound Ranging. Shortly after his arrival at General Headquarters in France, he received word that his father and himself had been honored with the 1915 Nobel Prize in Physics.

William Henry Bragg was engaged by the Admiralty in research on submarine detection and hydrophone development during the war. Five years later, when he took over as President of the Royal Institution, that society's Davy-Faraday Laboratory became even more world-famous as a center of research into the problems of crystal structure. William Henry Bragg was knighted in 1920; William Lawrence Bragg was knighted in 1941. Because of World War One the customary Nobel lecture had to be postponed in 1915 and was finally delivered in September, 1922, by W. L. Bragg. There was no Nobel award in Physics in 1916.

Soung Ranging had a two part function. The first problem was to locate the position of enemy guns and the second problem was to convince your own artillery that they should train their fire on the positions you had spotted. Minor artillery officers were harder to convince than generals. Artillery Information Officers were often found to be unshakably wedded to methods, of location by air photographs. The 100 Kilometer Gun, which was firing on Paris and generally known to Americans as "Big Bertha," was accurately spotted by five French Sound Ranging Stations. But Freneh Artillery never fired a shot at the Sound Ranging l0eation. All their shelling was directed at a well-camouflaged dummy location, pinpointed by aerial photography. This artillery error was not discovered until after the war was over.

The Freneh used several methods of Sound Ranging whieh they called, "Reparage par le son." In 1915, Sound Ranging experimentation was mostly French but had a number of overtones supplied by British physicists. By 1916, when the method began to have field suceess, the French Sound Ranging and the British Sound Ranging went separate ways. Colonel Driencourt was in charge of the French efforts with the title of Direetor of the "Service Geographique de 1'Armee." Under one French method, the sounds were received by a microphooe and the currents actuated a pen which wrote on smoked paper. The physicists, Cotton and Weiss, were associated with this system.

The second method was named after its inventor and designated as "Systeme Defour." Professor Charles Bazzoni has stated that the Defour system, although complicated, could have been perfected if more time had been devoted to it. The Defour system was never brought to completion.

A third method and the one adopted by the British, used a string Einthoveo galvanometer as the recording instrument. The shadows of the strings fell across a slit behind which cinematograph film moved. The time marking was made by interrupting the light a hundred times a second with a toothed wheel. The microphones were of the carbon grain type, usually called the Paris-Rome model.

The method adopted by the French was called T. M. (Telegraphic Militaire). T. M. was a system in which sounds were registered by means of mechanical levers, thus doing away with all photographic necessities. It was much less sensitive than the British string galvanometer methode.

In the late summer of 1915, Colonel Winterbottom and other officers of the Royal Engineers witnessed a demonstration of French Sound Ranging methods. It took considerable persuasion on Colonel Winterbottom's part to interest the War Office and British Artillery in having a Sound Ranging Section of their own, but eventually the Colonel's tenacity won out and Captain Lawrence Bragg was sent to the front in September of 1915 to take charge of an experimental section. He first saw ranging in action in the Vosges and then set up a station on Kemmel HilI near Ypres. The original British Sound Ranging group consisted of two officers and six men.

The inventor of the British system has the name of Lucien Bull and we will learn the pertinent facts about Mr. Bull at the close of this chapter. The principle of the Bull system is this. A number of microphones, usually six, were spread along a base of about 4,000 meters, parallel to the front line and about 1,500 meters (metric mile) behind it. The microphones were connected to a recording station at some convenient spot behind the base. In front of the base, one or two forward observers were stationed. When a forward observer heard a gun fired he pressed a button which set the recording appararus going and telephoned a report to headquarters, saying as weIl as he could roughly where the gun was firing from, what calibre it was, and what it was shelling . He was sufficiently in front of the base to make sure that he heard the gun in time to set the recording apparatus in motion before the sound reached the base. The time intervals between the arrival of the sound at the six microphones was measured on the sensitized film. By applying corrections for wind and temperature, it was possible to locate the source of the sound waves which corresponded with these intervals.

When the forward observer pushed the switch he started in motion the camera motor, the magnetizing coil of the electro-magnet of the galvanometer and the lampe. It would not be possible to switch on the tuning fork and time wheel in this way. These were started when work began for the day and kept running all the time that locations were likely to be made. After once closing the switch, the forward observer kept the switch closed until sufficient time had elapsed to take the sound records, then released the key to stop the galvanometer-camera system. The Einthoven string galvanometer was invented in 1909 and put to use by Lucien Bull in 1910 in his physiological study of electro-cardlography. In 1914, Bull constructed a smaller and simpler model for sound-ranging purposes. In the original Bull camera, the film simply passed into a light-tight box from which it could be removed and developed and fixed by hand. This was later modified and the camera fitted with an automatic developer. In this device the film passed into a sloping guide at the top of which developing solution was dropped on it and was spread onto it as it passed down the guide by three light rollers. At the bottom of the developer guide, the film passed onto an endless belt which carried it through the fixing bath, and it emerged from the apparatus developed and fixed as fast as the motor drove it through.

Sir Lawrenee Bragg reports that for the first year of British Sound Ranging operations, the system did not work very weIl for the specific reason that the microphone was extremely sensitive to rifle fire, traffie and any other noises near it. The microphone was relatively insensitive to the "boom" of the gun itself, which was the very thing which was desired to be recorded. The invention of the Tucker microphone changed sound-ranging into an efficient operation. As fortune would have it, a member of the soundranging section had been performing an experiment during peace times which had a direct bearing on the problem at hand. Lance Corporal Tucker had been on the staff of the Imperial College in London and had been conducting research on the cooling of Wollaston wire by small air currents. Bragg and Tueker talked over the possibilities of employing this effect to make a better microphone. They ordered some Wollaston wire from England. When it arrived they drilled a hole in a wooden ammunition box, stretched a bit of Wollaston wire across it and heated the wire. This set-up became one arm of a Wheatstone bridge. The air current through the hole, due to the pressure wave, cooled the wire and upset the balance of the bridge. High frequeney sounds had little effect because they did not disturb the hot air around the bridge. So the hot-wire Tucker microphone was born; it picked up the lowfrequency gun sound and ignored high-frequency sounds.

One more problem needed to be solved, providing the wind protection around the microphone. The answer was found to be one or more fairly open hedges made of branches or the equivalent which turned the air flow into steady stream lines. The bugbear was a west wind blowing toward the enemy lines which made sound-ranging impossible. Sound-ranging worked like a charm when there was an east wind and even better in still, foggy weather.

From a single section with eight men, Britisq Sound Ranging grew by leaps and bounds with a final personnel of better than five hundred. Bragg was promoted to Major and Tucker became a lieutenant. There were some forty sections in all. Each section had a mechanic, a watchmakers lathe, a kit of tools and a box of scrap metal so that each section could try out its own experiments. The entire Sound Ranging Section met every two months to exchange information and swap stories. Sir Lawrence Bragg ascribes the rapid development of British Sound Ranging to the fact that the work was done in the front lines under practical conditions and not at an experimental station in England. In 1918, Major Bragg was awarded the Military Cross and made an Officer of the British Empire.

Is Lucien Bull, the man who designed all the British Sound Ranging apparatus except the Tucker microphone, an Englishman or a Frenchman? From the facts at hand, I would surmise that he should either be called a French Englishman or an English Frenchman. Bull was born ninety years ago in Dublin, Ireland, of an English father and a French mother. After studying at Belvedere College in Dublin, Lucien Bull was sent to Paris at the age of 18 to perfect his knowledge of the French language. Living with the maternal branch of the family, he received an introduction to the leading French physiologist of those days, Professor Marey. Bull was soon working in the "Institut Marey." Lucien Bull, already weIl versed in physics and mathematics, took his degree at the Sorbonne in 1908 in zoology, botany and geology; thus becoming one of the most versatile scientists of all time.

I have already related how Bull perfected and adapted the Einthoven harp galvanometer for sound-ranging purposes. This he accomplished late in 1914, loog before the British were even interested in sound-ranging. The Bull apparatus was operating at the French front in the spring of 1915, many months before any other system was tried out. In the late summer of 1915, when Colonel Winterbottom made a detailed study of all French methods for the British War Office, he liked the Bull system and together with Captain Bragg selected it in preference to the other French systems. The French army finally decided to standardize on the T. M. system.

Lucien Bull has never lived in England, although he has always held English citizenship. His residence was Dublin until the age of 18 and his residence has been Paris for the past 72 years. After Bull's graduation from the Sorbonne he returned to the Marey Institute as Assistant Director. In the year 1965 he still works in the same laboratory. Publication of work on high-speed cinematography brought him into prominence in 1904. Since that time he has engaged in a wide variety of physiological experiments and his delight in experimentation persists to this day.