Is this a Faraday cage?

[DavidBoettcher]

Wristwatches sometimes used to have the movement encapsulated in a soft iron housing to prevent the movement, particularly the steel balance spring, becoming magnetised. In the case of the IWC Mark 11 used by the RAF after WW2 this took the form of an iron ring around the movement with an iron back plate and iron dial plate so that the movement was completely enclosed in soft iron.

The use of soft iron, a ferromagnetic material, was intended to capture any magnetic field that the watch might encounter, keeping the lines of magnetic flux in the iron and channelling them around and away from the movement.

This encapsulation is often described as a Faraday cage. Is that the correct name for it, i.e. is it a special case of a Faraday cage made of ferromagnetic material?

Thanks for any help!

Regards - David

 

[VEBill]

Not exactly. A Faraday cage is a fully-enclosing conductive box.

I'm not sure what they'd call what you've described.

 

[DavidBoettcher]

That's what I thought. A Faraday cage can be made of any conductive material, not necessarily ferromagnetic, and it has to be earthed, right?

 

[VEBill]

...has to be earthed...

Absolutely not.

Almost any portable electronics (especially anything with a radio receiver inside) will contain some shielded sections. Such hand held devices are obviously not 'earthed'.

Airplanes, and satellites in orbit, will have many shielded sections (Faraday cages) and are obviously not earthed.

Another point worth considering is that at higher frequencies it becomes very difficult to achieve an earth ground due to the length of the ground wire. Even if the bottom end is earthed, a quarter wavelength along the wire it will be high impedance.

Faraday cages do not depend upon earth grounding. In fact they don't even depend on localized grounding.

 

[DavidBoettcher]

Ah, OK, I was wrong about the earthing . . .

Would it be true to say that a Faraday cage will shield from an electrical field but not a magnetic field - Wikipedia says that a compass will still work inside a Faraday cage for example.

I'm not sure that it is right to draw a distinction between an electrical field and a magnetic field, maybe I mean fluctuating as opposed to static electromagnetic field . . .

 

[VEBill]

This topic was the subject of another recent thread.
See:

http://www.eng-tips.com/viewthread.cfm?qid=308520

(The above simply references some related Wiki articles.)

Low frequency (including 'DC') magnetic fields are (in general) not blocked by Faraday cages.