Magnets dissipate RF noise
Q: On many electrical cables, particularly those for computers, there is a cylindrical chunk of ferrite material slipped over the
end adjacent to the plug. I understand these things somehow filter out undesirable electrical interference but, how do they work?
-- James D. Hooker, Swansea, UK
A: Ferrite is an iron-like material commonly used to make magnets. It draws flux lines into it like a sponge. The
figure shows iron filings aligned along a magnet's flux lines.
[NASA/Marshal Space Flight Center] Lines of flux radiating from a bar magnet
Computers can generate electrical signals from any of their cables that interfere with other devices, such as a
television set. For example, personal computers provide brilliant crisp images that require fast timing circuits. These
circuits operate at rapid frequencies (from 30 to 130 megahertz)-- about the same as television broadcast signals.
The high-frequency signals carried by the video cable radiate an electromagnetic field. The TV antenna receives the
energy and the TV screen goes fuzzy: a herringbone pattern caused by electromagnetic interference (EMI).
In this case, the culprit is the video-circuitry-generated electromagnetic field radiating from the video cable.
Slip a ferrite doughnut over the end of the computer cable and Whoosh! The ferrite draws in the interfering
flux lines and disrupts the undesired high-frequency current that's clobbering the TV picture. The ferrite acts
like a high-frequency resistor by dissipating the interference.
[audioquest] Ferrite, clamped around a cable, blocks interfering RF
(Answered Aug. 8, 2002)
Repairfaq.org: RFI sources
Steward EMI Suppression & Inductive Components: EMI caused by computer video cables
Elmac Services: Ferrites for interference suppression