A Variac, is an autotransformer with a single smoothly variable tapped winding, wound around a wrapped sheet coil toroid core. I'll bet that lucid one liner left you as puzzled about Variacs as you were when you clicked on this page. Ok, first the wrapped sheet coil toroid core, lets say you have a 50 foot long, 4 inch wide ribbon of mild steel, the kind of steel they use to make "E" and "I" laminations out of, for building transformers, and you roll it up like aluminum foil, when you get done, you have a doughnut shaped hunk of steel that weighs about ten pounds, is about 10 inches in diameter 4 inches tall, and has a wall thickness of about 3 quarters of an inch. Next you wrap enameled copper wire, usually red in color, through the center of the torrid, around the outside, back through the center, back around the outside, over, and over, and over... until you have a single evenly spaced copper winding, that spans most of the steel toroid, with this enameled copper wire. Next you glue the enameled copper wire securely to the toroid steel core. After the glue has hardened you get out your belt sander, and carefully sand away a small patch of the insulating enamel, on the top edge of the toroid, and construct a spring loaded electrical contact, that we call the "wiper arm" that when rotated allows contact with any single winding at the top end of the toroid. Next you apply 120 Volts AC across 80 percent of the coil, this allows the knob that controls the wiper arm, to select any voltage, from zero, to about 150 volts or so. If your drill runs too fast to do delicate work, it's a simple matter to plug it into the Variac and reduce the voltage, to a point where the savage beast is tame. Going to the other extreme, plug in a Light Bulb, and crank it up to 150 Volts, Wow Edison really out did him self on that bulb, talk about bright, don't look too long at a 100 watt bulb running that way, or latent super bright light bulb images will float in your visual field for minutes afterward. Oh, and by the way... A bulb so over powered, only lasts about one one-hundredth it's life. Anyway Variacs are neat toys, get one soon.