[USMA:22960] Re: 'metric' versus 'SI'

[kilopascal]

2002-10-27

 

Here is an example:

 

Both Gauss (G) and Tesla (T) are 'metric' units.  Both are units of magnetic flux density.  Gauss is a 'cgs metric' unit defined as the 1 maxwell per square centimetre and tesla is an 'mks metric' unit defined as 1 weber per square metre.  1 T = 10 000 G.

 

As you see 'metric' has two distinct units defining magnetic flux density.  Kind of confusing.  In order to rid metric of redundancy SI was created out of mks units and all other 'metric' units not coherent or consistent with the 7 SI base units are deprecated and not recommended for use.  The deprecated units may be metric and have a relationship to an SI unit, but they are not SI. 

 

Gauss is not SI because it is defined from either prefixed units (square centimetre instead of square meter) or non-SI units.  The maxwell really has no comprehensible definition.  It has to be compared to the weber.  The weber can be traced back to base units.  Non-SI metric units, like FFU are random and incoherent.

 

maxwell (Mx)

a CGS unit of magnetic flux, equal to 10^-8 weber. In a magnetic field of strength one gauss, one maxwell is the total flux across a surface of one square centimeter perpendicular to the field. This unit was formerly called the line [2]. The newer name honors the British physicist James Clerk Maxwell (1831-1879), who presented the unified theory of electromagnetism is 1864.

 

line (li) [2]

a former name for the maxwell, the CGS unit of magnetic flux. The unit was called the line because magnetic fields were traditionally represented by lines depicting the direction of the field; the idea was to quantify the strength of these lines. This is a small unit, so fields were often measured in megalines; one megaline is equal to 0.01 weber.

 

weber (Wb)

the SI unit of magnetic flux. "Flux" is the rate (per unit of time) in which something crosses a surface perpendicular to the flow. If the something is a magnetic field, then the magnetic flux across a perpendicular surface is the product of the magnetic flux density, in teslas, and the surface area, in square meters. If a varying magnetic field passes perpendicularly through a circular loop of conducting material, the variation in the field induces a electric potential in the loop. If the flux is changing at a uniform rate of one weber per second, the induced potential is one volt. This means that numerically the flux in webers is equal to the potential, in volts, that would be created by collapsing the field uniformly to zero in one second. One weber is the flux induced in this way by a current varying at the uniform rate of one ampere per second. The weber is a large unit, equal to 10⁸ maxwells, and practical fluxes are usually fractions of one weber. (Because of this, when we want to induce an electric potential in a conductor with a changing field, as we do in all electric generators, transformers and electric motors, we loop the conductor into hundreds of coils, thus adding together the small voltages induced in each loop by the changing field.) The unit honors the German physicist Wilhelm Eduard Weber (1804-1891), one of the early researchers of magnetism.

 

 

John

 

 

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

From: <[EMAIL PROTECTED]>

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

Sent: Sunday, 2002-10-27 10:43

Subject: [USMA:22959] Re: 'metric' versus 'SI'

> [EMAIL PROTECTED] wrote:
> >I'm sure someone will correct me if I'm wrong, but there isn't one
> >(beyond a system based around the metre). This is why SI was
> >developed. The BIPM/CGPM have been the guardians of the metric system,
> >and it became clear that the original simple system had been overtaken
> >by special-interest groups who had formed their own units (albeit
> >metric-based) to suit their own applications which hadn't existed at
> >the genesis of the metre. They therefore determined to review the
> >metric 'system' (which was no longer really a system, in the same way
> >that imperial units do not constitute a system) and SI was the result
> >of the overhaul.
>
> Thanks. So the various metric systems in use in past were a mess and
> CGPM came in and created SI. I am still confused about how other people
> are interpreting the definition of 'modern form of the metric system'. I
> appear to be the only person who reads it one particular way.
>
> "the International System of Units (SI), which is the modern form of the
> metric system."
>
> Does it mean that
> 'modern form of the metric system' = 'SI'
> ?
>
>
> --
> Terry Simpson
> Human Factors Consultant
> [EMAIL PROTECTED]
> www.connected-systems.com
> Phone: +44 7850 511794
>
>
>