Most students of electricity begin their study with what is known as direct current (DC), which is electricity °owing in a constant direction, and/or possessing a voltage with constant polarity. DC is the kind of electricity made by a battery (with de¯nite positive and negative terminals), or the kind of charge generated by rubbing certain types of materials against each other. As useful and as easy to understand as DC is, it is not the only \kind" of electricity in use. Certain sources of electricity (most notably, rotary electro-mechanical generators) naturally produce voltages alternating in polarity, reversing positive and negative over time. Either as a voltage.
