A coil suspended in a space produces an AC voltage proportional to any alternating field or signal passing through it, because of the Induction Principle.
This voltage is electronically amplified to produce a visual response on a display and an audible response from a loudspeaker.
Note that the coil is only influenced by that small proportion of the total field in space which actually passes through the coil. However, a ferrite rod inside the coil will channel more of the field through the coil through it instead of around it. When locating plastic or concrete pipes, ducts and drains there is no way of detecting and tracing them electromagnetically by current flow, unless a tracer wire is inserted or laid along to the line. This can then have a signal imposed upon it by active location methods.
The alternative is to insert a signal transmitter known as a Sonde into the line, and use a suitably tuned receiver to detect and pinpoint its position from above ground. The sonde element comprises nothing more than ferromagnetic rod surrounded by a coil, which is energised with a closely controlled signal frequency by a battery powered oscillator. Rod, oscillator and battery are usually built as a self contained watertight unit, designed to be attached to a drain rod, floats, or other appropriate means of traversing along a length of pipe. The frequency used can be chosen to match that of a signal generator, so that the same receiver can be used to locate the position of the sonde, and trace its progress from above ground.
There are certain important differences in the characteristics of the magnetic field produced by a sonde in comparison with that produced by a current in a length of line.
The significance of them is dealt with in the Sonde Signal Characteristics article. The distance over which a sonde can be detected is a function of its transmitting power, which is necessarily related to its size. A tiny sonde for small diameter lines inevitably has a limited detection range by comparison with a large sonde for use in deep sewers.
The aerials of Radiodetection instruments are constructed this way. How they perform is a function of their orientation in relation to the radiated field from the signal source.