Coil about 30 turns of a thin copper wire around a glass tank (like the ones used for aquariums) and connect t

Coil about 30 turns of a thin copper wire around a glass tank (like the ones used for aquariums) and connect the ends to four 1.5-V dry cells arranged in series. Magnetize eight needles and thrust each needle horizontally through thin wafers of cork. Float the corks one at a time in the water tank. Explain the pattern produced.