Before RADAR was invented the armed forces of most nations needed a way of detecting approaching enemy aircraft before they got to the target area. The only way was to listen for them and since the human ear has it's limitations then the hearing had to be amplified. America, Britain, France and Germany all came up with their own versions of these devices during WWI and worked on improving their ability before WWII and the late 1930s invention of RADAR. Here are 12 different such devices used by those countries.
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