If you had one what would you use it for
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From: MooseFET on 21 Oct 2009 21:53 On Oct 21, 12:23 pm, "n...(a)bid.nes" <alien8...(a)gmail.com> wrote: > On Oct 21, 6:21 am, MooseFET <kensm...(a)rahul.net> wrote: > > > Imagine you had a sensor that could measure very small magnetic > > fields. It measures with a noise floor of about 0.1fT. Unfortunately > > the band width is only a few hundred Hz and it only works inside a > > shield. > > Point-one FEMTO Tesla? Yipes! Yes. > > Um, at room temp? Not cooled. It is heated but not all that hot. > > What's the spec on the shield? Many layers. We need the field to be under about 100nT for it to work. Lower is better. > > > What would you use such a sensor for? The best I've thought of is > > detecting the flow of current in a PCB to find a short circuit. > > Hm. How fast does it respond, and what sort of output? It has a bandwidth of a few 100 Hz. The output is analog voltage. > > It'd make a dandy "somebody opened the box with the DO NOT OPEN > label" sensor, if nothing else. The little circuit with a Sonalert, 4 9V batteries and an SCR does a dandy job of that. > > Mark L. Fergerson
From: MooseFET on 21 Oct 2009 21:54 On Oct 21, 1:39 pm, Spehro Pefhany <speffS...(a)interlogDOTyou.knowwhat> wrote: > On Wed, 21 Oct 2009 12:23:53 -0700 (PDT), "n...(a)bid.nes" > > <alien8...(a)gmail.com> wrote: > >On Oct 21, 6:21 am, MooseFET <kensm...(a)rahul.net> wrote: > >> Imagine you had a sensor that could measure very small magnetic > >> fields. It measures with a noise floor of about 0.1fT. Unfortunately > >> the band width is only a few hundred Hz and it only works inside a > >> shield. > > > Point-one FEMTO Tesla? Yipes! > > > Um, at room temp? > > > What's the spec on the shield? > > Has to be superconducting. ;-) No supermagnetic. Infinite mu. It must not conduct at all. > > >> What would you use such a sensor for? The best I've thought of is > >> detecting the flow of current in a PCB to find a short circuit. > > > Hm. How fast does it respond, and what sort of output? > > > It'd make a dandy "somebody opened the box with the DO NOT OPEN > >label" sensor, if nothing else. > > > Mark L. Fergerson > >
From: John Larkin on 22 Oct 2009 15:21 On Wed, 21 Oct 2009 18:54:59 -0700 (PDT), MooseFET <kensmith(a)rahul.net> wrote: >On Oct 21, 1:39�pm, Spehro Pefhany <speffS...(a)interlogDOTyou.knowwhat> >wrote: >> On Wed, 21 Oct 2009 12:23:53 -0700 (PDT), "n...(a)bid.nes" >> >> <alien8...(a)gmail.com> wrote: >> >On Oct 21, 6:21�am, MooseFET <kensm...(a)rahul.net> wrote: >> >> Imagine you had a sensor that could measure very small magnetic >> >> fields. �It measures with a noise floor of about 0.1fT. �Unfortunately >> >> the band width is only a few hundred Hz and it only works inside a >> >> shield. >> >> > �Point-one FEMTO Tesla? Yipes! >> >> > �Um, at room temp? >> >> > �What's the spec on the shield? >> >> Has to be superconducting. ;-) > >No supermagnetic. Infinite mu. It must not conduct at all. http://en.wikipedia.org/wiki/Meissner_effect John
From: MooseFET on 22 Oct 2009 21:33 On Oct 22, 12:21 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 21 Oct 2009 18:54:59 -0700 (PDT), MooseFET > > > > <kensm...(a)rahul.net> wrote: > >On Oct 21, 1:39 pm, Spehro Pefhany <speffS...(a)interlogDOTyou.knowwhat> > >wrote: > >> On Wed, 21 Oct 2009 12:23:53 -0700 (PDT), "n...(a)bid.nes" > > >> <alien8...(a)gmail.com> wrote: > >> >On Oct 21, 6:21 am, MooseFET <kensm...(a)rahul.net> wrote: > >> >> Imagine you had a sensor that could measure very small magnetic > >> >> fields. It measures with a noise floor of about 0.1fT. Unfortunately > >> >> the band width is only a few hundred Hz and it only works inside a > >> >> shield. > > >> > Point-one FEMTO Tesla? Yipes! > > >> > Um, at room temp? > > >> > What's the spec on the shield? > > >> Has to be superconducting. ;-) > > >No supermagnetic. Infinite mu. It must not conduct at all. > > http://en.wikipedia.org/wiki/Meissner_effect Unfortunately, I think it only applies "on the average". There is a quantum physical probability to the motion of the Cooper pairs. I believe that when they move, they create a magnetic field. I am not sure about this but it seems right. I do know that high temperature super conductors make noise that varies with temperature. LN2 isn't quite cold enough to make a high performance SQUID with them. > > John
From: John Larkin on 22 Oct 2009 22:04 On Thu, 22 Oct 2009 18:33:40 -0700 (PDT), MooseFET <kensmith(a)rahul.net> wrote: >On Oct 22, 12:21�pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Wed, 21 Oct 2009 18:54:59 -0700 (PDT), MooseFET >> >> >> >> <kensm...(a)rahul.net> wrote: >> >On Oct 21, 1:39�pm, Spehro Pefhany <speffS...(a)interlogDOTyou.knowwhat> >> >wrote: >> >> On Wed, 21 Oct 2009 12:23:53 -0700 (PDT), "n...(a)bid.nes" >> >> >> <alien8...(a)gmail.com> wrote: >> >> >On Oct 21, 6:21�am, MooseFET <kensm...(a)rahul.net> wrote: >> >> >> Imagine you had a sensor that could measure very small magnetic >> >> >> fields. �It measures with a noise floor of about 0.1fT. �Unfortunately >> >> >> the band width is only a few hundred Hz and it only works inside a >> >> >> shield. >> >> >> > �Point-one FEMTO Tesla? Yipes! >> >> >> > �Um, at room temp? >> >> >> > �What's the spec on the shield? >> >> >> Has to be superconducting. ;-) >> >> >No supermagnetic. �Infinite mu. �It must not conduct at all. >> >> http://en.wikipedia.org/wiki/Meissner_effect > >Unfortunately, I think it only applies "on the average". There is a >quantum physical probability to the motion of the Cooper pairs. I >believe that when they move, they create a magnetic field. I am not >sure about this but it seems right. > >I do know that high temperature super conductors make noise that >varies with temperature. LN2 isn't quite cold enough to make a high >performance SQUID with them. >> >> John There's a way to make a volume with a very low DC field. Flatten a metal bag or one-end-closed tube, cool it to be superconductive, then expand it. The interior will have very little flux and it will be shielded from outside fields. Several of these can be nested. John
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