Science Puzzle
How Does a Lightning Rod Protect a Building?
Tall buildings and church spires carry a pointed metal rod on top, joined to a thick strip of copper that runs down the outside of the building and into the ground. Before they were invented, church towers were regularly wrecked by lightning: stone burst apart and wooden roofs went up in flames.
How does a lightning rod protect a building?
The Answer
It gives the lightning an easy path to the ground that goes round the outside of the building, instead of through it.
A lightning bolt is an enormous electric current, tens of thousands of amps, flowing between a charged cloud and the ground for a fraction of a second. It tends to strike the highest points. Stone, brick and wood conduct electricity badly, so a current forced through them heats them violently. Water trapped in the stone turns instantly to steam and blasts it apart, and dry wood catches fire.
Copper conducts electricity extremely well. A strike that hits the rod flows down the thick copper strip and spreads out into the ground through a buried metal plate or rods, without heating anything much on the way. The rod doesn't attract lightning from miles around; it just makes sure that a strike on the building goes where it can do no harm.
Benjamin Franklin, who showed that lightning is electricity, invented the lightning rod in the early 1750s. The bell tower of St Mark's in Venice had been damaged by lightning again and again over the centuries. A rod was finally fitted in 1766, and lightning stopped wrecking it.
The principle: A safe path to earth. Electric current follows the path that conducts best, so a metal rod and a copper strip carry a lightning strike down the outside of a building instead of through its walls.