A transformer is a very common and efficient electric machine. Huge transformers are used in power houses while smaller ones are used in radio sets. A transformer is used mainly to increase or decrease the voltage of an alternating current. The basic principle on which transformer works is that when electric current passes through a coil of wire it behaves like a magnet. Similarly, as long as a wire keeps cutting a magnetic field, an electric current is generated in it. The product of voltage and current remains more or less constant. It is called ‘power’. When voltage increases, current decreases and vice versa. In alternating currents of voltages, currents and magnetic fields keep changing many times in a second. It is because of this reason that a transformer works only on alternating currents. This whole phenomenon is known as ‘electromagnetic induction’. A transformer, usually, consists of a laminated core of soft iron with two coils wound around it. A sheet of paper or a coating of varnish insulates each layer from the next. The number of turns of wire in the two coils is different. We give alternating power to one coil, which is called the ‘primary coil’. And we take electric power from the other coil, which is called the ‘secondary coil’. When the number of turns of wire in the secondary coil is more than that in the primary, we get a higher voltage. Such a transformer is called a ‘step-up transformer’. On the other hand, when the number of turns in the secondary coil is less, we get a lower voltage. This type of transformer is called as a ‘step-down transformer’. The same transformer can be made to function either as a step-up or as a step-down transformer. Steppingup or stepping-down depends on the ratio of the turns of the coils. In a step-up transformer voltage in the output increases but the current decreases. In a step-down transformer voltage in the output decreases but current increases. There are many types of transformers and they are used for different purposes. They handle power from a fraction of a watt to millions of watts. These transformers differ greatly in construction, materials, size and power handling capacity. Transformers are used to produce electricity at a very high voltage for long distance transmission. This voltage is then lowered through many transformers before it reaches our homes. A transformer cannot increase or decrease a direct voltage because it cannot produce induced voltage.