Editing Metal detector (section)

===Discriminators===
The biggest technical change in detectors was the development of a tunable induction system. This system involved two coils that are electro-magnetically tuned. One coil acts as an [[RF]] transmitter, the other as a receiver; in some cases these can be tuned to between 3 and 100&nbsp;kHz. When metal is in their vicinity, a signal is detected owing to eddy currents induced in the metal.  What allowed detectors to discriminate between metals was the fact that every metal has a different [[phase response]] when exposed to alternating current: longer waves (low frequency) penetrate deeper into the ground, and select for high-conductivity targets like silver and copper; shorter waves (higher frequency) are less ground-penetrating, and select for low-conductivity targets like iron. Unfortunately, high frequency is also sensitive to ground [[Mineralization (geology)|mineralization]] interference. This selectivity or discrimination allowed detectors to be developed that could selectively detect desirable metals, while ignoring undesirable ones.

Even with discriminators, it was still a challenge to avoid undesirable metals, because some of them have similar phase responses (e.g. tinfoil and gold), particularly in alloy form. Thus, improperly tuning out certain metals increased the risk of passing over a valuable find. Another disadvantage of discriminators was that they reduced the sensitivity of the machines.