In the old days when the TV station went off air at night there was a small white dot in the middle of the screen. What was the purpose of this?

gmackematix

I'm not a mechanical expert but I do know that a TV picture is formed by a cathode ray tube firing a beam of electrons at light-emitting phosphors on the screen. Two vertical metal plates lie either side of the screen. These have an alternating current running through them which creates a variable magntic field between them. This pulls the electron beam horizontally along the screen forwards and backwards. Meanwhile, two plates above and below the screen have a similar magnetic field between them that pulls the electron beam from top to bottom in the time it takes the beam to cross the screen horizontally 625 timee (405 in the US). This happens about 25 times a second to create a TV picture. If the electron beam is still on when the current to the electromagnetic plates has been switched off, the beam, unaffected by magnetism, will point at the middle of the screen. I take it this was the white dot of old. [Jun 30 04 7:25 PM] gmackematix writes: And for those who like their TV sets switched off permanently, there's always: http://www.whitedot.org/ Jun 30 04, 6:20 PM