A device used to ensure uniform fields and to define the sensitive volume in absolute electrometers and standard capacitors. It consists of a metal plate surrounding and coplanar with a smaller plate; a narrow air gap separates the two. The diagram shows the lines of force between a plate A surrounded by a guard ring RR, and a parallel earthed plate P. Variations in field at the edges affect only the guard ring RR.

A similar device in the form of an auxiliary electrode is commonly used in semiconductor devices and vacuum tubes.

Principle of guard ring