Randall and Boot have they actually invented the cavity resonator construction?

A historical contribution


The reason for this contribution is the most lukewarm response I got from e-mail addressees, after they received most rare patent document copies. I truly believe, that writing history needs the objective of being sensible for hard and true facts. Sadly, this perception is not always appreciated! 


Digesting some recent discussions (April/May 2010), I got the quite strong impression that some do not know the state of affairs on magnetron developments just before the war started. Thus before Randall and Boot’s break-troughs.


Attached you find three patents: DE748551, DE763494 and DE938196 


Both first two patents most of you will never have seen before. These actually are copies of the original patent application documents. Regarding their serial number, they have been granted during the war, though had been confiscated by the US Patent Office and which returned about late 1970s or early 1980s to the German Patent Office again. After Espacenet started with bringing the European patents on the web, these patents were left out by the Germans. After their major job had been completed, the Germans have also put these “Not Available” or lost documents on their “Depatisnet” site only.


This message was quite often shown on the German patent microfilms at the British Patent Office (about the 700,000 series). Later on step by step patents became available on the world wide web 

However, these ominous patent documents cannot be traced by regular search systems yet, as they never have been registered appropriately (being classified). I have searched for more than a decade (actually thirteen years) in the British Patent Office and got always the answer that these so-called lost patents were being victim of Allied bombardments. I always wondered how smart Allied bombs must have been, that mostly only the interesting patents were “not available”, thus being lost. In springtime 2006 the computer system at BL in London was not working well and they advised me to log-on to the German Depatisnet system instead. And Eureka! (At BL they were also astonished!)     How could I know about patent numbers that did not exist?

These kinds of volumes are a most valuable source of information. I have read thoroughly all relevant volumes. Of course not the ones dealing with, for instance, mining, heavy industry, chemistry, etc!

It was Tom Going, he is the one who brought me in the patent research in 1995. He showed me these so-called “blue volumes”. These volumes give abridgements of all German patent applications between about 1930-1945, which were made on behalf of British and US authorities between say 1946-49. Most are showing the application reference numbers only, though, some are also providing a patent number. Other references are indicating that it was an additional claim to an already existing patent (Zusatz …).

We have heard during the CAVMAG Conference of encountered problems of back-heating.  

This problem is the subject of  DE748551, date of application 24 November 1939!

DE763494 is more or less identical to DE938197. It is, however, from the historical point striking as it shows clearly the cavity construction (figure 1) as its application date was 15 November 1938. This file was also kept from being published by US Patent Office for decades.

For me rather curious is, that Dr. Karl Fritz did refer in this paper presentation during the German Magnetron Conference held March 1944 to Samuel’s early cavity patent application of 1934 and to the Russian 4 cavity magnetron construction, which was published in 1940, but not to Engbert’s magnetron construction, although both were engaged at Telefunken. Please regard the photo shown in Karl Fritz contribution.

There has to be also an US application of this patent of 1938, but I lost the document, so cannot give the actual reference yet.

One point is clear, the construction and principle of cavity magnetron resonators was quite well known and probably understood as well, before Boot and Randall considered their constructions.

Also some of its implications was thus pre-war knowledge.

It might, however, also indicate that theoretically it was understood, but likely not being thoroughly investigated. It was, quite common to take as soon as possible a patent claim. Though, materialisation was left for a later occasion. Their claims did not mention high power application. However, this phenomenon was also not expected directly by the Birmingham crew, as their first sample destroyed itself by providing far more energy than was expected (predicted).


That some of the German magnetron related patents, among many others, was confiscated might indicate that by doing so future patent objections was not to be feared! They simply did not exist, as documents were being kept closed. For some time regarded being a “Trophy of War”.

Please, also try to look at the coming into being of a patent document. For instance, DE748551 where one of Engbert’s constructions is being rejected.

After digesting the content of this web page, I have come to the, maybe harsh, conclusion that the subject of the recent CAVMAG2010 Conference should not have been on: 70 Years of Cavity Magnetron, though, more relevantly on "70 Years of Oxide Cathode in Cavity Magnetron". As it was the application of oxide cathodes that allowed high output power, without an oxide cathode they never would have obtained it. It was Gutton who brought this knowledge to Megaw and others. And, it was Megaw who finally transformed the rudimentary Birmingham device into a versatile, call it decisive, device. The claim that Randall and Boot have invented the cavity resonator magnetron must be regarded being simply nonsense!  This technique was, as we have seen, known before the war. Maybe not always focussing on high output power, though it existed! It was the combination of oxide cathode in conjunction with cavity resonators that made the break-through which allowed highly effective cm radars. Certainly a war winning device!

To be honest - one very significant point of the Randall and Boot group was that they introduced the only appropriate way of picking up the generated EM force by means of an inductive loop placed inside the output cavity. The Lorenz cavity magnetron as well as the Japanese cavity power magnetron of the 1930s were only having electrical pickup stubs. Which is very ineffective in an inductive environment!


I hope you can enjoy this digging into historical aspects as I do.


Arthur O. Bauer


Please consider also the German wartime magnetron conference papers: Breslau 1944

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