how do u measure the increase or decrease of transformers and do they work for only dc or only ac or both ?

Answer 1

Transformers either step up or step down voltage. So the ratio of the output voltage to the input voltage usually measures how much voltage is stepped up or down. This is usually the same as the ratio of the number of turns of the coil winding on the output side to the number of turns of coil winding on the input side.

Transformers are AC components. There are DC transformers, but they don't work in quite the same way as the AC ones do.