I seem to recall someone doing a study on these a while back but I can't remember... Anyways, I just did some experiments with wands of lightning and rings/[[cloaks ]] of electricity resistance. I was bouncing lightning bolts off walls two spaces away from myself. I found some interesting things...
With no resistance I consistantly consistently took 18-19 damage per shot.
With 15 resistance (one ring) there was a dramatic, but irregular drop -- damage taken now ranged from 1 to 17, tending to be closer to 8 or 9 points.
With 30 resistance (two rings) I took nearly no damage. I spent probably 16 shots, and took only 3 points from the first one and 1 point from a later one. Other than that I took no damage at all.