In 1940, at the University of Birmingham in the UK, John Randall and Harry Boot produced a working prototype of a cavity magnetron that produced about 400 W. Within a week this had improved to 1 kW, and within the next few months, with the addition of water cooling and many detail changes, this had improved to 10 and then 25 kW. To deal with its drifting frequency, they sampled the output signal and synchronized their receiver to whatever frequency was actually being generated. In 1941, the problem of frequency instability was solved by James Sayers coupling ("strapping") alternate cavities within the magnetron which reduced the instability by a factor of 5–6. (For an overview of early magnetron designs, including that of Boot and Randall, see ) According to Andy Manning from the RAF Air Defence Radar Museum, Randall and Boot's discovery was "a massive, massive breakthrough" and "deemed by many, even now, to be the most important invention that came out of the Second World War", while professor of military history at the University of Victoria in British Columbia, David Zimmerman, states: