in order to provide a pipette device which is easier and more accurate to use than those using known pipette fillers , a piston , which is preferably flexible , is provided to fit into the pipette bore . the plastics piston rod sits conveniently in a guide cap at the mouth of the pipette tube and the piston rod is operated by stroking its upper surface , which is serrated along its length , with a digit ; thus dispensing by a positive displacement method .