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Tech Tip 06

Magnetic Fields Can Distort Computer Screens


  • Distortion of characters on computer screen


  • Magnetic fields (EMF) affecting screen
  • Electric fan near screen
  • Screen is near power transformer or other "heavy" electrical distribution


  • Relocate monitor away from EMF source
  • Relocate EMF source
  • Shield monitor from fields
  • Change monitor refresh rate


An individual reported a computer monitor screen was "jumping around". An investigation revealed that an Electromagnetic Field (EMF) of 11 mG was interfering with the screen. The distortion appeared as a wavy screen and oscillating of text primarily around the edges of the screen. A similar screen is illustrated in figure 1. Measurements in the area suggested that the EMF was due to electrical distribution circuits running under the floor.

Magnetic field interference to monitor screens does not usually occur until a level of roughly 10 mG. Some high resolution screens have detected image distortion at 3-4 mG.


Magnetic field strength decreases rapidly as you move away from the source. Making use of this principle may assist in eliminating monitor screen distortion without the need for shielding.

Additional measurements revealed that the magnetic field strength was about 7-8 mG near an empty workstation. Relocating the monitor to this table eliminated the screen distortion.

Magnetic field strength will vary with the amount of current flowing in the source of the EMF. Relocating sensitive monitors may not always provide a permanent solution. In some cases it may be necessary to install a shield made of a ferromagnetic material around the monitor. A shield for a typical monitor costs approximately $800.

When applicable, changing the monitor refresh rate to 60 Hz has decreased image distortion. The change will place the monitorís CRT in synchronization with the 60 Hz current induced magnetic field.


Figure 1: Wavy screen caused by an electric fan held close.

The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.