New coil designs

New coil designs

Coil designers also tried out innovative designs. The original Induction Balance coil system consisted of two identical coils placed on top of one another. Compass Electronics produced a new design; the two coils were made in a D shape, and were mounted back-to-back to form a circle. This system was widely used in the 1970s, and both concentric and D type (and Wide scan as they became known) had their fans.

Another development was the invention of detectors which could cancel out the effect of mineralization in the ground. This gave greater depth, but was a non-discriminate mode. It worked best at lower frequencies than those used before, and frequencies of 3 to 20 kHz were found to produce the best results. Many detectors in the 1970s had a switch which enabled the user to switch between the discriminate mode and the non-discriminate mode.

Later developments switched electronically between both modes. The development of the Induction Balance detector would ultimately result in the Motion detector, which constantly checked and balanced the background mineralization. At the same time, developers were looking at using a different technique in metal detection called Pulse Induction. Unlike the Beat Frequency Oscillator or the Induction Balance machines which both used a uniform alternating current at a low frequency, the pulse induction machine simply fired a high-voltage pulse of signal into the ground. In the absence of metal, the ‘spike’ decayed at a uniform rate, and the time it took to fall to zero volts could be accurately measured.

However, if metal was present when the machine fired, a small current would flow in the metal, and the time for the voltage to drop to zero would be increased. Pulse Induction detectors are now widely used in the construction industry; the Whites PI-150 is an industrial machine which can detect large objects to 10 feet, using a 12 or 15 inch coil.