What it shows:
A 2000 μF capacitor is discharged by a carbon steel wire. The surge of current literally vaporizes the wire and it explodes into a spectacular arc of sparks that span the front of the lecture hall.
How it works:
The wire (aka "music wire") is 0.051" diameter, spring temper, and phosphate coated (to reduce corrosion). It is cut to a length of 1 meter (2/3 meter when performed in a small lecture hall).
The capacitor is a bank of five 400 μF capacitors wired up in parallel to give a total of 2000 μF. In a large lecture hall, this capacitor bank is charged to 3.4 kV, resulting in about 12 kJ of stored electrical energy. In a small lecture hall we charge to 2.7 kV, to reduce the stored energy to about 7.3 kJ.
Setting it up:
This demonstration requires space, time, and care to set up and perform. The lab benches should be moved out of the way so that the apparatus can be set up directly in front of the blackboard and as far away from the audience as possible. The capacitor bank is very heavy and takes extra time to wheel in from the Prep Room. Charging takes about 5 minutes. To avoid injury from flying debris the first few rows should be evacuated and flammable material should be cleared from the area. For maximum effect turn the hall lights down. Although the wire dissipates most of the energy stored in the capacitors, a dangerous amount of voltage usually remains across them and time and care is required to safely discharge them.