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From 1870 there were various attempts to build a machine that could detect metal or ore rich rocks as this would be a great mining tool. All these attempts ended up with a clumsy power hungry device that was extremely unreliable and practically useless. No major breakthroughs were made until in 1931 Gerhard Fisher a scientist working in Los Angeles on aircraft directional equipment made an observation that prompted him to make metal detectors. He noticed that interference between a radio transmitter and receiver was caused by ore deposits and by passing a piece of metal between them the signal was altered. He developed the first metal detector that worked on this principle in his shed and started producing them for buyers. A patent was given to him for this device in 1937 and he went on to create what is today Fisher Detectors.
Gerhard's device was heavy and bulky due to using vacuum tube valves as electronics were very basic at this time and it also used a lot of power requiring large batteries. In world war 2 a lighter version was needed and it was developed in secret by a polish Lieutenant stationed in Scotland called Josef Stanislaw Kosacki and his involvement in building it remained a war secret for 50 years. These detectors were light, compact and could work for longer on a smaller battery than Gerhard's device and were used extensively for clearing minefields. Many of them ended up on sale as surplus after the war and this was when people started to view metal detecting as a hobby.
In the 50s a few new companies also started to produce detectors and the only two notable developments were the Oremaster Geiger Counter made by whites and the introduction of the Beat Frequency Oscillation system by Garrett detectors. Then when the transistor appeared in the late 50s thus allowing more design options and lighter machines needing less power many more manufacturers suddenly appeared making the market very competitive. The next breakthrough was the induction balance system or discrimination as it's more commonly known. This was a method where using two coils that were balanced electrically you could use the return signal to tell what kind of metal was in the ground. The problem with using this system was that it reduced the overall sensitivity of the machine. The discrimination mode on detectors was constantly refined throughout the 70s with many detectors fitted with the ability to switch this ability on and off to allow sensitive searching as well. Eventually electronics automatically balanced and checked signals removing the need to switch between modes.
Pulse induction was a new system which worked in a completely different way to the Beat Frequency Oscillation method. It worked by sending a pulse into the ground and monitoring how long it took to disappear, if nothing was blocking it's path it would disappear quickly but if there was metal it would take longer and it was this time which told if a find was made. This method would work in areas that the Beat Oscillation System couldn't for example in black sand and highly mineralized sites. At first it was not possible to discriminate between metals with this method until Eric Foster found a way and built the goldscan detector which had all of the advantages of the pulse induction system and was able to discriminate between metals.
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