How can we change the magnitude …

If you have an inductance in the circuit the current will rise until the emf generated by the inductance equals the applied voltage. The current will then continue at a rate determined by the circuit resistance. The current may be varied by varying the input voltage, or by varying the resistance of the circuit. In order to lower the amperage of an electrical circuit, you must either lower the circuit's voltage or increase its resistance. Lowering amperage is done by applying Ohm's law, given by the formula I = V/R, where I is the circuit's total current in amperes, V is the voltage and R is the resistance. If too much current flows, the fuse wire rises to a higher temperature and melts, thus opening the circuit, but destroying the fuse. A circuit breaker opens a circuit as soon as the current climbs to unsafe levels, and can be used repeatedly.