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The National MagLab is funded by the National Science Foundation and the State of Florida.


Transformers are devices that transfer a voltage from one circuit to another circuit via induction.

Transformers vary in size and design, but in their basic form they consist of two coils of wire.

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. The main use of transformers is to channel electricity of high voltage to a circuit in which a lower voltage is needed.


  1. Take a look at the simple vertical transformer. It uses two 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.
  2. Only a short distance filled with air divides the coils. The magnetic field of the primary coil, reversing directions dozens of times a second, generates enough voltage in the secondary coil to power the light bulb.
  3. To see the magnetic fields for either, click on the button for primary coil or secondary coil.
  4. arying the distance between the coils, as well as changing the medium between them, can affect the inductance of the coils. These properties can be altered by moving the separation slider and selecting different options from the medium pull-down menu.

Notice that increasing the separation between the coils 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. You’ll see this if you leave the field lines on.

Notice also that changing the medium from air to water does not impact the inductance.

But 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 or water. So the magnetic field lines, as they emanate from north to south poles, prefer to travel through the steel, and avoid the air.