Question: I have an old CRT connected to laptop as a secondary display. As you know, when CRT turns on it degausses itself; remember that sound when you turn it on, or force degaussing through menu.
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CRTs have a copper, or often in the case of cheaper appliances, ?aluminum, coil wrapped around the front of the display, known as the ?degaussing coil. Tubes without an internal coil can be degaussed using ?an external hand held version. Internal degaussing coils in CRTs are ?generally much weaker than external degaussing coils, since a better ?degaussing coil takes up more space. A degauss causes a magnetic ?field inside the tube to oscillate rapidly, with decreasing ?amplitude.
I searched everywhere but couldn t find if degaussing has effects on nearby hard drives? Is it dangerous to have CRT and laptop close (about 7-8 inches) ?
Answer: It takes a tremendous field gradient to flip the magnetic domains on a hard drive. The hard drive can do it because the heads are so close to the surface and the gaps are so small. The magnets inside the drive’s spindle motor and arm actuator produce a stronger field than will the external coil… but because of the motor designs, they don’t put a high field gradient near the surfaces.
That’s theory.
I have a bulk tape eraser left over from my days of owning quarter-inch reel-to-reel audio recorders. It pulls 8.5 amps from 120 VAC, which is far more current than an entire CRT monitor uses, let alone the degaussing coil inside it. (Magnetic field strength is proportional to current.) And besides having a stronger basic field strength, its magnetic field is far more concentrated than that of a degaussing coil (since the latter has no pole pieces).
Some time ago I had a stack of 18 too-small-to-be-useful laptop hard drives (4.3 GB). There being no market any more for such small drives, I decided to try an experiment.
Keep in mind that hard drives include an embedded servo signal (created by so-called “low level formatting” at the factory) which is essential to the drive’s operation. If that is weakened too much, not only is data unrecoverable, the drive is toast.
So I tried to get the degausser to affect these hard drives.
It did not, not even a little. Even after thorough attempts at degaussing, holding the eraser’s pole pieces against both sides of the drive and using a “wiping” movement, even though the poor drives were vibrating madly from the 60 Hz field… ?all 18 drives’ entire surfaces were still perfectly readable and writeable afterwards. (n.b.: It doesn’t take long to run a read/write/read surface scan on 4.3 GB!)
4.3 GB HDs are a much more primitive technology than modern HDs. But more recent hard drives require an even higher field gradient to flip the domains. (That’s because the domains are smaller, packed closer together… they would self-erase themselves if it was easy.) If a device that deliberately makes a highly concentrated magnetic field, designed to erase magnetic media, can’t affect those old drives, I mightily doubt that a CRT TV or monitor’s degaussing coil can affect a modern multi-TB drive at all.