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.
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
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
have heard during the CAVMAG Conference of encountered problems
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.
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.
There has to be also an
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
Also some of its implications was thus pre-war knowledge.
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
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!
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!
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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