Many treasure hunters swear by the devices; however, skeptics have examined the internals of many such devices and found those that have been examined to be incapable of operating as advertised, and have dismissed them as overpriced dowsing rods or similarly useless (the skeptical view on dowsing is that it does not stand up to a properly controlled test). Virtually all such devices claim to operate on a resonant frequency principle where the device is said to emit an electromagnet signal, either through an antenna or a probe, that will respond to a specific substance such as gold, silver, or sometimes even paper money, and that the device will indicate the presence of such material by indicating a change in direction relative to the operator.
It is generally believed that there is no physical principle by which such a device can operate meaningfully; many such devices contain non-functional circuitry or naively constructed approximations of radio transmitters. A few do have functional circuitry, putting out a weak signal with a function generator or a simple timer circuit, but are still largely useless in comparison with a coil-based metal detector; others have been found to contain intentionally obfuscated or completely superfluous components, indicative of intentional fraud by the designer.
Author Tom Clancy came under fire for including the DKL Lifeguard, a long range locator purported to be useful for detecting people, in critical passages of his novel Rainbow Six. A study by Sandia National Laboratories proved the Lifeguard to be completely useless, and other designs by the Lifeguard's creator Thomas Afilani have been shown to contain numerous dummy components with no clear function.