arc sodium What is the meaning of "arc sodium" "A little while later, as they filed us out for morning chow, I glanced into his cell and saw Rita over his bunk in all her swimsuited glory, one hand behind her head, her eyes half-closed, those soft, satiny lips parted. It was over his bunk when he could look at her nights, after lights out, in the glow of the arc sodiums in the exercise yard." (from Rita Hayworth and Shawshank Redemption by Stephen King)