Next let the canister be touched with the finger, the leaves collapse, but diverge again when the ball is withdrawn.
Even if a charged and insulated conductor, such as an open canister or deep cup, is not perfectly closed, it will be found that a proof-plane consisting of a small disk of gilt paper carried at the end of a rod of gum-lac will not bring away any charge if applied to the deep inside portions.
If, before withdrawing the ball, after touching the outside of the canister for a moment the ball is touched against the inside of the canister, then on withdrawing it the ball and canister are found to be discharged.
A test will show that in this last case the canister is left negatively electrified.
On the plate of a gold-leaf electroscope place a metal canister having a loose lid.
This was followed by two whistling sounds of canister shot, one after another.
Let the canister be touched with the finger to discharge it perfectly.
If before the ball is withdrawn, after touching the outside of the canister with the finger, the ball is tilted over to make it touch the inside of the canister, then on withdrawing it the canister and ball are found to be perfectly discharged.
On touching the canister this last charge goes to earth.
Let a metal ball be suspended by a silk thread, and the canister lid so fixed to the thread that when the lid is in place the ball hangs in the centre of the canister.