Paul Langevin and the Discovery of Active Sonar or Asdic introduced by David Zimmerman Pendant la première guerre mondiale, le «National Academy of Sciences » des États-Unis a créé le « Research Information Service» dans le but de recueillir des informations militaires européennes pour les forces armées américaines dans les domaines techniques et scientifiques. Le professeur David Zimmerman a trouvé le document suivant dans les dossiers du «Research Information Service» conservés dans les archives nationales au College Park, Maryland. Ils 'agit du rapport de 1918 par Paul Langevin concernant ses recherches importantes et innovatrices sur ce qu'on considère aujourd'hui comme étant le sonar actif Le document original de Langevin rédigé en français fut traduit en anglais par les «Forces navales des États-Unis» au moment où il avait été reçu. During the First World War U-boats came close to winning the war. Unlike the later Battle of the Atlantic, technological innovation was not a key ingredient in the Allies victory over the U-boat campaign against shipping. Instead, the German submarines were defeated by the widespread application of convoys to shepherd ships through waters patrolled by submarines, radio intelligence which directed convoys away from submarines, and the implementation of huge ship building programs. Despite a large scientific and engineering research effo rt, submerged submarines remained a particularly elusive prey through to the end of the war. Yet, there was a significant legacy left by the First World War research, which would prove to be the foundation for anti-submarine warfare (ASW) in the Second World War. Prior to 1914, little thought had been given to the use of sound waves either for military or scientific purposes. Scientific research on the properties of underwater acoustics had been mainly confined to studies of the speed of sound in water. Most of the early research in ocean acoustics was undertaken by empirical inventors so common in the era of Bell and Edison. Their work focused on the commercial development of underwater signaling devices. Dwight R. Messimer, Find and Destroy, (Annapolis: Naval Institute Press, 2001). The Northern Mariner/Le marin du nord, XII, No. 1, (January 2002), 39-52. 39 24 40 The Northern Mariner/Le marin du nord The best known research was carried out by The Submarine Signal Comp any, which was founded by three inventors in 1901. The company developed a system of underwater bells placed at lighthouses and lightships which could be heard by ships equipped with underwater microphones or hydrophones over 15 kilometers away. According to Hackmann, "by 1912 this system was well established; bells had been installed along the coasts of major importance, and many trans-Atlantic ships were equipped with the receiving apparatus.2 There had also been some efforts made to utilize subsurface sound waves in collision avoidance systems, work that was given extra impetus by the sinking of the Titanic. The day after the Titanic went down, M.L. Fry Richardson took out a B ritish patent on one such device. Richardsons sound detector was quite sophisticated since it provided a means to accurately determine the direction of an object. Unfortunately, he relied on bells and whistles which produced a sound with too long a wavelength to produce sufficient reflection from a submerged object. In April 1914, Canadian electrical engineer Professor R.A. Fessenden tested a device used in the transmission of underwater morse code messages as an echo sounder. The sound was generated by `a reciprocating induction motor moving 540 times per second attached to a large steel diaphragm. It could detect an iceberg up to two miles away, but because it produced continuous oscillations rather then intermittent pulses of sound it tended to create its own interference.4 Most of the wartime research in underwater detection of submarines focussed on finding either a mechanical means, such as using underwater nets or grappling devices, or in perfecting passive listening devices, hydrophones.. A large number of hydrophones types were developed in Great Britain, the United States, and France. These included simple devices involving one or two microphones, to multiple arrays involving dozens of listening devices. Some of the devices were towed or mounted on towers fixed to the seabed in order to minimize interference. The best indicator of the overall performance of these devices is their comparative lack of operational success. Almost 10,000 hydrophones were delivered to the Royal Navy during the war. By October 1917, there were some 1,201 small RN ASW vessels equipped with the devices, a number that would grew dramatically in the last year of the war. Added to this must be the large number of American and French warships assigned to ASW duties. Given the considerable numbers involved, surprisingly few U-Boats were sunk by attacks initiated or completed through passive acoustical listening. Only 3 or 4 German submarines were sunk in actions in which ship-borne hydrophone detection played a role. One, or possibly two, other U-boats were sunk by mines after being detected by a fixed mounted harbour defence hydrophones. This compares unfavourably to the number of U-boats sunk Hackmann, 6. Interallied Conference on Submarine Detection by Means of Supersonics: An Account of the Research Work Carried out in France, October 1918, National Archives RG 189 Records of the National Academy of Sciences Research Information Office, Entry 2, Pa ris Office Box 11/161. Hackmann, 6. 6 Langevin and the Discovery of Active Sonar 41 by another new military technology, aircraft, which sent at least nine submarines to the bottom. The passive listening systems suffered from a lack of tactical flexibility, particularly since most devices required ships and all nearby vessels to shut off their engines and drift while listening. While good directional sense was developed, dist ance could generally only be estimated by triangulation using more then one ASW vessel or by a rough measure of sound intensity. Allied navies were also convinced that German tactics, which include sitting on the bottom or moving off on the surface at high speed, also greatly reduced the number of successful hunts. "The contribution of hydrophones" to the anti-submarine campaign, concluded Hezlett, "cannot be classified as higher than a useful bonus."5 The comparative lack of success of hydrophones fuelled interest in developing other methods for detecting submarines. Right from the start of the war, some inventors proposed the development of active acoustical systems based on submarine signalling or navigation devices. In active sonar, a pulse of high frequency sound is transmitted through the water. This sound wave is reflected from a submerged target back to the transmitting vessel. It was well understood from prewar work on collision avoidance systems that the use of a narrow sound beam on a trainable transmitter would give the direction. The length of time necessary for the reflection to return to the transmitting ship would provides an indication of range. The main technical difficulty was in developing a way to produce a powerful acoustical signal of the right wavelength. Serious research on underwater supersonics began in February 1915, when M. Constantin Chilowsky, a Russian inventor, submitted to the French government a plan to detect submarines by transmitting through the water "elastic high frequency waves by transforming the electric oscillations of high frequency, commonly used in wireless telegraphy." Chilowsky proposed to do this by using the medium of magnetic attraction produced by an electric current "acting synchronously on all points of the internal face of a plate of soft iron, finely laminated."6 Chilowskys proposal was forwarded by the French government to Professor Paul Langevin, an early supporter of the theory of relativity and an expert on paramagnetism, diamagnetism, secondary x-rays, and the behaviour of ions. Langevin concluded that Chilowskys basic idea had merit, but that his means to produce a suitable sound wave was unlikely to succeed. Langevin decided to begin research into developing a practical means to create an intense pulse of high frequency sound. He asked Chilowsky to join him and a small team of scientists working at his laboratory at the School of Physics and Chemistry in Paris. By April 1916 results were so promising that the French Navy transferred their work to 5 Arthur Hezlett, The Electron and Sea Power (London: Peter Davies, 1975) 151-2; Hackmann, 48, 64-71. Interallied Conference on Submarine Detection by Means of Supersonics: An Account of Research Work Carried out In France, 31 October 1918, NA RG 189 Records of the National Academy of Sciences, Research Information Office, Entry 2, Paris Office Box 11/161. Hackmann, 78. 8 42 The Northern Mariner/Le marin du nord Toulon so that experiments could be undertaken at sea. In January 1917, a clear echo was detected for the first time using a cumbersome apparatus that relied on the electrical excitation of mica for transmissions and microphones for reception. Langevin realized that this method offered neither the range nor the reliability required for a submarine detector. hi February 1917, after numerous experiments with various apparatus and mediums, he turned to utilizing the piezo-electric properties of qua rtz to act as a receiver. The quartz would transform sound waves into electrical oscillations which could be amplified and played into a headphone. After developing the receiver, Langevin realized that the dual nature of the piezo-electric effect would allow quartz to be used as a transmitter as well, since when the crystal was fed an electrical oscillation, supersonic elastic sound waves would be produced. The French scientists developed a transmitter/receiver which consisted of a sheet of quartz about four millimetres thick, composed of pieces in a mosaic, placed parallel to each other and perpendicular to an electric arc.