Two types of current are used. There is direct current, such as that provided from a battery, which is designed to provide a certain amount of voltage until it is exhausted. For example, a 9-volt radio battery will always deliver 9 volts. But appliances use power at different rates. If an appliance uses little power, it may draw on the energy slowly, at say one amp per hour. A different appliance may draw on the energy more quickly, at two amps per hour, meaning that the battery will be consumed twice as fast.

In these dc battery circuits, current flows in one direction, which we define as the negative side of the battery to the positive. Then there is alternating current, which travels in one direction and then reverses.

Alternating current can be stepped up, or stepped down with a transformer, which permits the transmission of high voltage along municipal power lines that is transformed to lower voltages at homes and businesses.

Alternating current is delivered to homes and businesses for appliances that are designed to accept it at either 110 volts or 220 volts. Both of these designations are averages, because most appliances use a range of 108 to 127 volts, and 215 to 250 volts. That’s why it’s common to see voltages for appliances expressed as 110, 115, 120, or 220, 230, 240.

Alternating current is also delivered at a certain rate. In the U.S., power is delivered at 60 cycles per second (60 hertz) whereas in Europe and other places in the world it is delivered at 50 hertz.