Ed

I agree with Curt and Erik. The force exerted by a magnet will
increase with the thickness of the metal it's attracted to, up to a
point. Beyond that more metal doesn't help. Making the metal bar
longer might help to reduce the distance that the returning magnetic
field lines need to travel, or it might not do anything depending on
how the magnets are configured.

We have some magnets at work (McMaster part number 5848K93 i believe).
By accident, two of them came together and it took two of us pulling
as hard as we could to get them apart. We made this mistake only once
and now store them separate wooden boxes. If you put one in your fist,
small pieces of metal will leap off a table a few inches away, but it
takes little force to remove the metal from the back of your hand. The
point being, it doesn't take much of an air gap to reduce the force a
magnet exerts.

An electromagnet allows for very high levels of force with the ability
to turn them on or off, but they aint cheap.

Thanks,
Chris