Flash Spotting and Sound Ranging in WW1



Artillery played a significant role during the First World War.  In general, the guns remained behind your lines and lobbed shells over your head to destroy the enemy and its defensive infrastructure.  Problem was that the enemy was doing the same.  The task was to seek out the location f the enemy guns and destroy them before they destroyed you.

I found this description of flash spotting and sound ranging in a military forum and, combined with the accompanying video, you can begin to understand the role that science played in warfare.

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Little science was used in WW1, the thermionic valve had been invented and its use in radio was just starting, mapping and survey were established but there was a barrier between military and scientific minds. The military felt that gadgets had no place in the field and the scientists could not understand why their ideas had no appeal to the military.

Sound ranging started in the French and German armies. The principle is simple, a series of listening posts in know positions and the time difference between the arrivals of sound at each is measured T1, T2 etc… Then draw a circle on the map about T2 with radius V(T2-T1) and the other posts. The great circle touching all circles is the gun report wave and the gun is at its centre.

The RE was impressed with French experiments, the RA was not but agreed that an officer not suitable for better could be employed. Brag then a 2 Lt with limited knowledge of horses was appointed. He found that French methods by which an operator pressed a key to record time of sound worked at close ranges but was too inaccurate for long ranges. This was changed to use ‘pens’ marking smoked paper to record the sound at a rear location connected to the microphones.

A further method used an Einthoven galvanometer in which six fine wires, located in a magnetic field, carried the currents from the microphones and the movement induced recorded by a cine film. By the end of 1914 experiments, by Lucien Bull and Charles Nordman, with three wires to record the signals from microphones 1 km apart was demonstrated as capable of locating a gun 4 km away to an accuracy of 5 m lateral and 25 m range. A portable 6 wire device was developed and came into service on the French front in Feb 1915. Photographic equipment and a dark room were needed, but it was capable of measuring time to less than 1/100 sec and at a later stage was used to time the passage of a shell through two screens 100 ft apart so as to calibrate guns. Another officer, H. Robinson, afterwards Professor of Physics and Vice Chancellor London University, was found. They went to Paris in Oct 1915 to take over a Bull apparatus and reported to Col Jack head of maps at GHQ St Omer to work on the BEF front.

The section was located on Kemmel Hill where they were joined by Cpl Tucker, sometime Physics Dept of Imperial College. For the first year, there were many problems due to the different characteristic of the sounds of different types of gun. Howitzers could be ranged under good conditions but not guns. Improvements were made making use of the shock effect of the shell and placing the microphones in heavy, ammunition, boxes. But the quantities of high-quality wire were attractive to other users. Buried wires were run over and developed leaks to earth. Wind interfered with the microphones until it was found that layers of camouflage netting steadied the flow of air.

Other improvements emerged such as the idea of putting the microphones at exactly equal intervals in a straight line which enabled easy distinguishing of friendly fire. Latter an arc of a circle with the centre behind the enemy lines in likely gun positions gave better results. These were placed by surveyors to an accuracy of 1 m. It was also possible to allow for the effects of wind and temperature by setting up a section behind the lines and setting off charges at about the range of enemy guns.

A German field order was published by Maps, GHQ for the benefit of Field Survey Companies ‘In consequence of the excellent sound ranging of the British. I forbid any battery to fire when the whole section is quiet, especially in an East wind. Should there be an occasion to fire the adjoining battery must be called on to fire a few rounds’

Similar systems were used in WW2 and it was found that microphones could be surveyed into position at the same time as the guns and proved to be very effective in a country where maps were inadequate.

Reference: Annex L (by Sir Lawrence Bragg) to General Fardales – History of the RA in the Great War