Lots of security agencies used wire recorders during spy activities. There was a brand in Austria creating extremely small types of wire recorders, called 'Miniphone'. Have a look at our website http://www.cryptomuseum.com/covert/minifon/mi51/index.htm to see one of those Miniphone recorders and also note the wrist-watch where a microphone is hidden inside. There was once a spy captured on an airport. Because he wore 2 watches, since this watch was not able to run on time (no watch mechanics inside this silly thing ;-)
Other examples are wire recorders to record in flight information. The Russians had a beautiful designed tiny wire recorder in their MIG airplanes. One advantage is that wire can withold high temperatures. the 'pack of wires' isolate those inside the 'pack', hence, big chance of signals recovery once a plane has been crashed...
Anyway, lots of excamples to be found and big fun to play with.
In the summer of 1953 I received a one month appointment as an Engineering Aid (GS-4, $3175.00 per annum) at one of the three Signal Corps Engineering Labs at Fort Monmouth, NJ following Signal Corps ROTC Summer Camp at Camp Gordon GA. My assignment was to run tests on a prototype wire recorder for field use built by General Electric. The wire was very springy like piano wire and had a tendancy to pop off the spools. If you had the missfortune to drop a spool, it took an hour or more to untangle and rewind the wire, not something that could be done in the field. The other problem I recall had to do with the variation in read signal as the wire twisted and the read head saw the side of the wire the record head had seen and then the other side.
A tape recorder was being evaluate at the same time and was vastly superior. In fact the wire recorder had already been rejected. In retrospect that may be reason I was given this task. Nevertheless it was a good experiance.
I messed around with primitive PWM amplifiers a little in the 80s, and always wanted to - but never got around to - replace the conventional HF bias + audio drive scheme in my home-made cassette deck with a straight square wave drive width-modulated with the audio.
In those days you had to roll your own, but these days there are some - already long in the tooth - PWM power amplifier configurations that can run at around 50kHz. Stick the audio signal into that and drive the resulting current into the head - should produce a myuch broader dip in the distortion curve and be less critical of bias level versus metal formulation. Simpler circuitry too - no separate bias power amp and record driver.
When (not if, Max!) this gets published in EET, I'd like to get hold of a table of the frequency response data, pre-equalization. I'm the first person to get excited about doing special filters, but this is a case where a digitally-implemented equalizer could probably do a super job. Implemented on the replay side, you could use a reel of calibration wire to give standardized excitation, and then everyone's wire recorder can be calibrated to have a super-flat frequency response! If that's what floats your boat, anyway.
@DrZuhoch: I still have one in my shop somewhere...I'll see if I can find it and send pictures.
Please do send the pictures to me at email@example.com and maybe we can do a follow-up column on this. I'd also love to see some pictures of a wire recorder being used as a form of computer memory as mentioned elsewhere in ths icomment thread.