Step-down transformers

If the first coil has more turns that the second coil, the secondary voltage is smaller than the primary voltage:

This is called a step-down transformer. If the second coil has half as many turns as the first coil, the secondary voltage will be half the size of the primary voltage; if the second coil has one tenth as many turns, it has one tenth the voltage. In general:

Secondary voltage ÷ Primary voltage = Number of turns in secondary ÷ Number of turns in primary

The current is transformed the opposite way—increased in size—in a step-down transformer:

Secondary current ÷ Primary current = Number of turns in primary ÷ Number of turns in secondary

So a step-down transformer with 100 coils in the primary and 10 coils in the secondary will reduce the voltage by a factor of 10 but multiply the current by a factor of 10 at the same time. The power in an electric current is equal to the current times the voltage (watts = volts x amps is one way to remember this), so you can see the power in the secondary coil is theoretically the same as the power in the primary coil. (In reality, there is some loss of power between the primary and the secondary because some of the "magnetic flux" leaks out of the core, some energy is lost because the core heats up, and so on.)