Given the same voltage and current, hence electrical power, what kind of core design can maximize the "reach" of an electromagnet? For example, with the magnet above a table, axis pointing down at the table, attempting to lift a small iron washer, is there a core design that significantly increases the distance the electromagnet can be raised and still lift the washer? Is just a cylindrical core best? Would a horseshoe core with a winding on each leg (though same electrical power as other cores) reach farther? Or could a conical core with the "down" end having a smaller face shape the magnetic lines of force to reach out farther, using the same electrical power as other cores?