In lecture 9 of the second volume of the Feynman Lectures on physics, it is written... How can we measure [the field in the air above the ground] if the field is changed by putting something there? There are several ways. One way is to place an insulated conductor at some distance above the ground and leave it there until it is at the same potential as the air. If we leave it there long enough, the very small conductivity in the air will let the charges leak off (or onto) the conductor until it comes to the potential at its level. Then bring it back to the ground, and measure the shift of its potential as we do so. I don't understand the procedure he describes. Firstly, what is an insulated conductor? The continues to describe another method: A faster way is to let the conductor be a bucket of water with a small leak. As the water drops out, it carries away any excess charges and the bucket will approach the same potential as the air. (The charges as you know, reside on the surface, and as the drops come off "pieces of surface" break off.) We can measure the potential of the bucket with an electrometer. What "excess charges" is he talking about"? Will the bucket become progressively electrically charged as the water escape?