Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!brutus.cs.uiuc.edu!apple!agate!shelby!decwrl!jumbo!dillon
From: dillon@jumbo.dec.com (John Dillon)
Newsgroups: sci.electronics
Subject: Re: Tesla vs gauss, and other obscure units
Message-ID: <14184@jumbo.dec.com>
Date: 31 Oct 89 17:49:34 GMT
References: <30339@buckaroo.mips.COM> <1914@sactoh0.UUCP> <5817@shlump.nac.dec.com>
Organization: DEC Systems Research Center, Palo Alto
Lines: 44

In article <5817@shlump.nac.dec.com>, koning@koning.dec.com (Paul Koning) writes:
> 
> Tesla is the SI unit, gauss the (officially obsolete) unit from the
> CGS system.  Why is it still used?  Partly because we're in the USA,
> where the metric system is only barely understood.  Partly because
> engineers and amateurs tend not to care much about consistency in
> units.

Ahh, unit bigotry!  Just what we need.  The author suggests that 
SI units are the answer regardless of the question.

It is true that several systems of units carry an enormous burden 
of historical chaos, but the CGS system is NOT one of them.  In fact, 
were it not for the burden of real-life test instruments calibrated 
in volts and amps, a person seriously schooled in classical 
electrodynamics would choose to work in either the CGS or 
Heaviside-Lorentz systems*.  If you wish to explore this further, 
I would suggest reading J.D. Jackson's "Classical Electrodynamics", 
which has an excellent appendix on units and dimensions.

> I still remember the grief I caught from my Physics prof. because I
> had the nerve to write up a lab report on gamma ray experiments with
> the energies listed in fJ (femtojoules) rather than his pet MeV 
> (megaelectronvolts, a mishmash if ever I saw one).

This makes as much sense as complaining about astronomers' use of 
parsecs and light-years.  The use of electron-volt (eV) and its 
derivatives in this assignment gives the student familiarity and 
appreciation of it's utility.  Ignoring the utility misses the point.  
That utility is the same as the real-world utility of SI units.

Sure, many folks have amused themselves by computing plate tectonic 
movement in mils per fortnight or microchip die area in femto-acres.  
To be saddled with systems of units where this a constant necessity 
would be a horror.  But even more horrible would be an educational 
system which did not teach students to cope with the various units 
found in the real world, or to take advantage of specialized units 
where practical.

-- John

* extra credit question: do you know why the vacuum permeability 
in rationalized MKSA (a subset of SI) is exactly 4*pi*1e-7 ?  If not, 
please direct flames to /dev/null.