guard ring

(electricity) A ring-shaped auxiliary electrode surrounding one of the plates of a parallel-plate capacitor to reduce edge effects.
(electronics) A ring-shaped auxiliary electrode used in an electron tube or other device to modify the electric field or reduce insulator leakage; in a counter tube or ionization chamber a guard ring may also serve to define the sensitive volume.
(thermodynamics) A device used in heat flow experiments to ensure an even distribution of heat, consisting of a ring that surrounds the specimen and is made of a similar material.