Adjustable Capacitor with Dielectric

MIT Physics Demo

First, an adjustable parallel-plate capacitor is held at a constant voltage. As the separation between the plates is widened, the electrometer shows charge (or current) flowing off of the plates, while the electroscope shows no change in voltage. 

Notice (by the deflection of the needle) that more charge flows off of the plates when they are closer together, and less when further apart. When the plates are brought back together, we see charge flowing back onto of the plates. Next, we deposit a fixed amount of charge onto the left plate. 

Now, when the separation is widened, the electroscope shows a rising voltage, even as the amount of charge stays constant. When a block of plexiglass (a dielectric) is inserted between the plates, the voltage drops. However, when the plexi is removed, the voltage rises back up again, showing that the charge is still there. This is one of the main reasons capacitors are built with dielectrics between their plates, more charge can be stored at a lower voltage.

Source: MIT Department of Physics