Re: [time-nuts] Rb Oscillator - rather fundamental question
David C. Partridge wrote:
No they cannot be - yet. At the point where (e.g.) the second is re-defined
in terms of the aluminium quantum clock, then the aluminium quantum clocks
are then by definition the primary standards of time, and all the Cs clocks
are now secondary standards as the second is no longer defined in terms of
the Cs beam clock.
Dave
Does that mean that there is no primary standard for the Ampere?
An Ampere is defined as the current which will produce an attractive force of 2
× 10–7 newtons per metre of length between two straight, parallel conductors of
infinite length and negligible circular cross section placed one metre apart in
a vacuum.
Since its impossible to build such a system, does that mean there is no primary
standard for an amp?
Dave
Gosh. I remember the ampere as the current that would deposit a given
weight of silver in a fixed time...
Also saw a note about one part in e-20. As the universe is apparently
about 5e17 sec old, can we make a standard that is good to 1 sec in 1e20
sec???
Don
Dr. David Kirkby
David C. Partridge wrote:
No they cannot be - yet. At the point where (e.g.) the second is
re-defined
in terms of the aluminium quantum clock, then the aluminium quantum
clocks
are then by definition the primary standards of time, and all the Cs
clocks
are now secondary standards as the second is no longer defined in terms
of
the Cs beam clock.
Dave
Does that mean that there is no primary standard for the Ampere?
An Ampere is defined as the current which will produce an attractive force
of 2
à 10â7 newtons per metre of length between two straight, parallel
conductors of
infinite length and negligible circular cross section placed one metre
apart in
a vacuum.
Since its impossible to build such a system, does that mean there is no
primary
standard for an amp?
Dave
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--
Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
www.lightningforensics.com
www.sixmilesystems.com
Mike S wrote:.
There is no difference between Cs and Rb in that regard, except perhaps
scale.
The frequency of a Cs is subject to gravitational and electromagnetic
effects. Which is why the definition of the second was clarified in
1997 by the CIPM to refer "to a caesium atom at rest at a temperature
of 0 K."
Since absolute rest and absolute zero are impossible conditions for a
real world clock, Cs clocks do not have a "known and invariant
frequency." If they did, why would they have a C-Field knob to twiddle,
and why would TAI be a weighted average of multiple Cs clocks? I'd
guess that Cs clocks can also be thrown off by trace gasses in the
tubes and numerous other effects, impossible to completely remove.
You covered many topics here. First, let me put the C-field knob
issue to rest. This was used to vary the frequency of older/cheaper
atomic standards, whether primary or secondary, before frequency
synthesizer technology allowed the C-field to be fixed at one value.
In a secondary standard such as Rb, the synthesizer is used to "set"
it on frequency against a reference. In a primary standard such
as a Cs beam standard, the synthesizer (if it is used at all) is used to
get a known offset from 10 MHz for various reasons unrelated to "setting"
it to a reference.
The TAI is a weighted average to improve short term stability and
to average out random frequency errors. The TAI is not a crutch
to make up for inferior clocks; rather, clocks have to earn their
way into TAI by being accurate. I haven't looked recently,
but traditionally something like 85% of the weight has been 5071A's.
Also, the "big" clocks that are better than the 5071A's are used more
as frequency standards and don't necessarily run continuously and
actually keep time as a clock. Most 5071A's run continuously,
because no one needs to baby sit them to keep them going.
Because the 5071's have non reversible beams, they are subject to a small
frequency error due to end to end cavity phase differences. Because
of various proprietary techniques used to machine the cavity, these
errors are very close to truly random and average to nearly zero
over a large group of 5071A's.
Unlike the buffer gas in an Rb standard, the inside of a cesium
beam tube is at a very high vacuum and is continuously pumped down.
Pressure shift due to trace gases is not a factor. Velocity effects
and doppler shift, etc in Cs standards have all been addressed
and a small upper bound has been established.
Rick Karlquist N6RK
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 measurement.
Now with tongue still planted firmly in cheek, I have to think that THE primary standard time interval reference is whatever NIST (and other such outfits) tells me it is. Anything else would seem to be a secondary standard as it would have to be compared to NIST to be sure it is right (OK, its error to NIST is known), especially based on all of your comments.
But as even NIST pointed out, primary vs. secondary seems to be in the context of the beholder.
Thanks for the enlightening discussion; you've really encouraged me to think about this a bit.
Regards,
Tom Holmes, N8ZM
Tipp City, OH
EM79xx
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On Behalf Of Don Latham
Sent: Tuesday, February 23, 2010 7:33 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Primary Standards...
Gosh. I remember the ampere as the current that would deposit a given
weight of silver in a fixed time...
Also saw a note about one part in e-20. As the universe is apparently
about 5e17 sec old, can we make a standard that is good to 1 sec in 1e20
sec???
Don
Dr. David Kirkby
David C. Partridge wrote:
No they cannot be - yet. At the point where (e.g.) the second is
re-defined
in terms of the aluminium quantum clock, then the aluminium quantum
clocks
are then by definition the primary standards of time, and all the Cs
clocks
are now secondary standards as the second is no longer defined in terms
of
the Cs beam clock.
Dave
Does that mean that there is no primary standard for the Ampere?
An Ampere is defined as the current which will produce an attractive force
of 2
× 10–7 newtons per metre of length between two straight, parallel
conductors of
infinite length and negligible circular cross section placed one metre
apart in
a vacuum.
Since its impossible to build such a system, does that mean there is no
primary
standard for an amp?
Dave
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
--
Dr. Don Latham AJ7LL
Six Mile Systems LLP
17850 Six Mile Road
POB 134
Huson, MT, 59846
VOX 406-626-4304
www.lightningforensics.com
www.sixmilesystems.com
time-nuts mailing list -- time-nuts@febo.com
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and follow the instructions there.
Tom Holmes, N8ZM wrote:
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 measurement.
That certainly was not the definition I learned during my EE degree, and neither
is it the one given on Wikipedia - not that I'd call Wikipedia a standard.
My recollection is the same as Wikipedia's - though I could not remeber the bit
about it needing to be a vacuum. But if you stuffed mu-metal between the wires,
it would tend to reduce the force, so I can well believe its defined in a vacuum.
I think as someone else said, this depends on one's definition of a "standard".
There's no one standard definition of a standard (pun intended).
Dave
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@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
measurement.
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
Dave...
I went back and checked in my college Physics textbook, Halliday & Resnick,
Vol. II, circa 1960, and you are correct about the Ampere being defined
based on the force between two parallel wires. However, H&R does not specify
a vacuum nor negligible wire cross section. The latter seems reasonable to
minimize the effects of geometry. They also say that at the time, NBS was
using a balance beam technique with a moving coil between two fixed coils as
the primary measurement standard.
Where Avogadro's number comes in is that 1 coulomb is defined as "the amount
of charge that flows through a given cross section of wire in one second IF
there is a steady current of one Ampere". In other words, if I moved a
coulomb of charge in one second, then the current must have been one Ampere.
Kind of a strange way to state it given that one of the equations given for
charge is Q= the integral of I*d(t), implying that current and time are the
are the measurables.
So I think in a way we are both correct: you have the definition of the
standard, and I cited an equivalence that is based on the fundamental units
of the mks system.
In a table in the appendix called "Symbols, Dimensions, and Units for
Physical Quantities" there are listed about 60 quantities and their primary
units (Length, Mass, Time, and Charge). For example, capacitance has
dimensions of T^2 * Q^2 / M^2 * L^2, with the derived unit Farad. Force has
dimensions of M*L/T^2, with derived units of newtons. This fits with F=MA,
that is, force is derived from mass, length, and time, all of which have
fundamental standards. The Kg is a slug of something carefully stashed in a
cave in France ( a little license here, please), the meter is a bunch of
wavelengths of a Krypton dance, and the second is based on...oh, wait, this
is the time-nuts forum.
So what is bugging me is that the Newton, a derived unit, is being used to
define the Ampere, which appears to be a fundamental part of the definition
of the Coulomb, a primary unit. This strikes me as backwards. However, it
does make sense that the method used to determine a 'standard' value for the
Ampere might not be possible using such a strict dependency on direct ties
to primary units.
OK, I think I have meandered far enough OT once again as to put this to rest
for now.
Regards,
Tom Holmes, N8ZM
Tipp City, OH
EM79xx
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Dr. David Kirkby
Sent: Tuesday, February 23, 2010 8:12 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Primary Standards...
Tom Holmes, N8ZM wrote:
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 measurement.
That certainly was not the definition I learned during my EE degree, and
neither
is it the one given on Wikipedia - not that I'd call Wikipedia a standard.
My recollection is the same as Wikipedia's - though I could not remember the
bit
about it needing to be a vacuum. But if you stuffed mu-metal between the
wires,
it would tend to reduce the force, so I can well believe its defined in a
vacuum.
I think as someone else said, this depends on one's definition of a
"standard".
There's no one standard definition of a standard (pun intended).
Dave
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
In my checkered past, I once legally owned, for about two minutes between
signatures, the British Standard Pound weight. It was a bar of platinum.
I confess I did wonder how easy it would be to sell a pound of platinum, as
it would have been a bit difficult to cut it up...
----- Original Message -----
From: "Tom Holmes, N8ZM" tholmes@woh.rr.com
To: "'Discussion of precise time and frequency measurement'"
time-nuts@febo.com
Sent: Wednesday, February 24, 2010 3:00 PM
Subject: Re: [time-nuts] Primary Standards...
Dave...
I went back and checked in my college Physics textbook, Halliday &
Resnick,
Vol. II, circa 1960, and you are correct about the Ampere being defined
based on the force between two parallel wires. However, H&R does not
specify
a vacuum nor negligible wire cross section. The latter seems reasonable to
minimize the effects of geometry. They also say that at the time, NBS was
using a balance beam technique with a moving coil between two fixed coils
as
the primary measurement standard.
Where Avogadro's number comes in is that 1 coulomb is defined as "the
amount
of charge that flows through a given cross section of wire in one second
IF
there is a steady current of one Ampere". In other words, if I moved a
coulomb of charge in one second, then the current must have been one
Ampere.
Kind of a strange way to state it given that one of the equations given
for
charge is Q= the integral of I*d(t), implying that current and time are
the
are the measurables.
So I think in a way we are both correct: you have the definition of the
standard, and I cited an equivalence that is based on the fundamental
units
of the mks system.
In a table in the appendix called "Symbols, Dimensions, and Units for
Physical Quantities" there are listed about 60 quantities and their
primary
units (Length, Mass, Time, and Charge). For example, capacitance has
dimensions of T^2 * Q^2 / M^2 * L^2, with the derived unit Farad. Force
has
dimensions of M*L/T^2, with derived units of newtons. This fits with F=MA,
that is, force is derived from mass, length, and time, all of which have
fundamental standards. The Kg is a slug of something carefully stashed in
a
cave in France ( a little license here, please), the meter is a bunch of
wavelengths of a Krypton dance, and the second is based on...oh, wait,
this
is the time-nuts forum.
So what is bugging me is that the Newton, a derived unit, is being used to
define the Ampere, which appears to be a fundamental part of the
definition
of the Coulomb, a primary unit. This strikes me as backwards. However, it
does make sense that the method used to determine a 'standard' value for
the
Ampere might not be possible using such a strict dependency on direct ties
to primary units.
OK, I think I have meandered far enough OT once again as to put this to
rest
for now.
Regards,
Tom Holmes, N8ZM
Tipp City, OH
EM79xx
-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Dr. David Kirkby
Sent: Tuesday, February 23, 2010 8:12 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Primary Standards...
Tom Holmes, N8ZM wrote:
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 measurement.
That certainly was not the definition I learned during my EE degree, and
neither
is it the one given on Wikipedia - not that I'd call Wikipedia a standard.
My recollection is the same as Wikipedia's - though I could not remember
the
bit
about it needing to be a vacuum. But if you stuffed mu-metal between the
wires,
it would tend to reduce the force, so I can well believe its defined in a
vacuum.
I think as someone else said, this depends on one's definition of a
"standard".
There's no one standard definition of a standard (pun intended).
Dave
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
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Poul-Henning Kamp wrote:
In message 20100223214204.EAE711174BE@hamburg.alientech.net, Mike S writes:
renamed, since the discussion has shifted.
"In the time and frequency field, the term primary standard is
sometimes used to refer to any cesium oscillator, [...]
That rhymes with and Karls and my perception of the term:
A Cs clock is primary because when you turn it on, it latches onto
the physical phenomenon of a known and invariant frequency subject
to no systematic errors.
No, that's not quite accurate. The key is that the systematic errors is
predictable and that you can compensate for them such that residue
systematic errors becomes very small. The C-field frequency shift is one
such systematic factor.
The phase-error of the Ramsey interregation cavities is another, which
is first-degree compensated for in some laboratory standards, but which
can be reduced by carefull design and compensated for if it can be
controlled to be repeateable and stable.
Even for Caesiums there exists numerous shifts. Caesium fountains is one
approach to address some of these issues, alongside that of thermal noise.
The reason the small Rb's do not qualify as primary is that each
unit has a slightly different frequency, due to vapour pressure,
isotopemix and other physical details, and thus you cannot know the
frequency of a particular unit, until you have measured it relative
a primary clock.
There are many factors for gas standards which makes them have
unpredictable systematic errors. They also show long term drift factors.
The motivation for them is much lower price, volume and power
consumption. Rubidium has proven a good choice for a gas standard.
In other words, Primary and Secondary has nothing to do with which
atoms, but depends a lot on the interogations mechanism used.
Agreed.
The reason Caesium is chosen is that when they where choosing, they
higher frequency of Thallium was considered problematic as it would
become harder to achieve the wanted repeatability from a technological
point of view at that time. Thallium showed however a lower sensitivity
to magnetic field than Caesium, so technically it is a better standard
and it was known at the time. Thus, the aspect of gentlemens agreement.
By todays knowledge, Rubidium fointain outperforms Caesium, so with that
technology scope Rubidium would be chosen and Caesium beams would be
handy secondary standards....
So the tiny 1cm^3 Cs standards are secondary, because they are also
subject to all sorts of pulls and offsets.
Agreed.
The "experimental" clocks based on lonely ions and quantum embraces
are very likely primary, once somebody has measured their intrinsic
frequency relative to Cs once.
Various clocks have been made for this purpose. The field is being
investigated. The Aluminium-ion clock that was reported on recently is
one among many different projects.
The way to find out if your new invention has a chance to become a
primary clock, is to build N of them, turn them on, and see if they
all find the same frequency once they are locked, if they do,
you're on your way to become famous.
That helps. However, unless someone is able to independently build
clocks and get equivalent levels of accuracy and stability, you still
have a problem to show the actual performance.
It is challenging not to remake the same systematic design mistake. For
hydrogen clocks use of different sized glas bulbs to investigate the
wall-shifts and be able to cancel the effect, which has been important
when measuring the unperturbed hydrogen frequency.
It is not an easy science, but it is a science which excels in
increasingly improved methods.
Cheers,
Magnus