Abstract: This electric circuit breaker contains electromagnetic-assist means for opposing the contact-repulsion forces developed when a high current flows through the breaker. The assist means comprises a rigid supporting member which can be restrained in a fixed position to render the assist means capable of transmitting closing force to the movable contact of the breaker but which supporting member is releasable to render said assist means generally ineffective to transmit closing force to said movable contact. The assist means further comprises a toggle connected between the supporting member and the movable contact for imparting a closing force to the movable contact when forced toward an in-line position while said supporting member is restrained.For forcing the toggle toward its in-line position, there is provided magnetic structure cooperating with a conductor connected in series with the circuit breaker contacts.