How do we know the charge of the electron? Here is a way to find out: imagine that you have made a uniform electric field in a region that points downwards(towards the ground), E=-Eˆz,where E is a constant. Spraying oil through an atomiser produces a cloud of tiny spherical droplets of oil, some of which will have a tiny charge (usually just a few extra electrons) due to friction with the atomiser. By adjusting the value of the field,E, you discover that some of the droplets can be madeto 'levitate'–they are suspended in the air without falling. What is happening?