Electromagnetic energy is created when alternating current passes through a metallic conductor.

This electrical energy rises and falls a certain number of times per second, which in turn causes a magnetic field to build and collapse around the conductor at a regular rate. This rate is known as the frequency of the generated current and of its magnetic field.

Frequency is expressed in terms of hertz (Hz), which means cycles per second, or kilohertz (kHz), which means thousands of cycles per second.

In terms of locating, the reason for choosing one frequency over another is how it behaves when applied to a utility line. If we want to trace an underground line we need to apply a signal that can be received above ground and travel the distance we want to trace. Generally we will want to trace one line only, the line we applied our signal to.

As we learned in the article on circuits, each utility line, or circuit, we want to trace can be good (low resistance) or it can be bad (high resistance). Knowing how the frequencies behave on good or bad circuits is important to locating successfully.