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Mic Bugs CIA Easy Chair
Covert microphone
FM-5 was a miniature dynamic microphone,
developed and manufactured from 1954
to approx. 1965 by hearing aid manufacturer Fortiphone in London (UK). The
microphone was developed for their new generation of transistorized hearing
aids, but was also used for covert listening devices.
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Fortiphone was established in 1929 and had its headquarters at Langham House,
408 Regent Street 1 London (UK) [1]. Before WWII, Fortiphone sold
hearing aids manufactured by Siemens in Germany. These devices contained
valve-based amplifiers and were large and power-hungry.
During WWII, the Fortiphone factories continued to produce hearing aids,
but were also adapted for the production of tank radio equipment [2].
Once the war was over, the roles were reversed and Siemens sold
Fortiphone's miniaturized valve-based hearing aids from 1949 onwards.
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This situation continued until 1951, when Siemens introduced its own
Phonophor Alpha, and became Fortiphone's main competitor [3]. Fortiphone
on the other and had meanwhile started the development of an even smaller
device, based on the newly introduced transistor technology.
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Several new parts were developed especially for the next generations of
hearing aids,
including the dynamic miniature FM-5 microphone shown in the image above.
Although it may seem large by today's standards, with its 21 x 21 x 8 mm
it was one of the smallest microphones of its time.
Apart from microphones, Fortiphone engineers also developed a range
of miniature chokes and transformers that greatly reduced the size and
weight of their transistorized pocket amplifiers.
This technology was patented in the
UK in 1954, and in 1955 also as US Patent 2,916,713 [4].
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In 1955, the NRP managed to secure a contract with the CIA for development
of a range of so-called Passive Elements (PEs), the first of
which was the Easy Chair Mark I,
introduced later that year.
Being of a passive nature, the bug did not require a local power source,
but was instead powered by a strong RF signal beamed at it by a
powerful transmitter in a nearby listening post.
The image on the right shows three versions of the PEs used in the
Easy Chair Mark I in 1955
and the Easy Chair Mark II in 1956.
The transformers are clearly visible in the rightmost one.
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In the rightmost example above, the FM-5 microphone is integrated
with the device.
In 1958, whilst developing the
Easy Chair Mark III,
NRP engineers even managed to build the
entire PE inside the antenna,
which was possible because of the tiny Fortiphone transformers.
For use in bugs, the Fortiphone FM-4 microphone was succeeded in the
early 1960s by the Shure MC-11
and MC-30, and eventually by smaller
and more sensitive ones from Knowles,
like the BA-1501.
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After WWII, the address was 247 Regent Street, London (UK).
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- Grace's Guide, Fortiphone
Retrieved April 2017.
- Grace Doe (née Seager), Working for the War Effort
BBC, WW2 People's War, 30 September 2005.
- Siemens, Die Geschichte der Siemens-Hörsysteme, Ein Rückblick
CC 1622 02144.5. Siemens AG, Febrary 2014. Page 15.
- Denis Lionel Johnston, US Patent 2,916,713
Miniature Transformers and Chokes. Filed on behalf of Fortiphone Ltd.
15 March 14955, priority date 17 March 1954.
- E. Dunkin & D. Johnston,
Subminiature Transformers and their Application to Junction-Transistor Circuits
IEEE. IRE Transactions on Components Parts 3(1):30. 44 May 1955.
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© Crypto Museum. Created: Wednesday 05 April 2017. Last changed: Wednesday, 07 February 2018 - 08:30 CET.
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