Originally Posted by WorldWind
The barrel adjuster does not adjust the cable at all, it changes the effective length of the cable housing that the cable moves in. .
You've got a good point but in reality it does effect tension because adjusting the derailleur with the barrel adjustor depresses or releases the derailleur spring. If the derailleur is adjusted all of the way to the low limit stop the cable becomes slack. With the derailleur in the big cog position there is more tension on the cable than when it is in the small cog position because the derailleur spring is fully depressed.
Al