How can I effectively increase the distance in which some electromagnet can attract a coin (for the sake of simplicity - coin).
Now to show that I've done my homework: According to Ampere's Law, the strength of the magnetic field is directly proportional to the number of coils. Therefore, more coils (of some sort) right? How about pressing the coils by virtue of hydraulic press? Wouldn't that allow me to coil more wire at less space? The denser the coil is the better eh?
Also the law tells us that more current will also increase the power of the electromagnet. Well I have 4 3000 mAh 20A/3.6V li-ion LG batteries, are they sufficient? How about using OP-amps? I am not afraid of draining the batteries down quickly.
Best answer will explain why electromagnets loose strength over distance by the cube of the radius distance from the magnet (as corrected by @Trevor), ways to increase the distance, possibly up to the length of a half hand and explain how to measure what can one achieve, given number of coils, current, coil density etc...