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).
The explosion created a percussion that broke the windows.
On the plate of a gold-leaf electroscope place a metal canister having a loose lid.
Next let the canister be touched with the finger, the leaves collapse, but diverge again when the ball is withdrawn.
If before the ball is withdrawn, after touching the outside of the canister with the finger, the ball is tilted over to make it touch the inside of the canister, then on withdrawing it the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.
Replace the ball again and touch the outside of the canister; the leaves will collapse.
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.
This was followed by two whistling sounds of canister shot, one after another.
A test will show that in this last case the canister is left negatively electrified.
Hence when the ball is touched against the inside of the canister before withdrawing it a second time, the fact that the system is found subsequently to be completely discharged proves that the charge - Q induced on the inside of the canister must be exactly equal to the charge +Q on the ball, and also that the inducing action of the charge -{-Q on the ball created equal quantities of electricity of opposite sign, one drawn to the inside and the other repelled to the outside of the canister.
