[ARC5] The International System of Units (SI)

[Bruce Long]

While we are bitcing about convention, who ever thought naming  permittivity and permeablity or enthalpy and entropy where good ideas?
  


      From: J Mcvey via ARC5 <arc5 at mailman.qth.net>
 To: Roy Morgan <k1lky68 at gmail.com>; Leslie Smith <vk2bcu at operamail.com> 
Cc: ARC-5 List <arc5 at mailman.qth.net>
 Sent: Sunday, February 14, 2016 9:54 AM
 Subject: Re: [ARC5] The International System of Units (SI)
   
That's all well and good, but who has a greek character font that they can readily inject into their text? Thus the convention of using "u" for mu?I never had a problem understanding the usage in that context. It's better and less confusing than m which is more universally used to denote milli. 
Since that was the case, why did they ever use mfd for microfarads when ufd kind of looks like mu and is now universally understood as such?
Where you can get tripped up with mHz (milli hertz) vs MHz (megahertz) , but that too can usually be figured out in the context.
There are lots of backwards conventions, like current flow diagrams,etc, but at the end of the day, it does the same thing so it doesn't matter. 

New or old way.I can get through it OK.



 

    On Sunday, February 14, 2016 8:42 AM, Roy Morgan <k1lky68 at gmail.com> wrote:
 

 
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

______________________________________________________________
ARC5 mailing list
Home: http://mailman.qth.net/mailman/listinfo/arc5
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:ARC5 at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

   
______________________________________________________________
ARC5 mailing list
Home: http://mailman.qth.net/mailman/listinfo/arc5
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:ARC5 at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

  
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20160214/8d190fbe/attachment-0001.html>