A pulse is a burst of current, voltage, or electromagnetic-field energy. In practical electronic and computer systems, a pulse may last from a fraction of a nanosecond up to several seconds or even minutes. In digital systems, pulses comprise brief bursts of DC (direct current) voltage, with each burst having an abrupt beginning (or rise) and an abrupt ending (or decay). In digital circuits, pulses can make the voltage either more positive or more negative. Usually, the more positive voltage is called the high state and the more negative voltage is called the low state. The length of time between the rise and the decay of a single pulse is called the pulse duration or pulse width. Multiple pulses often occur in a sequence called a pulse train, where the length of time from the beginning of one pulse to the beginning of the next is called the pulse interval. Digital pulses usually have well-defined shapes (voltage-vs.-time graphs, as might be observed on an oscilloscope) such as rectangular or triangular. In nature, however, pulses can have irregular shapes and can occur at random intervals. A good example is an EMP(electromagnetic pulse) generated by a lightning discharge in a thunderstorm, a solar flare, or a transient "voltage spike" that can occasionally occur on a utility power line