Abstract: An electromagnetic circuit breaker is provided having a flux producing solenoid and a movable armature attractable to the solenoid upon the occurrence of a predetermined current level and duration. A flux path in the form of members of low reluctance are arranged so that the flux circuit comprises a plurality of loops, each loop having one or more variable reluctances which are dependent upon the position of the armature and a movable core located within the solenoid. Increased pulse resistance is obtained without substantially effecting the operation of the switch upon sustained overloads of a predetermined level and duration. The bobbin on which the solenoid coil is wound is provided with a groove with which a member of low reluctance is lockingly engaged. Other low reluctance members are configured to facilitate assembly of the circuit breaker.