What is a Faraday Cage?

Here's the science behind Doctor Who's ghostbusting technology

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The Doctor ain’t afraid of no ghosts. But it never hurts to have a place to hide. 

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In Under the Lake, the Time Lord and friends avoid the ghosts by taking shelter in a sealed room that, apparently, acts as a ‘Faraday Cage’. But what is a Faraday Cage, and why would it protect you from sci-fi spirits? 

Here’s another question for you: what happens when an aeroplane is struck by lightning? 

To answer that, let’s talk about former president Ben Franklin. A polymath, Franklin was not only a statesman but a scientist and inventor (he came up with bifocal glasses).

One long standing investigation was into the nature of electricity and conductivity: his most famous (and foolhardy) experiment was attaching a key to a kite and flying it during a lightning storm. (Coincidentally, he also invented the lightning rod. Don’t try it at home kids.)

One of his less dangerous pastimes was running a charge through a tin can and waving a piece of cork around it. He found that while the cork was attracted to the outside of the can, it didn’t react when dangled through a gap in the top, or retain a charge when it touched the metal. An intriguing find, but Franklin was at a loss to explain it, writing “You require the reason; I do not know it. Perhaps you may discover it, and then you will be so good as to communicate it to me.” This was in 1755.

It took 81 years before British scientist Michael Faraday replicated the effects with an ice bucket and a brass ball, then scaled it up, inventing the first ‘Faraday Cage’ large enough to stand in. Made of conductive material, a Faraday Cage redistributes any electrical charge across its outer surface, leaving the interior untouched. (Technically the electrons within the cage rearrange themselves based on their polarity, cancelling out the external charge, but you don’t need to worry too much about it.)

The important thing to remember is that a sealed coating of conductive material (like, say, the skin of an aeroplane) will block even powerful electric charges. How powerful?

Pretty powerful. As Faraday Cages will also block electromagnetic radiation and radio signals, they have a variety of uses: for instance, creating sealed rooms to test electronics, avoiding eavesdropping, or turning them into a suit and having crazy wizard fights.

But why would a Faraday Cage work on ghosts? Presumably because they’re not ghosts at all, but some kind of sci-fi jiggery-pokery like a hologram.

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Still spooky though. Mega spooky.