> >Back to reality, another way of obtaining galvanically isolated supply
> >is with piezoelectric 'transformers'.
>
> Let's not forget the dynamotor: an ac motor driving a DC generator.  For
> extra isolation, use a rubber belt to connect the two.
>
> And has anybody tried an AC fan driving a windmill connected to a DC
 generator?
>
> Or an AC powered Gerbil running in a wheel connected to a DC motor?

While your latter suggestions are getting a bit silly, the use of a motor/
generator setup was indeed common in some applications, especially before
the invention of good rectifiers.  In particular, many pipe organs circa
1950 or so used a motor/generator to produce the DC for the solenoids (the
same motor also provided the wind for the pipes).  Given that a decent-sized
motor was needed anyway (for the air supply) and that there weren't any good
technologies for producing 24v DC at moderate current, this was not a bad
method (interestingly, I was once practicing on such an organ when there was
a brief power failure.  The power supply switch is, I think, a latching relay
and so I was able to play through the power failure without interruption but
the organ quit about ten seconds after the power came back on!  It was only
then than I realized I had to push the "on" button again to restart the thing.