[ARC5] The International System of Units (SI)

[Roy Morgan]

On Feb 14, 2016, at 5:50 AM, Leslie Smith <vk2bcu at operamail.com> wrote:

Re: [ARC5] The BC-221 low frequency tank circuit puzzle.

> ... - the 10.4 uH vs 10.4mH difference nicely illustrates one of
> my pet peeves.  This is  a failure to distinguish corretly between unit
> designators.   By this I refer to those pesky prefixes - such as "u"
> (properly mu, not "u") or 10e-6 and "m" (milli, one one-thousandth),
> Mega (x10e6)  and so on.  

(From an earlier post I made on the Collins list, and expanded for this post):

> …  By the way GHz is spelled GHz and not Ghz! 

For the authoritative standard way to use abbreviations of this sort, see:

http://www.nist.gov/pml/div684/fcdc/si-units.cfm
International System of Units (SI)
"The International System of Units (SI) provides definitions of units of measurement that are widely accepted in science and technology …”

Guides to the SI:
...
A practical description of the SI is Guide for the Use of the International System of Units (SI), 2008 ed. (U.S. Government Printing Office, Washington, DC, 2008) [NIST Special Publication 811].
http://www.nist.gov/pml/div684/fcdc/upload/sp811.pdf

Wherein you will find:

Table 3. The 22 SI coherent derived units with special names and symbols.

Derived quantity   Special name   Special symbol
...
frequency   hertz   Hz

(Note: Table 3 contains many of the units named after famous persons such as Newton and Pascal.  I was under the impression that most, but not all symbols for such units are capitalized but that at least one was not.  I cannot find that example however. The “special names” in Table 3 are not capitalized except for “degree Celsius".)

and 

Decimal multiples and submultiples of SI units: SI prefixes
Table 5.  SI prefixes

Factor Prefix Symbol
…
10 ^ 9 = (10^3)^3  giga  G
and
10^-6 = (10^3)^-2 micro m(in the table this character is greek letter lower case mu)

I searched for the abbreviation “mu” that we associate with permeability and found
"Certain quantities, such as refractive index, relative permeability, and mass fraction, are defined
as the ratio of two mutually comparable quantities and thus are of dimension one”

7.10.1 Decimal multiples and submultiples of the unit one
Because SI prefix symbols cannot be attached to the unit one (see Sec. 6.2.6), powers of 10 are
used to express decimal multiples and submultiples of the unit one.

Example : mr = 1.2 3 1026 but not : mr = 1.2 m (the “1026” is not correctly rendered here_
Note : mr is the quantity symbol for relative permeability.

(In the document the “mr” above appears as the greek letter lower case mu with the subscript of the greek letter capital tau- I think)

Roy
Retired NIST employee

Roy Morgan
k1lky68 at gmail.com
K1LKY Since 1958