Any liquid which is under pressure will have the ability to flow. If the rate of the flow is detected by a meter, then the quantity of liquid proceeding past can be measured. The water meter operates in a similar way to the rotary pump, but in reverse. As the water streams through the meter, it causes the blades of an impeller to rotate. The impeller has a shaft which revolves a worm gear, and in turn decreases the velocity of the impeller. Groups of gears are then activated which turn a pointer and the counters, (a series of toothed drums) in order to register the total amount of water used.
This value is periodically `read' in the same manner as gas and electricity meters for billing purposes. Whereas most homes have a gas and electricity meter, the use of water meters at the moment is still fairly uncommon.
