How Stuff Works: Faraday Cage
If you know anything about lightning, you probably know that you're safe from lightning inside a car–at least as long as you aren't touching anything metal.
A lot of people believe that the car's rubber tires act as an insulator, and that this is the reason a car is safe during a lightning storm. This is a common myth; the truth is actually a little more complex.
Air is an insulator, which is why you can walk by a power outlet in the wall without getting shocked. (If air conducted electricity, we'd all be in a lot of trouble!) A lightning bolt can reach eight miles from the leading edge of a storm and hit someone standing under clear, sunny sky without a cloud in sight; if eight miles of an insulator like air won't stop a lightning bolt, a few inches of an insulator like rubber certainly won't. For this reason, you are not safe from lightning by wearing rubber-soled shoes, another common (and dangerous) myth about lightning.
The reason a car protects you from lightning is that the metal skin of the car acts like a Faraday cage. We briefly talked about Faraday cages in an earlier post, so now we're going to talk a little bit more about them.
Put most simply, a Faraday cage is any enclosure that's made of a conducting material such as metal. Electrical fields are conducted around the outside of a Faraday cage without penetrating the enclosure. Faraday cages can be made of metal mesh, or any other conducting material, and are very effective at preventing electrical and electromagnetic fields from penetrating them.
If you own a microwave oven, the inside of the oven is a Faraday cage designed to prevent microwave-frequency electromagnetic signals from escaping; that's why the window in the door has a metal mesh over it. In Japan, some movie theaters are paneled with wood panelling that has a fine copper mesh inlaid in the back; the copper mesh acts as a Faraday cage to block radio-frequency signals used by cellular telephones.
Faraday cages are named after physicist and chemist Michael Faraday (1791-1867), who was one of the principle discoverers of electromagnetism and who invented the first working electric motor. Faraday proposed that a conductive shell might block electromagnetic signals and protect the objects inside it from strong electrical fields; he was the first scientist to propose that electromagnetic fields extend from electrical conductors, and developed the forerunners of modern electrical transformers and generators. He was also the first scientist to propose that light and magnetism are related.
A car's outer surface is made of sheet metal; for that reason, a car acts as a crude Faraday cage, protecting its occupants from lightning by conducting the electrical discharge around its outer surface, away from the interior. Cars are not terribly efficient Faraday cages; the large glass windows offer a way for radio-frequency signals to enter the car, which is why your cell phone still works when you're inside. But for a huge electrical surge on the order of a lightning bolt, which can carry 30 kA or more of electrical current, a car works just fine.
Today, Faraday cages are so ubiquitous that there is even specialized clothing used in industrial applications that's designed to protect its wearer from high-strength electromagnetic fields. Workers on high-tension lines use this clothing to protect themselves from electrocution, as this interesting video shows: