In MKS as well SI system, the length, mass and time are measured in the units meter, kilogram and second, respectively. Then how the SI units are different from MKS units? What are the key differences?
$\begingroup$
$\endgroup$
1
-
3$\begingroup$ It is a progenitor of the SI system and had variants/extensions of its own to include an additional base unit to describe electromagnetism (Ampere - MKSA or Statcoulomb - RMKS). With the introduction of SI is became obsolete. What exactly is at question here. The history of the linked MKS wikipedia page kind of says it all no? $\endgroup$– N0vaCommented May 19 at 13:05
Add a comment
|
2 Answers
$\begingroup$
$\endgroup$
The history section of the SI page on Wikipedia covers the history quite well. Basically, MKS came before SI. It was later upgraded to include electromagnetic units, creating a new system called MKSA. Eventually SI was constructed, containing the base units we are used to today.
So a meter is a meter, a kilogram is a kilogram. However, a candela (a SI unit) has no corresponding unit in MKS. There simply isn't a defined unit for luminous intensity in MKS.
And as JEB points out, SI has evolved to be increasingly based on physical constants that we believe are uniform everywhere in the universe.
$\begingroup$
$\endgroup$
The main difference is the standards.
MKS is/was based on the traditional standards, e.g. a kg was the mass of one liter of water at a specified temperature, a meter was 1/10,000 the equator-to-pole distance on Earth$^1$. Time was derived from Earth's rotation and orbital properties.
S.I. ties these things to fundamental constants as much as possible, so mass is tied to $\hbar$ and length to $c$, with a time standard that is based on an atomic transition providing the scaling factor.
A proper detailed answer could probably be a book.
[1] this makes the Earth's circumference about 24,000 miles, which is convenient since there are 24 hours in day. And any experimental nuclear/particle physicist who has plugged in a BNC or NIM cable knows the speed of light is 1 foot per nanosecond. Gotta love Imperial units.
