Charging an electroscope by induction.

Charging an electroscope by induction. A charged plate (see D+10+18) is brought close to, but not touching the top plate of the electroscope. The metal pointer deflects. Remove the plate, and the pointer returns to its discharged position. The charged plate displaces free electrons in the electroscope. If the plate is positive, electrons are temporarily drawn from the pointer into the top disk, and a positive charge temporarily results in the pointer, as long as the charged plate is in position. The top disk of the electroscope is touched by a finger. At the same time a charged plate is brought nearby (but not touching). The finger is withdrawn, then the charged plate is withdrawn. The electroscope will be left with a charge whose sign is opposite that of the charged plate. If the plate is positive, positive charges are repelled into the hand touching the top disk, and negative charges are drawn into the pointer and top disk. Removing the hand leaves the pointer negatively charged.