Answer: For example, two sliding rheostat with the same length, one with a maximum resistance of 1000Ω and the other with a maximum resistance of 100Ω. If the two rheostat are connected in series, the former will be adjusted as coarse adjustment, and the speed adjustment of the latter will be fine adjustment. The reason is not difficult to understand: if the moving slide has the same length, the former will change more, and the background will change less resistance.