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Monitors based on cathode-ray tube (CRT) technology, including those found in iMac computers, use precisely controlled magnetic fields to direct the flow of electrons to the red, green, and blue light emitting phosphors on the monitor. The earth's magnetic field varies in intensity throughout the world, which can affect the path of this electron beam. During manufacturing, CRT-based monitors are aligned in special areas called helmholtz cages that simulate the magnetic field the monitor is being aligned for. Monitors are typically aligned for the Northern Hemisphere or the Southern Hemisphere, and sometimes for the equatorial region.



A CRT-based monitor purchased in the Northern Hemisphere may not perform correctly if it is moved to the Southern Hemisphere. The reverse is also true because the earth's magnetic fields are not the same in each hemisphere. A monitor moved to another hemisphere may encounter color purity problems that cannot be adjusted out. It is technically possible to realign a monitor to work in a different hemisphere, but the skills and specialized equipment required make it prohibitively expensive for most purposes.



Liquid crystal displays, such as the Apple Cinema Display and the display of an iBook computer, are not affected by the earth's magnetic fields, and can be moved between the Northern and Southern Hemispheres.