Microwave ovens have little in common with the above engraving from a sixteenth century bible. They do, however, contain parts which enable the manufacture of a classic electrical device. A Jacob`s Ladder.

Useless & Extremely Dangerous, Jacob`s Ladders are seen on screen, whenever an evil laboratory scene is required.

Either an arc or a high frequency spark discharge can be used. Two nearly vertical metal rods are arranged so that they are closer at the bottom than the top. High voltage is applied to the rods, a discharge starts at the bottom. The spark or arc heats up the air surrounding the discharge channel, and convection moves the discharge up the rods. At the top the discharge bows upward until it cannot be sustained. A new discharge then begins at the bottom.

The dangers inherent in the operation of a Jacob`s Ladder are :-

Electrocution, probably fatal Burns of a possibly horrific nature Poisoning of the lungs by oxides of nitrogen Possible destruction of property by burning Tripping of electrical supply probable Complaints from the neighbours

With these inconveniences in mind, a microwave oven was ripped apart and it`s 2500 volt transformer unleashed. Hastily wired up, more thrown together, the result is shown here. (NOT for those of a nervous disposition. Dr. Diode)

Another device which uses convection to move an arc has it`s electrodes diverging at a much greater degree than a Jacob`s Ladder (since it`s purpose is arc extinction): -

"A gap in an electrical circuit, of the form illustrated in the diagram. An arc, starting at the narrow part, is driven up the horns by convection and/or electromagnetic action, and is ultimately extinguished.

Gap between horns is set so that the normal voltage is just unable to cause a spark, whereas a surge is dissipated in the arrester and resistor, while the resistor also serves to limit current if an arc occures between live horns of adjacent gaps."