I TRIED TO MAKE A CAPACITOR BY WRAPPING A 70 MM DIA BOROSILACATE GLASS TUBE, 2 MM THICK. I WRAPPED 50 mm WIDE AL FOIL ON THE OUTER SURFACE AS WELL AS 50 mm WIDE AL FOIL ON THE INNER SURFACE OF THE GLASS TUBE. I EXPECTED A CAPACITANCE VALUE IN THE RANGE OF ABOUT 20 nano F and while I got about 250 pico F only. I tried many tricks to check whether the aluminum foil was oxidised , thus resulting in poor cotact. I tried making capacitor using copper foil etc. But everytime I found capacitance vaue was just about 1 to 2 % of the value computed using standard formula for C given in text books. I think Borosilicate has relative dielectric ocnstant of about 4.5 to 5. Where did I go wrorng? Is simple wrapping of a foil sufficient or should the glass be coated to avoid thin layer of air?
How can I make a capacitor which has avlue close to what is given by the standard formula?