A magnetic breaker has a coil through which the load current passes. The core of the coil is a sealed cylinder containing a ferrous piston and a spring in hydraulic fluid. An over-current through the coil generates enough magnetic flux to cause the piston to begin moving toward the cylinder cap. This, in turn, attracts the armature toward the cylinder cap and trips a mechanism that causes the contacts to snap open, breaking the circuit. By varying the viscosity of the fluid in the cylinder, the manufacturer can vary the amount of time that passes before the breaker trips. This permits brief over-currents to occur without tripping the breaker, such as the inrush when starting a motor, to occur without tripping the breaker. The inrush over-current ends before the breaker trips. In the case of relatively high over-currents, there is no delay, as the coil generates enough magnetic flux to attract the armature without the piston needing to move to the top of the cylinder.