Flashboard

This was a device to permit military observers to pinpoint an enemy gun location by observing gun flashes. With a single gun firing several observers spaced apart could easily take bearings on the gun flashes and determine its location by triangulation. However, with multiple batteries firing continuously it was very hard for separated observers to be sure they were observing the same gun flash.
The flash board located at an HQ comprised a buzzer and light for each observer. The observers each had morse keys connected by wire to their specific buzzer and light. When depressed the key would cause their specific light to flash and the buzzer to sound.
The operation was as follows:
Suppose there are three or more observers observing 5 batteries. Observer B first sees a flash - he depresses his morse key and this sounds his buzzer and lights a light in the spotting HQ. This buzzer is also simultaneously heard by the other observers via the open microphone of the telephonists located at HQ.
The officer in charge of the HQ flash board will then instruct Observer B to lead and silences the other buzzers. He also plots the bearing of the gun from B on the plotting table and decides where the gun on this line is likely to be. He instructs each of the other observers where to look.
Every time B sees his particular gun flash again he depresses his key, his light is lit, and the buzzer sounds and is heard by the other observers over the telephone line. When another observer, say C hears B buzzer and sees a flash and he depresses his key to activate his light. Gradually all three observers will be flashing in time with B. At this point the Officer turns off B's buzzer. If all the other observers continue to flash in time with B then the Officer can be sure that all observers are seeing the same gun flash and the exact position can be computed by triangulation.

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