Transformers are devices that transfer a voltage from one circuit to another circuit via induction. To induce a voltage in the second circuit, the first circuit usually uses alternating current (AC) which, unlike direct current (DC), constantly reverses direction, thereby causing the magnetic flux it generates to constantly shift. Transformers vary in size and design, but in their basic form they consist of two coils of wire, sometimes sharing a single iron core. The coil directly linked to the power supply is the primary coil; the other, in which a current is induced, is termed the secondary coil. Generally, the primary and secondary voltages should be equivalent (ignoring heat losses) if the number of turns in each coil is the same. But if the secondary coil has fewer coils than the primary, it will take on less voltage. (In fact, the main use of transformers is to channel electricity of high voltage to a circuit in which a lower voltage is needed.) Other factors may affect the voltage induced in the secondary coil, as demonstrated in this tutorial.
Presented in the tutorial is a simple vertical transformer using two separate coils with iron cores inside. The coil on the left is linked in a circuit to an AC power supply, rendering it the primary. The other coil, linked to a small light bulb, is the secondary. When the applet initializes, only a short distance filled with air divides the coils. The magnetic field of the primary coil, reversing directions dozens of times a second, easily generates enough voltage in the secondary coil to power the light bulb. To see the magnetic fields, click the Show Magnetic Fields boxes (the fluctuation of these lines isn't depicted). Again, even though there is no physical contact between the coils, the alternating magnetic field from the primary coil induces current in the secondary. Varying the distance between the coils, as well as changing the medium between them, can affect the inductance of the coils, however. These properties can be altered by moving the Separation slider and selecting different options from the Medium pull-down menu.
Notice that the magnetic field of the primary coil passes through Water and Air, inducing a current sufficient to light the bulb in the secondary coil when the two coils are close. But increasing their separation dims and eventually turns off the light. This is because when the circuits are too far apart, too little (if any) of the flux generated by the first coil is able to reach the second coil, resulting in poor inductance and insufficient voltage to power the bulb. When a plate of Steel divides the coils, no magnetic field lines reach the second coil, and the bulb will not light up. This illustrates a property called permeability. Steel is highly permeable to magnetic flux– much more so than air. So the magnetic field lines, as they emanate from north to south poles, prefer to travel through the steel, and avoid the air. Due to this property of steel, it is often used to shield, or shunt, stray magnetic field generated by strong magnets from surrounding areas.