Well, last night’s thunderstorm was a bit of a feeble affair after the fireworks of Wednesday. There were a few flashes, the odd rumble, and a bit of rain, but it cleared away after an hour. Maybe somewhere else got the drenching this time. Still, it makes four days in a row of the same daily weather, almost hour-by-hour. And this morning….is shaping up the same way…
A student of mine sent me these photos of his television screen on Wednesday night (used with permission). There was a lightning strike close to his house, which seems to have magnetized the television. The picture is there, but the colour is wrong.
In an old CRT television, each point on the screen contains a phosphor – and there are three different colours, red, green and blue. To make up the colour picture, the phosphors are stimulated by the required amount, by a beam of electrons that hit the screen from behind. The beams are scanned across the screen using magnetic fields (charged particles like electrons are bent by electromagnetic fields), and, in order to hit the right phosphor, need to emerge in the exactly the right direction. What I think has happened in this case is that the lightning has permanently magnetized some part of the television. This is hardly surprising given that lightning carries a huge direct current, and consequently creates a nice magnetic field around it.
So as the electrons pass, they are bent slightly but consistently off course, and hit the wrong colour phosphor. Hence the image looks right, but just with the wrong colours. De-gaussing the TV removed the colour shift, putting it back how it should be.
Lindsay says:
I’m a little surprised that lightning could generate enough current within the set to magnetise the screen without frying the electronics. Still, lightning has done very many surprising things.
One part of the CRT which is quite easy to magnetise is the Shadow Mask, a perforated sheet of metal just behind the screen front, which prevents the electron beams from hitting the wrong colour phosphors.
I discovered this for myself when I was playing with a magnet in front of a friend’s somewhat expensive computer monitor and making strange colour effects. When I took the magnet away the effects stayed, which was a bit embarrassing until I did some heavy-duty degaussing to fix it, and before he spotted it.