It seems that your hand is the source of negative charge and the metal plate is the source of positive charge. Another source of negative charge is the plastic plate, in fact is has much more negative charge. Why can't you put the leyden jar on the plastic plate? The outside still gets negatively charged by the plastic plate and the ball still gets positively charged by the metal plate...
@Heads Mess Why holding the outside of the jar , removing electron of the disc ? I think the electron is removed by touching the disc. What happened to electrons inside the jar ?
A capacitor, especially the size of the one in this video, can hold a lot of energy. A charged capacitor can be very dangerous. You could easily have the equivalent discharge as a taser. In a defibrillator, the energy is stored on a capacitor, www.anaesthesiauk.com/documents/4_1_29.pdf
