Reservoir seal for writing instrument

Claim 1

THE COMBINATION OF A TUBULAR LIQUID RESERVOIR AND A SEALING MEMBER COMPRISING TWO PISTON ELEMENTS EACH HAVING A PERIPHERAL SURFACE WHICH FORMS A CLOSE SLIDING FIT IN THE TUBULAR RESERVOIR TO PROVIDE AN ANNULAR CLEARANCE OF CAPILLARY DIMENSIONS WITH THE RESERVOIR WALL AND WHICH TERMINATES IN A RELATIVELY SHARP CORNER AT THE END ADJACENT TO THE OTHER PISTON ELEMENT, MEANS SPACED FROM THE RESERVOIR WALL AT A SUBSTANTIALLY GREATER DISTANCE THAN THE PISTON ELEMENTS AND JOINING THE TWO PISTON ELEMENTS TO HOLD THEM IN SPACED RELATION, A BODY OF LIQUID CONTAINED IN THE RESERVOIR EXTENDING INTO THE ANNULAR CLEARANCE SURROUNDING A FIRST OF THE TWO PISTON ELEMENTS, A SECOND BODY OF LIQUID SEPARATED FROM THE RESERVOIR LIQUID EXTENDING INTO THE ANNULAR CLEARANCE SURROUNDING THE SECOND OF THE TWO PISTON ELEMENTS, A BODY OF GAS TRAPPED IN THE SPACE BETWEEN THE TWO PISTON ELEMENTS COMPLETELY SEPARATING THE TWO BODIES OF LIQUID, ONE OF THE PISTON ELEMENTS INCLUDING AN EXPANSION DUCT PORTION ADJACENT TO THE SPACE BETWEEN THE TWO ELEMENTS, THE EXPANSION DUCT PORTION INCLUDING EXPANSION DUCT MEANS FORMING A LIQUID-SEALED DUCT OF CAPILLARY DIMENSIONS COMMUNICATING DIRECTLY WITH THE SPACE BETWEEN THE PISTON ELEMENTS, THE MEANS FORMING THE DUCT BEING SPACED FROM THE RESERVOIR WALL THROUGHOUT ITS LENGTH BY A DISTANCE GREATER THAN SAID ANNULAR CLEARANCE BETWEEN THE PISTON ELEMENTS AND THE RESERVOIR WALL, THEREBY PERMITTING EXPANSION OR CONTRACTION OF THE TRAPPED BODY OF GAS WHILE MAINTAINING AN ANNULUS OF LIQUID IN THE SPACE SURROUNDING EACH PISTON ELEMENT.