Exposing a CRT (cathode ray tube) monitor to a moderately strong electric field will cause the shadow mask in the tube to become partly magnetized, which will cause distortion of the image and the colours. Certain parts in the transports of audio and video tape recorders are also prone to becoming magnetized, which can adversely affect the material recorded on the tape. There is a brief article on degaussing on Wikipedia (http://en.wikipedia.org/wiki/Degaussing). Many monitors include a built-in degaussing function, which can be initiated by selecting an entry in the setup menu in the monitor (software) or via a control switch (hardware). This usually clears up the problem, but may need to be repeated several times. If this does not clear up the problem or the monitor lacks a degauss function, you will need to use an external electrical degausser. AC (alternating current) degaussers incorporate a large magnetic coil in which the polarity of the magnetic field is flipped back and forth many times per second. These are not unlike bulk tape erasers that are used to erase entire magnetic tapes at once. A decent electronics repair shop should be able to degauss your monitor for a reasonable cost - or at least for less than the cost of the degausser itself. Manually degaussing equipment takes a little practice to avoid leaving residual magnetic fields.

u can reduce it buy waving a magnet to one side

My 3 year old son just decided to place an "Oidz" (extremely powerful hemanite kids toy magnet things) directly onto tv screen (CRT) leaving a green oblong shaped shadow - after referring to this link and reading that degaussing would correct this I decided to revisit the screen with the tool of destruction - the "Oid" itself. After a couple of passing waves in front of the screen with the "Oid" - screen going a mixture of colours, with emphasis on the area where the green oblong shadow was located - the shadow faded at first, then after the third attempt, disappeared. Shame really as I was almost at the point of convincing my wife that we needed a new TV and nearly got ourselves a HD Ready LCD 40". Nevermind - hope this helps some of you with this problem.