Arc

crank up a storm. keep it off the roofs.

crank a Wimshurst machine to charge a model storm cloud, and slide a lightning rod so every spark hits it and not the model houses

Air is an insulator until the voltage across a gap gets too high; then it breaks down all at once and a spark carries the charge across. Between blunt ends at sea level that takes about 3 kV for every millimetre near a centimetre (a little more per millimetre for short gaps), so double the gap and you need about double the voltage. The spark takes whichever gap gives way first: the nearest one, or a sharp one, because the field crowds onto a point and a needle gap sparks at well under half the voltage a pair of balls needs. The card computes every number from two published curves for air: the uniform-field formula and the CRC Handbook needle-gap table. Thunder houses were real: in the 1770s lecturers blew model houses apart with a Leyden jar and a pinch of gunpowder to show what a lightning rod is for, and in 1777 Benjamin Wilson staged the quarrel over knobbed or pointed rods for George III. Real lightning is subtler than a bench: in a seven-year field test (Moore and colleagues, 2000) moderately blunt rods were struck and sharp ones were not.

Concepts: electricity.

An interactive you play with one thumb, on Wundr.