One use of a variac is to power up long-unused vintage gear. If, say, you buy an amp that's been sitting in an attic for 10 years, the electrolytic caps in it are completely dry. If you plug it straight into the wall, the inrush current is liable to destroy them, so it's critical to "ramp up" the voltage slowly over hours to give the old caps time to reform.
Another use is to test gear whose condition is unknown. If it has an internal short, again the last thing you want to do is plug it into the wall. By bringing up the voltage slowly from zero while monitoring current with an ammeter, you'll see very quickly if the current is abnormally high. In that case you can turn off the power before the unit sustains any more damage.
Another use is to test gear whose condition is unknown. If it has an internal short, again the last thing you want to do is plug it into the wall. By bringing up the voltage slowly from zero while monitoring current with an ammeter, you'll see very quickly if the current is abnormally high. In that case you can turn off the power before the unit sustains any more damage.
