Abstract: A circuit breaker includes a tripping device in which a magnetic core is contained in a vessel of viscous fluid in a magnetic frame. An armature is disposed adjacent the core and is connected to a toggle mechanism. Under conditions of moderate overloads, the armature and the core ineract and the tripping device provides a time delay before it operates. Under severe fault conditions the armature is activated directly and the effect of the core is of less significance. The core or the armature are tapered, to provide a desired balance between the delayed and instantaneous operating characteristics.