3 ms·

Huh? "In fact, of the seven fundamental metric units — the kilogram, meter, second, ampere, kelvin, mole, and candela — only the kilogram is still dependent on

Huh? "In fact, of the seven fundamental metric units — the kilogram, meter, second, ampere, kelvin, mole, and candela — only the kilogram is still dependent on a physical artifact." Followed by: "And the luminosity of light, or candela, is measured in terms of power, designated in watts, or joules per second." I'm not an SI expert, but Wikipedia suggests the candela is in fact an SI base unit with a definition based on physical phenomenon of the universe, so the second sentence seems erroneous. [edit: clarity]

aprescott 15y ago
Or not! http://en.wikipedia.org/wiki/File:SI_base_unit.svg http://en.wikipedia.org/wiki/File:SI_base_unit.svg
quanticle 15y ago
What I don't get is why ampere is a fundamental unit while coulomb isn't. I mean, it seems more natural to think of amperes and coulombs per second rather than to define coulombs as the amount of charged carried by a 1 amp current in 1 second.
- jessriedel 15y ago
  Techinically, neither the Coulomb nor the Ampere are fundmental. Both are derived from the meter, second, and kilogram. http://en.wikipedia.org/wiki/File:SI_base_unit.svg http://en.wikipedia.org/wiki/File:SI_base_unit.svg Now, in the abstract, yours is the more natural definition. But experimentally, it turns out to not give the best accuracy. I believe one starts with (1) the meter (which is based on a second defined using a atomic transition, plus light) and (2) the Newton (which is the force required to accelerate the all-important kilogram at 1 meter per second squared). Then an Ampere is the amount of current flowing through two parallel wires required to produce a force of one Newton per meter between the wires. http://en.wikipedia.org/wiki/Ampere#Definition http://en.wikipedia.org/wiki/Ampere#Definition Now, the technology is apparently improving enough that it may be possible to actually count elementary charges like you suggest. See here: http://en.wikipedia.org/wiki/Ampere#Proposed_future_definition http://en.wikipedia.org/wiki/Ampere#Proposed_future_definiti...
deleted 15y ago
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wlesieutre 15y ago
The candela is based on Watts, but weighted to adjust for human perception. While the "standard human eye" could be considered a physical artifact, we use experimentally determined photopic (in bright light) and scotopic (in low light) response curves, which are supposedly fixed. It isn't quite as fixed as some measurements since the average human's vision could continue to evolve and change the curve, or because we could find out that we measured it wrong, but it's not dependent on a single physical sample like the kilogram.

aprescott 15y ago

Or not! http://en.wikipedia.org/wiki/File:SI_base_unit.svg http://en.wikipedia.org/wiki/File:SI_base_unit.svg

quanticle 15y ago

What I don't get is why ampere is a fundamental unit while coulomb isn't. I mean, it seems more natural to think of amperes and coulombs per second rather than to define coulombs as the amount of charged carried by a 1 amp current in 1 second.

jessriedel 15y ago
Techinically, neither the Coulomb nor the Ampere are fundmental. Both are derived from the meter, second, and kilogram. http://en.wikipedia.org/wiki/File:SI_base_unit.svg http://en.wikipedia.org/wiki/File:SI_base_unit.svg Now, in the abstract, yours is the more natural definition. But experimentally, it turns out to not give the best accuracy. I believe one starts with (1) the meter (which is based on a second defined using a atomic transition, plus light) and (2) the Newton (which is the force required to accelerate the all-important kilogram at 1 meter per second squared). Then an Ampere is the amount of current flowing through two parallel wires required to produce a force of one Newton per meter between the wires. http://en.wikipedia.org/wiki/Ampere#Definition http://en.wikipedia.org/wiki/Ampere#Definition Now, the technology is apparently improving enough that it may be possible to actually count elementary charges like you suggest. See here: http://en.wikipedia.org/wiki/Ampere#Proposed_future_definition http://en.wikipedia.org/wiki/Ampere#Proposed_future_definiti...

deleted 15y ago

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wlesieutre 15y ago

The candela is based on Watts, but weighted to adjust for human perception. While the "standard human eye" could be considered a physical artifact, we use experimentally determined photopic (in bright light) and scotopic (in low light) response curves, which are supposedly fixed. It isn't quite as fixed as some measurements since the average human's vision could continue to evolve and change the curve, or because we could find out that we measured it wrong, but it's not dependent on a single physical sample like the kilogram.