There is a pressure sensitive governor at the top of the drinker with a hose that connects the drinker to a water source.  In my case that is a small water tank running off the chicken shed.  The drinker is suspended by a cord so that its weight is entirely carried by the pressure sensitive governor. There is a shallow circular trough at the base of the drinker designed to hold water. 

As water flows via the connecting hose into the circular trough the drinker gets heavier, which places pressure on the pressure sensitive governor.  When the weight is heavy enough it completely shuts off the inflow.  As chickens drink the water the drinker gets lighter, which intern causes the governor to release more water.  The water level in the drinker can be adjusted by adjusting the pressure sensitive governor. 

The specifications say that this hanging drinker can provide enough water for up to twenty five adult chickens, which is more than enough for any backyard chicken run.

Maintenance is minimal.  There is a filter inside the drinker that needs cleaning every couple of months.  It also benefits from an occasional scrubdown.

• Low maintenance and automatic.
   
• Offers cleaner water than an open bowl.
   
• Relatively cheap.
   
  
• Robust, I have had mine for some ten years now and it is still going strong.
  
• Easy  to see if water is flowing into the drinker.
  

• If sparrows are allowed to perch on the hook holding the drinker up there is a likelihood that they will defecate into the water.  (I got around this problem by putting a floppy lid on top of the hook that is difficult for birds to perch on.)
   
  
• As it still only offers a single water source and the amount of water available in the trough is small the chickens would run out of water quite quickly if there was a blockage.  (I get by this by filling a temporary container of water as a backup whenever I go away for more than a day)