State the underlying principle of a transformer. How is the large scale transmission of electric energy over long distances done with the use of transformers?  

The principle of transformer is based upon the principle of mutual induction which states that due to continuous change in the current in the primary coil an emf gets induced across the secondary coil. At the power generating station, the step up transformers step up the output voltage which reduces the current through the cables and hence reduce resistive power loss. Then, at the consumer end, a step down transformer steps down the voltage. Therefore, the large scale transmission of electric energy over long distances is done with the uses of transformers.