RE: UNITS

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Jim Lux <jimlux-at-earthlink-dot-net> 

At 05:05 PM 2/2/2004 -0700, you wrote:
>Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com>
> >Because we can walk right into a motor rewind shop and get wire with 0.01mm
> >difference.
>
>You can't, not in Britain anyway :( The wire here is labelled in millimetre
>gauges, but the range of sizes available is just the same as the old British
>Standard Wire Gauge. So you get 0.315 and 0.125mm wire, etc.
>
> >In the SI system, all of the "Garbage"
> >is gathered into ľo, thereby simplifying the SI equations.
>
>The SI system has other oddities, like the "Impedance of free space". I
>never understood how thin air could have an impedance of 377 ohms. In fact,
>since my DMM doesn't read 377 ohms when the prods are held up in the air, I
>suspect it's just another mathematical fudge ;)

Impedance of free space..
Impedance, in this sense, applies to a travelling wave of some sort (as 
opposed to DC resistance).. It's more like the characteristic impedance of 
a coaxial cable (hook an ohmmeter across some coax, and you read infinite...)

Impedance is in ohms, because it's the ratio  of voltage and 
current:  Z=E/I, or, more properly, in the case of a propagating 
electromagnetic wave, the ratio of the electric field and magnetic field.

As for where the value of 377 comes from, it has to do with the ratio of 
the permittivity of free space - (usually epsilon sub zero) (having to do 
with electric fields) and with the permeability of free space  (usually mu 
sub zero) (having to do with magnetic fields).  In turn, those are related 
by the propagation velocity of the wave (which is the speed of light, some 
3E8 meters/sec).  377 ohms is no more a fudge than the fact that the speed 
of light happens to be 300 million meters/sec, etc... it's just falls out 
of the units chosen.  The meter happens to be approximately 1/10,000,000 of 
the distance from the equator to the pole, and the second happens to be 
about one heart beat.

One could just as easily choose other units for voltage, current, length, 
etc.  Furlongs per fortnight (=0.166 mm/sec) has a persistent popularity 
(and oddly, would be a decently scaled unit for talking about the speed of 
the current Mars rovers which zoom about at a bustling 5 cm/sec (300 f/f))

I have no idea where the Volt came from, or the Ampere, for that matter. I 
suspect they were totally arbitrary (although, with dimensional analysis, 
you can turn volts (or amps) into some combination of mass, length, time, 
and charge).  Most single electrochemical reactions put out about a volt, 
so that set the basic order of magnitude.  Amps are probably similar.