# The Science Guys

### August 2003

Why do magnets held close to television screens cause the picture to distort?

We first need to know what is going on inside of a TV picture tube. The picture tube on a TV has an electron gun in the back that constantly ’fires’ electrons at the screen. This stream of electrons is swept back and forth, and up and down across the screen many times a second. The actual speed of the electrons can be varied and the position the electron hits the back of the screen is also controlled.

The screen is coated with a material that glows when struck by an electron. In color tubes there are different tiny spots that glow red, green, or blue, with an electron gun allocated for each color. With all the spots glowing at slightly different times, places, and with varying intensities, a picture is painted on the screen. The speed and position the electrons strike are all controlled electronically. When you view a TV you do not notice the individual spots or the sweeping of the beam across the screen. Your eye simply combines all of this to give you a clear picture.

Now how does a magnet affect all of this? A magnet does not exert a force on a stationary electron. However, if the electron moves, a force suddenly appears! This force is proportional to the charge on the electron, the speed with which the electron moves, and the strength of the magnetic field. In the television tube the electrons forming or painting the picture are moving as they are shot from the back of the tube to the front. When a magnet is brought close to the picture tube, the interaction between the flying electrons and the magnetic field creates a force that throws the electrons off course. Now the electrons are hitting the screen in places they were not intended to strike and the picture becomes distorted.

Many televisions have a device called a degaussing coil that removes small magnetic fields that may form in the picture tube. Manufacturers often use what is called a degaussing coil. The word degaussing comes from the fact that the strength of magnetic fields is sometimes given in units of Gauss. This is one reason you should not place a video tape directly on top of the television. The degaussing equipment might distort some of the material stored on the magnetic tape.

The forces that magnetic fields create on moving charges are of critical importance in science. Such forces are used in cyclotrons (machines used to probe the nature of matter) and atomic mass spectrometers (devices that measure atomic masses.) This force is even responsible for trapping the solar wind particles resulting in the brilliant auroras at our polar regions.

Caution: Do not hold a magnet too close to the TV screen. It could permanently distort the screen which cannot be corrected by degaussing equipment.