JONNY PHILLIPS: Magnets.  A lot of fun but pretty harmless.  Or are they?  Could it be possible for this small bar of metal to wipe the memory off this laptop, destroying my life's work?

The theory is that if you leave your laptop near a sufficiently big magnet it will erase or scramble everything on its hard drive.  To understand how this could happen, we need to go back to basics.

Hard drives store information magnetically.

You can see how this magnet can change the directions of these iron filings.

Well, a hard drive works in a similar way.  The disc is coated with a magnetically sensitive material that can be re-orientated by this read-write head to form numerous patterns the machine interprets as information.

This information can be added to or deleted in the form of programs, text and images. So if it's susceptible to an internal magnetic source, surely it should be affected by an external one? But just how powerful does the magnet have to be?

This is a kid's toy.  Its two small elliptical magnets that rattle together.

Noisy. But not very powerful.  The question is, would they bother our machine?

The effect - if there's to be one - will be instantaneous.

That doesn't seem to be doing anything.  This is roughly where the hard drive is ... so, no.  This obviously isn't working.  We're gonna have to up the ante, and use a bigger magnet.

This is a gauss meter, which measures magnetic force.  It shows this big magnet is over three times more powerful than the toys. It can pick up several hundred paperclips without a sweat.  Let's give it a go.

So, no. Now I'm slightly relieved my data's still in tact, but in the interests of science, I'm gonna give the magnet a bit of a boost.

By wrapping a coil of wire around a magnet and passing a current through it, you create a more powerful electro-magnet.  In this case around 30% stronger than before.

But our little computer didn't even blink.  It was time to get really serious. This company builds massive electro-magnets - the type used to pick up cars in scrap yards.  Surprisingly, it's only twice as powerful as our small home made job, but can apply that force over a much larger area.

After 22 seconds of raw electro-magnetic force had it survived?

I powered it up and....nothing.  Was it really dead?  We had to be sure, so we had it checked out by a computer data retrieval company.

COMPUTER TECHNICIAN: This one's dead.

JONNY PHILLIPS:  By scrambling every particle on the surface of the disk, the big magnet had instantly wiped out every operating program, and every file stored on the computer.  There was literally no information left for it to read. But it had taken some fairly extreme measures to achieve it.

So it seems our magnetic data is safe from everyday life, just be sure to avoid any giant electro-magnets.