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.
Let the canister be touched with the finger to discharge it perfectly.
The explanation is as follows: the charge (-}- Q) of positive electricity on the ball creates by induction an equal charge (- Q) on the inside of the canister when placed in it, and repels to the exterior surface of the canister an equal charge (+ Q).
This was followed by two whistling sounds of canister shot, one after another.