"Tim Williams" wrote: (clip) Bit of trivia: that's exactly how they do
tachs and speedometers. The needle is spring-loaded and has a copper disc
on it; a spinning magnet (which is feed by whatever shaft) transmits torque
to the spring based on RPM, thereby causing the needle to deflect (based on
the spring constant).
^^^^^^^^^^^^
Once the needle reaches the balance point, the angular velocity is zero, so
the efficiency drops to zero. But, since the device is transmitting
information to the driver, the power-in is being converted to
intelligence--it ought to be possible to derive a conversion factor for
energy to information G.