# Induction and Inductance

### After reading this section you will be able to do the following:

- Explain how current can be induced in a conductor without making contact.
- Describe the process of induction.

## Induction

In 1824, Oersted discovered that current passing though a coil created a **magnetic field** capable of shifting a compass needle. Seven years later, Faraday and Henry discovered just the opposite. They noticed that a moving magnetic field would induce current in an electrical conductor. This process of generating electrical current in a conductor by placing the conductor in a changing magnetic field is called **electromagnetic induction** or just **induction**. It is called induction because the current is said to be *induced* in the conductor by the magnetic field.

Induction is measured in unit of **Henries (H)**.

## Inductance

When induction occurs in an electrical circuit and affects the flow of electricity it is called **inductance ( L)**.

**Self-inductance**, or simply inductance, is the property of a circuit whereby a change in current causes a change in voltage in the same circuit. When one circuit induces current flow in a second nearby circuit, it is known as

**mutual-inductance**. The image to the right shows an example of mutual-inductance. When an AC current is flowing through a piece of wire in a circuit, an electromagnetic field is produced that is constantly growing and shrinking and changing direction due to the constantly changing current in the wire. This changing magnetic field will induce electrical current in another wire or circuit that is brought close to the wire in the primary circuit. The current in the second wire will also be AC and in fact will look very similar to the current flowing in the first wire. An electrical transformer uses inductance to change the voltage of electricity into a different voltage level. In nondestructive testing, inductance is used to generate eddy currents in the test piece.

It should be noted that since it is the changing magnetic field that is responsible for inductance, it is only present in AC circuits. High frequency AC will result in greater **inductive reactance** since the magnetic field is changing more rapidly.

Self-inductance and mutual-inductance will be discussed in more detail in the following pages.

### Review

- The process of generating current in a conductor by placing it in a changing magnetic field is called
**induction**. - Inductance is only present in AC circuits.