The easiest way to change the output of a transformer is by what method?

Changing the number of turns in the windings of a transformer directly alters the transformer's output voltage. This is based on the principle of electromagnetic induction, where the voltage output is proportional to the ratio of the number of turns in the primary and secondary windings. By increasing or decreasing the number of turns in either winding, you can effectively modify the voltage ratio, thus changing the output voltage.

This method is fundamental because it allows for precise control over the transformer's performance characteristics. If you need a specific output voltage, adjusting the turns can be done relatively easily during the manufacturing process or during a rewind, making it a straightforward method for output voltage change compared to other methods that might be less efficient or more complex.

The other options involve methods that either do not provide direct control over the output voltage or require more complex adjustments which might not be as effective or feasible in all scenarios.