N pole (red), S pole (blue)Coil ABCD on the axle, slip rings R1 R2, brushes B1 B2Centre-zero galvanometer, 1 division = 0.5 mATurn the crank and the coil's sides AB and CD cut the field lines between the poles: one goes up while the other goes down, so a current flows round the coil, out through the slip rings and brushes to the galvanometer. Every half turn the sides swap and the current reverses: alternating current, e = NBAω sin θ. It is zero when the coil stands across the field and biggest when it lies level. Faster, more turns or stronger magnets: a bigger emf. A split ring turns it into direct current.