The figure shows the schematic of an absolute electrometer. The potential difference that is to be measured is applied between the plates `1 and 2`, with the upper plate connected to one arm of a balance beam.* The pan connected to the other arm is loaded with weights until balance is achieved, that is, when the upper plate begins to move upward. In this way the force acting between the charged plates is measured, and this enables one to determine the magnitude of the potential di fference between the plates. It the aquiltbrium in the electrometer stable or unstable? • The figure does not show the protecting rings around plates 1 and 2 with the same potentials. Theso are used to ensure that the field is as uniform as possible