CREATING A TRANSFORMER
What is a transformer composed of?
As you just read in the last page, it is common to wrap the wire into a coil to concentrate the strength of the magnetic field at the ends of the coil. Wrapping the coil around an iron bar will further concentrate the magnetic field in the iron bar. The magnetic field will be strongest inside the bar and at its ends (poles).
If we were to take this concept one step further and wind the second wire into a coil on the same iron bar as the first coil, we would create the strongest magnetic coupling of the two wires. When an AC current is flowing in one of the coils, a similar current is induced into the second coil. Even though no direct electrical connection exists between the two coils, we can induce electrical current in this manner. We often use this arrangement of coils to take the electrical current flowing in the first coil and change it in someway that is more useful for doing work. This is what is called a transformer.
As we can see in the above experiment, transformers have at least two windings or coils. One is called the primary, the other the secondary. The primary coil is where AC current is fed in. The secondary coil is where the current is induced to perform some sort of transfer of energy. In this case the current is used to light a light bulb. There are many types of transformers in existence. This is a very simple example. The iron bar core helps to transfer more of the magnetic energy from the primary coil to the secondary coil.
How does mutual induction work?
The secondary coil is also generating a magnetic field that is growing and shrinking just like the field in the primary coil. This coupling of magnetic energy between these two coils is called mutual induction. Mutual induction describes the fact that these two coils share the magnetic lines of force that are being generated by both coils. In other words, both coils are being affected by each others induced magnetic fields. The results of this mutual coupling can be quite complex. We will only deal with this concept as it applies to nondestructive testing principles.