AI got a warm reception from Air Chief Marshal Hugh Dowding, and even more so from Churchill. This proved problematic. Its accuracy, dependent on the height of the aircraft, meant that CH, capable of only , was not accurate enough to place an aircraft within its detection range, and an additional system was required. Its wooden chassis had a disturbing tendency to catch fire (even with attention from expert technicians), so much so that Dowding, when told that Watson-Watt could provide hundreds of sets, demanded "ten that work". The Cossor and MetroVick sets were overweight for aircraft use and the RAF lacked night fighter pilots, observers, and suitable aircraft. In 1940, John Randall and Harry Boot developed the cavity magnetron, which made ten-centimetre ( wavelength ) radar a reality. This device, the size of a small dinner plate, could be carried easily on aircraft and the short wavelength meant the antenna would also be small and hence suitable for mounting on aircraft. The short wavelength and high power made it very effective at spotting submarines from the air. To aid Chain Home in making height calculations, at Dowding's request, the Electrical Calculator Type Q (commonly called the "Fruit Machine") was introduced in 1940. The solution to night intercepts would be provided by Dr. W. B. "Ben" Lewis, who proposed a new, more accurate ground control display, the Plan Position Indicator (PPI), a new Ground-Controlled Interception (GCI) radar, and reliable AI radar. The AI sets would ultimately be built by EMI. GCI was unquestionably delayed by Watson-Watt's opposition to it and his belief that CH was sufficient, as well as by Bowen's preference for using ASV for navigation, despite Bomber Command disclaiming a need for it, and by Tizard's reliance on the faulty Silhouette system. Air Ministry In March 1936, the work at Orfordness was moved to Bawdsey Manor, nearby on the mainland. Until this time, the work had officially still been under the DSIR, but was now transferred to the Air Ministry. At the new Bawdsey Research Station, the Chain Home (CH) equipment was assembled as a prototype. There were equipment problems when the Royal Air Force (RAF) first exercised the prototype station in September 1936. These were cleared by the next April, and the Air Ministry started plans for a larger network of stations. Initial hardware at CH stations was as follows: The transmitter operated on four pre-selected frequencies