A transformer transform
Right Answer is:
Voltage & current
A transformer is a static device that is used to convert the voltage or current to a higher (step-up) or lower (step down) level.
In general, an electrical transformer is one that transfers electrical variables (voltage, current) on one side of the transformer to the other side, with the unique feature of not having a conductive connection between the two sides (except in an ‘auto transformer’).
The transformer, transforms the level of voltage and or current on one side, to different levels on the other side.
Depending upon the variables of transformation, transformers may transfer power or transform only voltage or current. The former is called Power Transformer and the latter is called Instrument Transformer.
The Distribution Transformer, Auto Transformer, and Tap changing Transformer basically belong to the family of Power Transformer with respective unique features.
In an Instrument Transformer, voltage or current on one side of the transformer, comparatively high (therefore not suitable for direct measurement), is scaled down to voltage or current to levels that are convenient for measurement with suitable instrumentation.
Some applications may call for constant current (as in welding) or constant voltage. Such transformers am called Constant Current and Constant Voltage Transformers.