bssc
Moderator Emeritus
- Joined
- Dec 27, 2005
- Messages
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It sells more tools. My wife has both English and Metric toolsets, depending on whether the equipment was made in the US or France.
But didn't Jesus speak English?
Kilogram - Wikipedia, the free encyclopediaBy definition, the error in the measured value of the IPK’s mass is exactly zero; the IPK is the kilogram. However, any changes in the IPK’s mass over time can be deduced by comparing its mass to that of its official copies stored throughout the world, a process called “periodic verification.” For instance, the U.S. owns three kilogram standards, two of which, K4 and K20, are from the original batch of 40 sister copies[citation needed] of the IPK delivered in 1884. The K20 standard was designated as the primary national standard of mass for the U.S. Both of these, as well as those from other nations, are periodically returned to the BIPM for verification.[13]
Note that the masses of the sister copies and replicas are not precisely equal to that of the IPK; their masses are calibrated and documented as offset values. For instance, K20, the U.S.’s primary standard, originally had an official mass of 1 kg – 39 µg in 1889; that is to say, K20 was 39 µg less than the IPK. A verification performed in 1948 showed a mass of 1 kg – 19 µg. The latest verification performed in 1999 shows a mass identical to its original 1889 value. The mass of K4, the U.S.’s check standard, as of 1999 was officially calibrated as 1 kg – 116 µg. However, it was 41 µg more massive (in comparison to the IPK) in 1889.
Since the IPK and its replicas are stored in air (albeit under two or more nested bell jars), they adsorb atmospheric contamination onto their surfaces and gain mass. Accordingly, they are cleaned in preparation for periodic verifications—a process the BIPM developed between 1939 and 1946 known as “the BIPM cleaning method” that includes steam cleaning, lightly rubbing with chemical-soaked chamois, and allowing the prototypes to settle for 7–10 days. Cleaning the prototypes removes between 5 and 60 µg of contamination depending largely on the time elapsed since the last cleaning. Further, a second cleaning can remove up to 10 µg more. After cleaning—even when they are stored in their bell jars—the IPK and its replicas immediately begin gaining mass again. The BIPM even developed a model of this gain and concluded that it averaged 1.11 µg per month for the first 3 months after cleaning and then decreased to an average of about 1 µg per year thereafter. Since check standards like K4 are not cleaned for routine calibrations of other mass standards—a precaution to minimize the potential for wear and handling damage—the BIPM’s model has been used as an “after cleaning” correction factor.
Because the first forty official copies are made of precisely the same alloy as the IPK and are stored under similar conditions, periodic verifications using a large number of replicas—especially the national primary standards, which are rarely used—can convincingly demonstrate the stability of the IPK. What has become clear after the third periodic verification performed between 1988 and 1992 is that the mass of the IPK lost perhaps 50 µg over the last century, and possibly significantly more, in comparison to its official copies.[14][15] The answer as to why this might be the case has proved elusive for physicists who have dedicated their careers to the SI unit of mass. No plausible mechanism has been proposed to explain either a steady decrease in the mass of the IPK, or an increase in that of its six sister copies and the others dispersed throughout the world.[16] Further, the IPK exhibits an instability of about 30 µg over a period of about a month in its after-cleaned mass.[17] The precise reason for this instability is not fully understood but is thought to entail surface effects: microscopic differences in their polished surfaces, unintentional differences in the cleaning process, and/or differences in the precise nature of the contamination. What is known is that the past assumption that the cleaning process reliably restores the prototypes to their original value is false and that the BIPM’s after-cleaning correction factor is useful only for long-term trends. Scientists are seeing far greater variability in the prototypes than previously believed. Further, there is no technical means available to know whether or not the entire worldwide ensemble of prototypes suffer from even greater long-term trends upwards or downwards because their mass “relative to an invariant of nature is unknown at a level below 1000 µg over a period of 100 or even 50 years.”[14]
The relative change in mass and the instability in the IPK has prompted research into improved methods to obtain a smooth surface finish using diamond-turning on newly manufactured replicas and has intensified the search for a new definition of the kilogram.
Yeah. I'm not sure I could get used to the switch from a "foot-long hot dog" to a "30.48 centimeter hot dog". Seems to lose a little something in the translation.
Time measurementAfter I realized that relativity slowed and speeds up time relative to the frame of reference I realized that time is not absolute. Why would we continue to measure time as the vibration of an atom. Wouldn't we start measuring time by the vibration of a photon? Since a photon is absolute no matter where in the universe it is and it stays constant for that wavelength.
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be very careful answering that. if you say "photons of blue visable light" then i would ask how you define blue? if blue photons are the photons that make it through some standardized blue filter, then you're back to defining this in terms of the atoms of some material.
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Why not measure time using as an unit the period of the electromagnetic oscillations taking place in the electromagnetic wave in which we are immersed? Besides that we should now which kind of time interval we measure: proper time or coordinate time.
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To measure time, you need photons of known fixed wavelength, so that you know the frequency. If you are moving in reference to the source, that changes the frequency (doppler effect), but thats not a problem in atomic clocks. So you use a specific atomic vibration, with a specific energy, to generate the photons of fixed frequency.
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which photon would you choose as a standard for a unit of time?
To measure cosmological time:
Select a photon sampled from the peak emission intensity of the CMB corrected for the Earth's dipole movement.
Then the universe woul be eternal, and if c is defined constant and used to measure radar distance, the universe would also be static, as in the Jordan conformal frame of SCC.
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I guess your right about the doppler effect. It just doesn't seem right to measure time with something that changes.
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You don't get any Doppler effect if you are not moving wrt the source. In metrology, your clock is supposed to be infinitely small and to lie at rest no farther away from your elbow. So, no Doppler effect, of ny sort.
you say, why use the wave length of light as a standard? Well, the wavelength is a physical property that can be replicated in labs across the world, and in space. Yes, we humans can't relate to it, but why does that matter?
Did everyone remember to set their greater-than-infinitesimally-small-and-thus-wildly-inaccurate clocks back?
ERD50.. I didn't think I was having an "argument"..
they pushed it back by another week to next Sunday.
Now I have to get confused about when I can can people in the States.
I noticed that too.Of course He did. He even spoke in red letters.
2Cor521
We don't observe DST in most of Arizona because who wants another hour of 40+ degree heat.Last time they tried that, I was one of the kids waiting for the bus. It wasnt that dark and I didnt have any problem with it, but I still remember the news furor over the PO'd parents.
I'd sure like DST all year round. I can use the daylight at the end of the day a lot more than in the morning. I've already had to tell Gabe a couple of times that we cant go to the park because its too late and almost dark already. Yank that back another hour next week and its gonna be a hassle.
I'd sure like DST all year round.