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

Inductive Reactance

Like resistance, reactance slows down an electrical current. This phenomenon occurs only in AC circuits.

When an electric current flows through a circuit, it is opposed by two things: resistance and reactance.

Resistance happens when the electrons that carry the current collide with each other, with the conductor material, or with a resistor built into the circuit.

Resistance is found in direct current (DC) and alternating current (AC).

Reactance is associated only with alternating current. Inductive reactance opposes electric current using an induced magnetic field, as shown here.


  1. Note the setup of the circuit including a power source, a copper coil with an iron core, and a light bank. Use the on/off button to control the knife switch and power the circuit.
  2. Start with DC power and see how the light bank illuminates. Use the slider to adjust the position of the iron core inside the copper coil. Note how its position does not affect the lights.
  3. Now switch to AC power. See how the lights dim as you slide the iron core into the coil.

Why is this? The current-carrying wire in the coil generates a magnetic field.

When the current changes direction under AC power, the magnetic field also changes direction.

This changing field creates a voltage which opposes the direction of the primary current.

This magnetic field is amplified by the presence of the iron core in the coil, limiting the current getting to the lamps.

The farther inside the coil the core penetrates, the greater the magnetic field produced, the greater the reactive inductance, and the more the lamps are dimmed.