Ezra Cornell, of Ithaca, New York, asks :-

I learned in physics that electricity stays on the outer surface of a metallic shell. Does that mean when a Medieval knight in armour is hit by lightning he would be OK?

John Campbell, a physicist at the University of Canterbury, responded.

Probably. I dont know of any examples from the historical record, though I am sure there must be some, so I will have to stick to modern examples.

The most important observation of the forces between electrical charges is that like charges repel. In a metal it is the electrons that are free to move. They repel each other so get as far away from each other as possible, ie to the outer surface of a metallic shell.

You can observe this repulsive force yourself by electrically charging your freshly washed and dried hair with a nylon comb. The hairs stand up because they repel each other. Or rub the comb on wool then hold it above very small pieces of paper. A piece of paper will first jump up to the comb then be repelled once it acquires the same type of charge the comb has.

So in the knight's suit of armour the current travels in the metallic shell, not through the human body. However, steel is a poor conductor so the large current flowing through a suit of armour will heat it, likely burning the knight should no clothes be between suit and body, and likely weld some flexable joints, which would place him at some disadvantage.

Some high voltage workers wear copper chain mail suits to help protect themselves. I recall various accidents to other high voltage workers which illustrate these points. A man working on a radio station transmitter managed to get his wedding ring finger between two contacts. The hot ring burnt off his finger but also cauterised the wound. It is one reason I dont wear jewelry. Careless power company workers have often survived shocks, but have charred flesh on the soles of their feet in spots where the nails in their boots were.

I used to give a lecture on "Death by Electric Shock", which captured the students attention like no other. In it I showed devices that had killed people and why. The interesting thing about electric shock is that it is the current that flows through the heart that does the damage. There is a peak amount that causes death. Above that people survive, but usually die about four days later of complications due to serious burns.