A Bottle top dispenser is a lab instrument with a piston mounted on the plunger. The Piston creates the required vacuum in the barrel to be able to aspirate and dispense liquids precisely. By an upward movement of the piston, the preset amount of liquid draws up into the dispenser cylinder. Consequently, the downward movement of the piston enables the release of the liquid into the container via a dispensing valve. 

Since the piston comes into direct contact with a liquid or chemical, it is important that the piston material be chemically inert to most reagents. 

Bottle top dispensers eliminate the need to set up a meniscus, which helps speed up the liquid transfer process.