[time-nuts] Primary Standards...
Lux, Jim (337C)
james.p.lux at jpl.nasa.gov
Wed Feb 24 01:16:38 UTC 2010
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of Tom Holmes, N8ZM
> Sent: Tuesday, February 23, 2010 4:57 PM
> To: 'Discussion of precise time and frequency measurement'
> Subject: Re: [time-nuts] Primary Standards...
> My recollection of the definition of an Ampere is 6.02 x 10^23 electrons per second (Avogadro's
> Number, I believe) passing a point in a conductor. To this day, I wonder how they managed to count all
> those electrons. But it does suggest that the silver deposit approach might be a better method of
> building a standard. Seems, though, like you'd have to make a darned high resolution weight
This is why Josephson junctions are useful. They have a frequency/voltage characteristic that is a fundamental property of physical constants. So, if you can measure frequency (using that primary standard for frequency or time), you can measure voltage.
So, to measure current, you have to turn voltage into current somehow, And you could use the Quantum Hall Effect as resistance standard, which like the Josephson, relies on fundamental physical properties, and is independent of most of the construction details (assuming it works at all)
You need a cryogenic system, and a high flux density (10 Tesla or so): http://www.warwick.ac.uk/~phsbm/qhe.htm
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