Abstract: Improvement in a circuit breaker employing a fixed pole piece associated with a movable magnetic core within a tube containing damping fluid wherein the side walls of the tube are transparent in order to facilitate the inspection of internal components. Calibrations can be provided on the side walls as well. Additionally, the tube is preferably made of resinous materials which is heat sealable to the end cap carrying the pole piece to which the breaker tripping armature is attracted by overload sensed by a coil surrounding the movable core and upon overload causing the armature to move to the pole piece and open the circuit breaker contact.