This is an activity about electromagnetism. Learners will use a simple circuit powered by a battery source to investigate the strength of the magnetic field produced by a coil of wire in the circuit. The strength will be indicated by the deflection of magnetic compass needles and by the distance a coil of wire was moved by the action of the circuit. This activity requires coils or spools of wire, a knife switch, three magnetic compasses, a source of electricity such as 3 D-cell batteries or an AC to DC power adapter, alligator-clipped wire, and a bar or cow magnet. This is the fourth lesson in the second session of the Exploring Magnetism teachers guide.