Exploding Wire
  • Detail of Exploding Wire
    Detail of Exploding Wire

This display demonstrates what happens when the electrical energy stored in a charged 500 mf capacitor heats, melts, and vaporizes a piece of 0.006 inch (#32 AWG) nichrome wire with a cold resistance of 3 W.

Instructions

Press the button in the lower left labeled ADVANCE until the wire advance mechanism starts. Also, press the RUN button in the upper right to reset the oscilloscope.

When the wire has finished loading, a red light will go on. Press the CHARGE button to charge the capacitor to the desired voltage. (900 volts maximum.)

Press the DISCHARGE button, and be prepared for a small explosion!

Explanation

The amount of energy stored in the capacitor will determine the intensity of the explosion you observe. At less than 400 volts, the wire will glow red briefly and may possibly burn out in one location, usually at a contact point. Between 400 and 600 volts, the wire will melt and burn out, forming molten droplets of metal.

When the capacitor is charged to around 800 volts, the capacitor is storing approximately 160 joules, and the average power is approximately 320 kilowatts. During the first 400 microseconds, the wire will melt and form molten droplets. The droplets separate from one another, and the current cannot flow between them. This is indicated by the brief dip in the current curve on the oscilloscope. However, the hot droplets vaporize, and the vapor becomes ionized in the electric field still present. The ionized vapor has a lower resistance (and hence better conductivity) than the solid wire, leading to a sudden spike in the current and a corresponding drop in the voltage. As the vapor expands and cools, the current drops off to zero.

In effect, you are observing a miniature flash of lightning in the box, since lightning also occurs from the process of electrical conduction through ionized gases and vapors.