How the speed of disc rotation can be e decrease by changing the position of brake magnet in energy metre

 

in this video I will explain you
1. how is speed of disc rotation of energy metre can be controlled by changing the position of permanent magnet
2. how is speed of disc rotation vary with distance of of magnet from the spindle

numerical question- in an induction type metre everything else remain same if the radial distance of the brake magnet poles from the spindle is decreased by 10% the rotational speed of the disc will approximately