We see transformers all along the roads. Without them it is impossible to make the electricity generated at power plants etc usable for individual houses. A transfomer simply transfers electric energy to our houses with a change in voltage.
A transformer works by electromagnetic induction. It consists of a core (normally made of soft iron or steel) in rectangular or cylindrical form. Two coils of wire (windings) are wound around the core. The coil that is connected to an alternating power source is the primary coil. Alternating current supply produces a constantly varying magnetic field in the primary coil. This varying field induces alternating current in the secondary coil which is connected to the load.
Depending on the number of windings in both the coils, the voltage is determined. For instance, if the primary has 1 and secondary 10 windings, the voltage induced in the secondary coil would be 10 times that of primary. Such transformers are called step up transformers. If on the other hand, if the number of windings in the primary is 10 and that of secondary is 1 then the voltage in the secondary windings is 1/10th of the primary voltage. Such transformers are called step down transformers.
Current generated at power plants is transmitted at high voltages in order to transport them long distances with a minimum loss due to heat. Such high voltages are not suitable for use at the houses. A step-up transformer is used to increase the voltage in the range of say 275 Kilovolts for transportation of electricity and step-down transformers are those seen on the roads that reduce the voltage to 220 Volts. This 220 Volts is the standard voltage in India (110 Volts in the US) necessary for running home appliances. The voltage standard is different in different countries. To run smaller devices like door bells etc, there would be a small step-down transformer put inside the door bell circuit to bring down the voltage to say 12-16 volts at which it operates.
Apart from step-up and step-down transformers, there are many other types of transformers like three phase and single phase transformers, two winding and auto transformer.
When there are heavy rains, lightning, thunder storms or over loads, the transformers may explode. Explosion occurs with a bright arc visible from several blocks away and a loud noise. There would always be a spare transformer which could be put to use until the damaged one gets repaired.