HI! I'M LOOKING FOR A ANTI-RECORDING DEVICE, WHICH WILL ASSURE ME THAT
NOBODY IS RECORDING ME DURING A CONVERSATION WITH ME. (I'M NOT TALKING
ABOUT A PHONE CONVERSATION!!!)

Ooh neat, this is high tech stuff.

For a diaphragm microphone, a changing magnetic field would be way too
electrically noisy for the recorder to function.  A tesla coil would
be ideal for this... or any other form of electromagnetic pulse. 
Unfortunately, diaphragm microphones are large (1" across -- and then
they're usually housed).

Another possibility is to saturate the environment with sound.  For
starters, playing loud music would overload the recorder, but not your
listener's ears.

The high-tech alternative is to use sounds that humans can't hear.  If
set up correctly, the recorder, which is usually sensitive to other
sounds, would remain overloaded.

An electret microphone, which is much more common on recorders, is
sensitive to high frequencies.  A frequency of 15-20KHz might be just
outside of your hearing, but if it was made loud enough, it could
overload the recording.

Still a theory though.

Just about anything you do is going to let the other party know that
you think you are being recorded.  Which tells me that you don't trust
them.  And they more than likely don't trust you.

So, you could always resort to pen and paper.  OH yeah, when you leave
- take the paper. :)

Modern telco switches allow 'the authorities' to listen in and record
whenever, quite undetectably.

Remember George Orwell?

Big Brother is watching you ...

And me ...

And all of us.

Can?t quite agree with purpleprogrammer optimism on magnetic field
sufficient to seriously disturb dynamic mikes. The field is pretty
tightly bound, often shielded too so you?d need an immense external
field. Not now used for surveillance anyway because of size and low
sensitivity.

Electrets do exhibit high bandwidth. Post amplification would be
bandwidth limited and automatically gain controlled (on any decent
surveillance system) but theoretically very high intensity ultrasonics
could saturate the internal FET amplifier. Dogs might not be too keen
though.

Crystal mikes used to be common in this sort of application but have
been superseded by electret.

Best