There are two ways to do this demonstration. One is using a high-voltage power supply of around 120 volts connected to a 10-20 thousand micro-farad capacitor. The power supply is used to charge the capacitor. When the capacitor is fully charged, a light bulb is placed across it. The bulb starts out bright, and then dims. The other way to do this is to use low-voltage light bulbs such as flashlight bulbs and a one-farad capacitor. This demo shows the RC time constant and exponential discharging of a capacitor visually, since the current from the capacitor can be seen from the brightness of the lightbulb.