Eripuit Coelo fulmen, it was said of Benjamin Franklin: "He snatched the lightning from the sky". The phrase, taken from his epitaph, recalls his invention of the lightning rod some 250 years ago.
Lightning occurs following the build-up of a very large negative electrical charge near the bottom of a thundercloud, and a corresponding positive charge on the surface of the Earth below. When the resulting electrical tension is sufficiently great, a lightning stroke is initiated - essentially a brief surge of electric current through the atmosphere which neutralises the opposing charges. As we know, it can cause horrendous damage, and Franklin's device - a sharp rod fixed to the highest point of a building, and connected to earth by means of a stout strip of iron or copper - provides the easiest path in the vicinity for the surge of energy. Once the path is established, the electric current passes harmlessly to earth through metal strips, without any damage to the structure.
But the idea was not without its share of controversy. Franklin himself was of the view that a sharp or pointed rod mounted atop the building was the more effective. But another Benjamin - a well-known English scientist of the day called Benjamin Wilson - had strongly held opposing views; he declared that pointed rods attracted lightning to a building, and that rounded rods should be preferred.
The controversy coincided with Britain's troubles with her American colonies, and so the rights and wrongs of it became almost a question of political allegiance. Swayed by Wilson's advice, King George III ordered all the "points" to be removed from the Palace of St. James, and to be replaced by blunted rods.
Now Franklin's own experiments suggested that Wilson, in fact, had got it right - that a pointed rod did indeed attract the lightning. But to Franklin this was an advantage, provided the rod was properly earthed to allow the charge to drain harmlessly away. Attracted to the pointed rod, the lightning avoided other vulnerable structures in the neighbourhood.
More recently, however, an American researcher called Charles Moore decided to undertake further experiments on the relative merits of blunt and pointed rods. He mounted a large selection of them on a mountain in New Mexico, ranging from sharp points to blunt ones 50 millimetres in diameter. The results acquired over seven thunderstorm seasons showed that the rods struck most frequently had tips 19 millimetres, or about three-quarters of an inch, in diameter; the rounded rods obviously attracted the lightning away from the pointed rods, none of which suffered any hits.
So perhaps George III was not so mad after all.