What should be the resistivity of materials used for electromagnets,and why?

$A$ metal detector works on the principle of:

Eddy currents are produced when

$A$ bar magnet falls from a height $h$ through a metal pipe. Its acceleration after coming out of the pipe is:

$A$ copper ring is held horizontally and a bar magnet is dropped through the ring with its length along the axis of the ring. The acceleration of the falling magnet while it is passing through the ring is

$A$ magnet is dropped down an infinitely long vertical copper tube.