The transformer core of the transformer is usually a complete ring

When an electric current is passed through a long, hollow coil, a strong magnetic field will exist inside the coil and a weaker field outside the coil. The lines of the magnetic field pattern go through the coil, unwind from the end, around the outside and out the other end. These are not real lines like a pencil drawing. They are lines we imagine, as shown in the sketch, to show the pattern of the magnetic field: the direction in which the iron sample is magnetized by the magnetic field. Where the field is strong, the lines are dense. With an empty coil, the lines form a complete loop. If there was an iron core in the coil, it would be magnetized and would appear to make the magnetic field strong when the current was on.

The transformer core of a transformer is usually a complete ring with two coils wound around it. One is connected to the power source and is called the "primary coil"; the other supplies power to the load and is called the "secondary coil". The magnetization caused by the current in the primary coil extends all the way around the ring. The primary and secondary coils can be wound in place on the ring because the iron transfers the change in magnetization from one coil to the other. There is no electrical connection between the two coils. However, they are connected by the magnetic field in the core.

When there is a steady current in the primary, the secondary current has no effect, but if there is a change in the current in the primary, then the secondary current has an effect. A change in current in the primary produces an emf in the secondary circuit. If the secondary is connected to the circuit, then there is a current. The Transformer Core itself is a thick secondary (like a coil of wire with one turn), and a change in the primary current induces a small circular voltage in the core. Therefore, the core is made of thin sheets sandwiched together, and the surface of each sheet is coated with a conductor to make it a poor conductor. The edges of the plates can be seen by looking at the edges of thetransformer core.