a lot of electricity is used in arc welding. some kinds of welding use alternating current like the electricity that buildings use. other kinds use direct current like the electricity in a car or most things with a battery. almost all kinds of welding use a lower voltage than the electricity that comes from a power plant. arc welding requires using a special power supply that makes the electricity from the power plant usable for welding. a power supply lowers the voltage and controls the amount of current. the power supply usually has controls on it that allow these things to be changed. for kinds of arc welding that use alternating current, sometimes the power supply can do special things to make the electricity alternate differently. some power supplies do not plug into a power plug, but instead generate their own electricity. these kind of power supplies have an engine that turns a generator head to make the electricity. the engine might run on gasoline, diesel fuel, or propane.