A wheel is spun clockwise in a vertical plane at a constant rate. At the place where the experiment is performed, the Earth’s magnetic field is in a horizontal direction. It acts into and perpendicular to the paper. The rim is the outside of the wheel, and the hub is the centre. A constant e.m.f. is induced across the length of each spoke. Label the hub and rim either plus or minus to show the polarity of the e.m.f. Using Flemings Right Hand rule, thumb points to centre, index finger into page, so induced emf is clockwise. How do I use this to determine whether the rim is positive or negative, and then for the hub?