[USMA:23425] Re: A quote that I want to counter

[kilopascal]



2002-11-17

 

By the late '40s it seemed that the metric system was starting to diverge into the same hodge-podge that is characteristic of FFU.  Something had to done.  The resultant is SI. 

 

Even if before the advent of SI in 1960, the absolute units (abunits) and the electrostatic units (see below) formed a competing subset of electrical and magnetic units.  Even then, there were additional subsets of units, including Gaussian, Heavy-side Lorentz and practical. I could not find any reference on the Rowlett page to indicate any variance in the ohm.  The variance may have been in the relationship between the ohm, the abohm and the statohm.  These all were dependent on the value of μ(0) (permeability of freespace) and μ(0) (permitivity of freespace) (meaning mu subzero and epsilon sub zero) and how accurately they were represented.  μ(0) is defined as 4π x 10^-7 Wb/A.m and ε(0) which is defined as 1/c^2.μ(0) F/m (c = speed of light) = 8.8542 x 10^-12 F/m.

 

If the ampere was then defined from the magnetic units and the value of μ(0), ε(0), π and c were not defined as accurate as they are today, I can see where such a variance would come in.  Even today the ampere, even though an SI base unit, is still defined according to the magnetic force present when conductors carry a current. 

 

See: http://www.unc.edu/~rowlett/units/index.html

 

See here for a table of the differences between cgs and SI:  http://www.unc.edu/~rowlett/units/cgsmks.html

 

See also http://www.scienceworld.wolfram.com/physics/topics/initsanddimensionalanalysis.html

 

 

ab-

a prefix indicating that an electrical unit is part of the CGS absolute electromagnetic system. These units are also indicated by the notation emu (as in "volt emu"). Although these units are defined naturally as part of the CGS system, all of them except the abampere and abcoulomb are either much too large or much too small for most applications. They have been replaced almost completely by the corresponding SI units. Following are the SI equivalents for each of the "ab" units:

Electric current: 1 abampere = 10 amperes
Electric charge: 1 abcoulomb = 10 coulombs
Capacitance: 1 abfarad = 10⁹ farads = 1 gigafarad
Inductance: 1 abhenry = 10^-9 henry = 1 nanohenry
Resistance: 1 abohm = 10^-9 ohm = 1 nanoohm
Conductance: 1 abmho = 10⁹ siemens
Magnetic flux density: 1 abtesla = 10^-4 tesla = 1 gauss
Potential: 1 abvolt = 10^-8 volt = 10 nanovolts
Power: 1 abwatt = 10^-7 watt = 0.1 microwatt
Magnetic flux: 1 abweber = 10^-8 weber = 1 maxwell.

stat-

a prefix indicating that an electrical unit is part of the CGS electrostatic system. These units are also indicated by the notation esu (as in "volt esu"). Although these units are defined naturally as part of the CGS system, they are usually much too large or much too small for most applications. They have been replaced almost completely by the corresponding SI units. Following are the SI equivalents for each of the "stat" units:

Electric current: 1 statampere = 3.3356 x 10^-10 amperes
Electric charge: 1 statcoulomb = 3.3356 x 10^-10 coulombs
Capacitance: 1 statfarad = 1.1126 x 10^-12 farads = 1.1126 picofarads
Inductance: 1 stathenry = 8.9876 x 10¹¹ henrys
Resistance: 1 statohm = 8.9876 x 10¹¹ ohms
Potential: 1 statvolt = 299.79 volts
Power: 1 statwatt = 10^-7 watts

 

John 

 

 

----- Original Message -----

From: "Carroll Hughes" <[EMAIL PROTECTED]>

To: "U.S. Metric Association" <[EMAIL PROTECTED]>

Sent: Sunday, 2002-11-17 11:11

Subject: [USMA:23419] Re: A quote that I want to counter

> > Matthew Zotter wrote:
> >
> > 2002 NOV 17 SUN
> >
> > Once I ask someone, "Why don't we use SI?"
> > Response, "Inch pounds put an american on the moon and dropped the
> > A-bomb on Japan, so they're good enough for me."
> >
> > So does anybody know if the Apollo missions were done in SI or US
> > Customary Units?
> > I know that a lot of Nuclear research is done in SI, but was the
> > Manhattan project in SI?  Was the first bomb made with is mix of SI
> > and inches?
> >
> > Matthew Zotter
>
> There was a major revision to the standards in 1948- as I recall the
> International Ohm was off by about 0.5%.- The international units were
> replaced by the absolute units we have today. The development of the SI
> as a coherent system of units began about this time and was adopted in
> the 1950's. The system did not exist for the Manhattan Project.
> --
>         
>          Carroll Hughes
>