箔検電器です。

左上に電荷のソースがあります。赤が正、青が負です。
そのとなりの棒が帯電棒で、マウスドラッグで動かします。
左下から出ている棒がアース電極です、マウスドラッグで左右にのみ動かせます。

帯電棒をドラッグして電荷ソースに触れると黒の電荷ゼロ状態から色が変わり、棒が帯電します。
これをそのまま検電器に近づけると、分極が起こり箔が開きます。
そのまま電極に棒を接触させると、検電器に電荷が移ります。
帯電棒の電荷を消したいときは、棒をアース電極に接触させてください。
検電器の電荷を消したいときは、アース電極をドラッグして検電器の電極に触れてください。

応用として、「帯電棒を近づけて検電器を分極させた状態でアースを行い、棒とは逆符号で検電器を帯電させる」、
「帯電した検電器と逆符号の帯電棒を近づけて、開いていた箔を閉じる」、
「同符号の帯電棒を近づけてさらに開く」などの動作を行うことができます。

There are two sources of charges on the upper left. The red/blue square indicates positive/negative charge.
There is a black bar beside the sources which can be electrified by touching the sources, and it can be moved by dragging it.
There is another black bar on the bottom of the left which represents the ground (earth).
The ground bar can be also moved by dragging it.

When you contact the bar on the source, the bar will be charged and the color will be changed either red or blue.
As the charged bar approaches to the plate of the detector, the foil will open according to its polarization.
The closer the charged bar approaches, the more polarization occurs.
Once the charged bar contact with the plate, the charge will transfer to the plate.
When you want to eliminate the charges, let the ground bar contact to the charged bar or the plate of detector.

Try these experiments and think why these phenomenons occur.
1. Put the charged bar close to the plate (not contacted) and then ground the plate. The foil will be charged which has opposite sign to the charged bar.
2. Electrify the foil, and let the bar be charged in an opposite sign to the foil. As the charged bar approaches to the plate, the foil will close.
3. Electrify the foil and the bar in same sign. As the charged bar approaches to the plate, the foil will open further.