[time-nuts] Optical link connects atomic clocks over 1400 km of fibre

[Alex Pummer]

to shield against DC and low frequency magnetic field usually high 
permeability magnetizable material -- Permalloy, Mu-metal or similar is 
used, the field concentrates in the high permeability material and 
"behind it" is no left over magnetic field

73

Alex


On 8/25/2016 11:10 AM, Magnus Danielson wrote:
> Hi,
>
> On 08/25/2016 11:04 AM, Bill Metzenthen wrote:
>> On 25/08/16 18:25, Poul-Henning Kamp wrote:
>>> --------
>>> In message
>>> <1057836989.2088307.1472104857885.JavaMail.yahoo at mail.yahoo.com>, Br
>>> uce Griffiths writes:
>>>
>>>> You'd need a rather thick copper jacket to shield effectively
>>>> against the 50Hz magnetic field.
>>> As in: A good-sized fraction of the wavelength if I recall :-)
>>>
>>> Electric fields are so much easier...
>>>
>>> One interesting thing here is that across distances like this,
>>> there would be significant longitudal currents in such a shield.
>>>
>>> Not as bad as metal spanning the Mississippi, but getting there.
>>>
>>>
>> Skin depth is probably a good place to start with in roughly estimating
>> the thickness needed.  In copper at 50 Hz, a quick calculation suggests
>> 9.5 mm, but this just the depth at which the E-M field decreases by 1/e
>> or 8.7 dB.  Thus to get 20 dB attenuation this implies a thickness of
>> about 22 mm, etc.
>
> While interesting, I think you are going overboard. The easy remedy is 
> to move the fiber of the power-cable and put it on some distance, just 
> choose a different path for it in the building.
>
> Cheers,
> Magnus
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