"_" wrote in message
...
I'm thinking of how to make a small clutch and was wondering if putting a
sheet of aluminium between a rotating magnet and a steel plate would
decrease the distance they would need to be separated for the same drag.

There is no way to stop a magnetic field. You can alter the flux path with
a ferrous metal however putting aluminum between a steel plate will do
nothing to increase or decrease the strength. Magnets lose their
attraction at 1/D squared.. in other words if we double the distance from
the magnet it will only have 1/4 the attraction
For many years Eddy Current clutches were used to do what you describe .