Sentence:  Liter quantities of Mallinckrodt , low sulfur , reagent grade carbon tetrachloride were saturated with * * f and * * f and illuminated for about 50 hours with a 1000 <WSD>watt</WSD> tungsten lamp at a distance of a few inches . 
Meaning of the word marked with <WSD> tags:  a unit of power equal to 1 joule per second; the power dissipated by a current of 1 ampere flowing across a resistance of 1 ohm