What does a transformer do when it 'transforms'?
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First, a quick tutorial on electricity. An electric current consists of a flow of electrons, and if you think of it as being analogous to a flow of water, then voltage is like water pressure.
Sometimes high voltages are useful (to send electricity long distances, for example), but at other times you need low voltages (to charge cellphones, for example), so you need to be able to change one to the other.
Have you ever made a little electromagnet by winding some wire around a nail and connecting the ends of the wire to a battery, so that the nail picks up paper clips? Had you wound two coils of wire on the same nail, you would have made a transformer.
Here’s how it works: When you change the voltage across one coil, the magnetism in the coil changes, and that makes a voltage appear across the other coil. It is easy to calculate how big that voltage is. You take the voltage you put in and multiply it by the ratio of the turns on the second coil to those on the first.
It’s important that the voltage changes with time - a steady voltage across one coil does nothing to the other. This is why the electricity delivered to your house - so-called alternating current - changes continuously and smoothly 60 times a second.
An example may help: If you have 100 turns on the first coil and 10 turns on the second one, and you connect the first coil to the 120-volt electrical outlet in your home or office, you get 120 volts multiplied by (10/100), or 12 volts.
The description is a little simplified, so don’t actually try this at home unless you really know what you’re doing, or you’re likely to get a shock, blow a fuse, and/or start a fire.
By changing the ratios of the turns, you can “transform’’ any voltage to any other.
Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail him at drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. 
