Permanent magnets
Tungsten steel, and alloys of iron, nickel, aluminium, cobalt, ceramic, and titanium are used. Iron oxides can also be used.The magnetic field surrounding a coil does not appear immediately the circuit is connected. It takes time to grow from nothing at the moment of switch-on to its maximum. The time taken for this depends on many factors, including the number of turns in the coil, the current, the core material, and the self-resistance of the coil. Similarly, when the current is switched off the field takes time to decay. It should be noted that the current in the coil takes time to rise to its maximum. This must be compared to the capacitor where at switch-on, the voltage across the capacitor takes time to rise to its maximum