Ablepinto,
The system works as a simple battery by using the chlorine in the water to oxidise a metal electrode. The ammount of current it generates (mA) is proportional to the amount of chlorine in the water (more amps, more chlorine). I have used Deplox units maufactured by Wallace and Tiernan. They use a platinum electrode to measure the chlorine with a built in refference electrode to correct the result for changes in pH.
I hope this is usefull.
Paul