Henri Busignies, working in the French division of the US's ITT Corporation. Their system motorized the search coil as well as a circular display card, which rotated in sync. A lamp on the display card was tied to the output of the goniometer, and flashed whenever it was in the right direction. When spinning quickly, about 120 RPM, the flashes merged into a single (wandering) dot that indicated the direction. The team destroyed all of their work in the French office and left France in 1940, just before Germany invaded, and continued the development in the US. Watson-Watt It had long been known that lightning emits radio signals. The signal is spread across many frequencies but is particularly strong in the longwave spectrum, which was one of the primary radio frequencies for long-range naval communications. Robert Watson-Watt had demonstrated that measurements of these radio signals could be used to track thunderstorms and provide useful long-range warning for pilots and ships. In some experiments he was able to detect thunderstorms over Africa, away. However, the lightning strikes lasted such a short time that traditional RDF systems using loop antennas could not determine the bearing before they vanished. All that could be determined was an average location that produced the best signal over a long period, incorporating the signal of many strikes. In 1916 Watt proposed that a cathode ray tube (CRT) could be used as an indicating element instead of mechanical systems, but did not have the ability to test this. Watt worked at the RAF's Met Office in Aldershot, but in 1924 they decided to return the location to use for the RAF. In July 1924 Watt moved to a new site at Ditton Park near Slough. This site already hosted the National Physical Laboratory (NPL) Radio Section research site. Watt was involved in the Atmospherics branch, making basic studies in the propagation of radio signals through the atmosphere, while the NPL were involved in field strength measurements in the field and direction finding investigations. NPL had two devices used in these studies that would prove critical to the development of huff-duff, an Adcock antenna and a modern oscilloscope. The Adcock antenna is an arrangement of four monopole masts that act as two virtual loop antennas arranged at right angles. By comparing the signals received on the two virtual loops, the direction to the signal can be determined using existing