Current

• Static electricity is "static" because it doesn't move. It's the build-up of electric charges on an object. Current occurs when electric charges flow from one place to another. The most dramatic example of this is lightning, but it also makes everything from toasters to iPods work.
  
  

Direct Current

• A direct current flows when electricity moves in one direction and at a steady rate. Batteries provide a source of DC. Each battery has a positive and negative terminal. When it's connected to a circuit, current flows from the positive to the negative side. The voltage and current are constant as long as the battery is good. Some types of electric generators produce DC, and most power supplies that plug into the wall also make DC.
  
  

Alternating Current

• The power from a household electrical outlet is AC. Unlike DC, AC has a rippling current that reverses direction at a 60-hertz frequency. Mechanical generators and some electronic circuits produce AC. Electric motors that run on AC, like the one in your vacuum cleaner, are simpler and more reliable than DC motors. The main advantage with AC is that you can use transformers to easily increase or decrease the voltage. Raising electricity to thousands of volts makes it more efficient to send through long-distance wires. When the electricity reaches towns and neighborhoods, other transformers reduce the voltage to a few hundred volts, then to 110 volts when it reaches your home.
  
  

War of the Currents

• In the late 1800s, when electrification of cities was in its earliest years, Thomas Edison promoted using DC for power and lighting. Nikola Tesla and George Westinghouse developed an AC motor and generators. With fortunes at stake, Edison waged a propaganda campaign against AC, exaggerating its dangers compared to DC. After years of competition and demonstrations, cities chose alternating current as the more efficient way to distribute power.
  
  

Mixed Signals

• A DC power supply, like the one that charges your cell phone, derives its power from the AC outlet. A small transformer first reduces the voltage, then electronic circuits convert the lower-voltage AC to DC. This process, while efficient, is imperfect. Small amounts of the original 60-hertz signal remain mixed in the DC. Sometimes AC and DC can coexist in the same current.