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Metal detecting involves a blend of physics and practice. Understanding the principles of magnetism and how detectors use it to locate metals can enhance the hobby or profession of metal detecting, making it a more fruitful and enjoyable activity. Whether for relic hunting or ensuring safety, the ability to effectively use a metal detector can be greatly improved with a foundational knowledge of the science behind it.

For tips on gold prospecting, silver, coins, jewelry and much more, check out our blog on Metal Detecting Tips: The Ultimate Guide. This article revisits and elaborates on how metal detectors work, expanding on previous blogs here. For more information on how to work with and read your detector, check out our post on Learn How to Read a Metal Detector. 

Basic Physics of Metal Detectors

Metal detectors are fascinating devices that leverage the principles of magnetism to detect metallic objects. They are invaluable for locating hidden treasures like coins and jewelry, and for ensuring safety by identifying potentially dangerous metal objects like weapons. While there are various types of detectors, such as magnetic locators, security and metal detectors, their fundamental functionality is comparable as it relates to ferrous metals. The device uses magnetism to distinguish ferrous and non-ferrous materials (ferrous originates from ferric, which means 'extracted from iron').

Magnetism in Metal Detection

Detectors use magnetism by producing a current from a transmitter coil. When the coil is “ON", it induces or “turns on” the magnetic field. The power source is used to either increase or decrease the current, or the strength, of the magnetic field. A separate coil, the receiver coil, is the “listening” coil. The receiver coil is tuned to identify when the current that is created is interrupted. So the coil that induces the magnetic field and the separate coil that listens to the disruption in the field can be thought of as echolocation by bats. The bat creates a sound (induces a field by emitting a high-frequency sound) and then the ear (the receiver coil) listens for the disruption.

Eddy Current in Metal Detection

Eddy current is a term for the electric current created in a metal by a magnetic field. When a metal detector's magnetic field interacts with a metal object, it induces small circular electric currents (eddy currents) in the metal, which create their own magnetic fields and can be detected by the receiver coil. Different sizes, shapes, textures, and materials of objects can disrupt the magnetic field. When a metal object, like a gold ring, is detected, it changes the constant magnetic field and creates an opposing field, much like how waves interact. Larger metal pieces and those with more iron produce stronger currents. Detectors use alerts, pins, or sounds to indicate these disruptions.

This is an example of what the induced and received process may look like via an illustration from Florida State University’s National High Magnetic Field Laboratory.

As illustrated in the graphic, the blue represents the magnetic field that the detector "turns on”. That field collides with the quarter, which is made out of copper/nickel, and has an eddy current (yellow) and creates an alteration in the signal (red). This returns a modified signal back to the receiver coil in the detector. Since metals are made of different materials, detectors have creatively created “Target IDs” to help read the returned frequencies to distinguish what may be buried underground. A frequency is simply how many waves occur within a second, so 1 Hz frequency is 1 wave per second (in detectors, it is typically reported as kHz, so 1,000 Hz per second). An illustration is for 1 to 25 Hz int he figure, demonstrating the number of waves per second. In some cases, this simplifies detecting but also makes it exciting because the process is not perfect and some materials cluster together.

Summary

Magnetism is at the heart of metal detection, making it possible to discover and identify metallic objects through the principles of electromagnetism. Whether for safety, hobby, or professional use, understanding how magnetism works can enhance the effective use of metal detectors in various scenarios.

Metal Detectors: From Origin to Modern-Day Use

The initial metal detector in medicine can be pinned to Alexander Graham Bell. Bell made history by developing the earliest version of a medical metal detector to try and locate the bullet in U.S. President James Garfield. While it was not successful in its initial use-case, magnetism and metal detectors are invaluable and successfully used today in screenings at airports, finding artifacts, identifying metals/textiles in food processing, and in medical practice. Some 40 years later, two inventors from Germany, Geffchen and Richter invented the first walk-through metal detector.

Dr. Gerhard Fisher, of Fisher Laboratories, is credited (but debated) for the first patent in the domain of metal detectors in 1931. The major breakthrough in expert and hobby metal detecting occurred in the 1970s, when Dr. Charles Garrett, of Garrett Metal Detectors, received the patent for the first handheld metal detector. As can be observed from the patent, it still resembles the modern-day relic hunting tools that we use. For a more elaborate history of metal detectors, check out our previous post on the History of the Metal Detector and the post from Explain That Stuff.

Modern-Day Metal Detectors

While the 1972 patent resembles modern-day detectors, several advancements have been made in the technology to improve your metal detecting experience. The original patent from Garrett was followed up in 1987 by Robert Podhrasky, of Garrett, to improve digital signal processing; in 1989 by a patent from Dr. David Johnson, of Fisher Research Lab, for important mechanism related to eddy currents; and in 1996 by Bruce Candy, of MineLab Electronics, a patent on discriminating ferrous and non-ferrous targets.

Discrimination in Detectors

Metal detectors have a range of ferrous and non-ferrous detection options. Some metal detectors, such as the Garrett AT Pro, can help you discriminate between types of metals to avoid digging up unwanted items. However, not all metal detector models have this very helpful feature. 

The video from Huygens Optics illustrates how Target IDs, used in discrimination, are sensitive to materials types and shapes/sizes and can be exploited to your advantage, saving time in the field. Here is an example of the target ID ranges from Garrett AT Pro’s instruction manual:

Metal detector discrimination can speed up the operator’s ability to distinguish gold, silver and/or jewelry. However, they do not work on non-magnetic items such as porcelain, bones, gemstones, paper, stone figures or pearls, which do not generate eddy currents. Nevertheless, modern detectors allow the operator to change the magnetic field to induce larger eddy currents in different metals that are more/less conductive, resulting in greater signal. In some applications, it may even be used to distinguish the thickness of metals (thicker metals produce higher signals, often used in industrial situations).

Ultimately, metal detectors can use a range of frequencies to help distinguish different objects by leveraging the induced magnetic fields. Your handheld metal detector may have the capability to discriminate between different types of metal objects, like iron, silver or gold. Metal detecting is a combination of art and science, with an emphasis on patience and interpretation. Depending on the detector, this may determine the depth of items that it can detect. Typically, coin-sized objects can be detected at depths of 4-12+ inches (10-25 cm). Mid-range deep-seeking detectors can detect items at depths up to 12-18 feet (30-45 cm) underground in ideal circumstances and some expert detectors can achieve depths of 65 feet (20 meters). Of course, these depths are altered by things such as conductance, positioning and interferences from the minerals and materials in the soil.

Checkout how DetectorMoe combines the power of a metal detector and pinpointer to locate a diamond ring on the beach!

Metal Detecting for Gold

Gold detection with modern metal detectors has become more precise and efficient. Gold nuggets are typically found in areas with historical gold production. Dredge tailings and placer deposits in riverbeds or stream beds are prime locations. While gold is often associated in US with states like California, Arizona, and Alaska, it has been discovered in many regions across the U.S and the globe (see our post on Best Places to Find Gold). Researching past gold-producing areas and understanding the geology of your target region can significantly improve your chances of finding gold. If you're a metal detecting enthusiast looking for a new challenge, searching for gold nuggets can be a rewarding pursuit. However, it's not a hobby for the faint-hearted; it demands time, patience, and the right equipment.

Gold is a highly conductive metal. Metal detectors for gold come in different frequencies, such as Pulse induction (PI) and very low frequency (VLF). PI metal detectors work by sending quick bursts of electricity through a coil to create magnetic fields. When these fields collapse, they generate brief voltage spikes that the detector can use to find metal objects. PI detectors are particularly good at finding metal in challenging conditions like salty or mineral-rich soils and underwater, making them great for gold prospecting, treasure hunting, and diving. These are typically more advanced and more expensive (Read our blog on how PI detectors work). VLF metal detectors send out low-frequency signals into the ground. When these signals hit metal objects, they bounce back to the detector. The detector then processes these bounced signals to locate and identify the metal objects. VLF detectors are greater for higher sensitivity to small(er) gold nuggets, coins and jewelry. VLF detectors are entry-to-mid models that are more affordable and easier to use (Read our blog on how VLF detectors work).

Two notable detectors for gold prospecting are the PI Garrett Axiom Metal Detector and the VLF Fisher Gold Bug-2.

Garrett Axiom Metal Detector Expert Gold Price: $3,995.00

The Garrett Axiom delivers top-tier performance for gold prospecting with advanced pulse timings and versatility. It's equipped with features to detect all sizes of gold, from large nuggets to tiny pieces, in challenging environments like mineralized soils and saltwater.

The Garrett Axiom is designed for serious gold prospectors, offering a range of features and settings for optimal performance. It includes two search coils (13"x11" DD and 11"x7" Mono) and wireless Garrett MS-3 headphones. The detector is lightweight, weighing just 4.2 lbs with the 11" mono coil, and collapses to 25 inches for easy transport. It operates with advanced Ultra-Pulse Technology, providing enhanced sensitivity and versatility in various conditions.

• Search Modes: Four modes—Fine, Normal, Large, and Salt—each optimized for different types of targets and conditions.
• Ultra-Pulse Technology: Advanced pulse timings for detecting all gold sizes in extreme environments.
• Iron Check: Identifies iron targets to reduce time spent on unwanted items.
• TERRA-SCAN: Independent dual-channel ground balance for comprehensive mineralization coverage.
• Lightweight Design: Only 4.2 lbs with the 11" coil, easy to handle and transport.
• Extreme Battery Life: Built-in high-capacity lithium-ion battery for extended use.
• Search Coil Options: Available in multiple sizes and configurations: 16" x 14", 13" x 11", 11" x 7".
• Adjustable Audio Options: Volume, tone, and audio type adjustments for better target identification.
• Backlight and Sensitivity Controls: Customizable settings for various environments.

Fisher Gold Bug 2 Metal Detector Price: $819.00

The Fisher Gold Bug 2 is renowned for its exceptional sensitivity to small gold nuggets, thanks to its ultra-high frequency of 71 kHz. It's a trusted choice for gold prospectors due to its advanced iron discrimination, dust and moisture resistance, and user-friendly design.

The Fisher Gold Bug 2 is a legendary gold nugget detector, offering superior performance and sensitivity. It includes either a 6.5" or 10" search coil, with both available as an option. The detector features a high-resolution digital display, audio boost for enhanced signal clarity, and a three-position mineralization switch to adapt to various ground conditions. Lightweight and durable, it is built to handle tough environments while providing precise target detection.

• Ultra-High Frequency: Operates at 71 kHz for exceptional sensitivity to small gold nuggets.
• Iron Discrimination Mode: Filters out iron and hot rocks to reduce trash targets.
• Dust and Moisture Resistance: Designed to perform reliably in challenging conditions.
• Audio Boost Mode: Enhances signals of small and deep targets.
• Adjustable Mineralization Switch: Adapts to different ground conditions with ease.
• Lightweight Design: Weighs only 2.9 lbs for comfortable use.
• Digital LCD Display: Provides detailed target information for precise adjustments.
• Convertible Mounting: Can be used as a hipmount or chest mount for convenience.
• Quartz-Crystal Locked Electronics: Ensures consistent performance in the field.
• Includes: Waterproof 10" Search Coil, 5-Year Warranty, and Detailed Instruction Manual.

Gold prospecting is an exciting and potentially rewarding hobby. To increase your chances of success, several steps that we covered in our step-by-step guide to find gold. Do Your Research: Study the geology and history of your target area; Consult mining reports, geological books, old newspapers, and local histories; Join forums and clubs to learn from experienced gold hunters. Understand Mining Geology: Learn about placer deposits (found in streams/rivers) and lode deposits (found in solid rock). Recognize Gold Indicators: Look for quartz veins, alteration zones, shear zones, black sand, and areas of contact metamorphism. Start River Panning: Use a pan to separate gold from sediment in rivers, especially after rainstorms when water levels are high. Utilize Online Resources: Research techniques and equipment online. YouTube videos and forums can be helpful. Understand Legalities: Get permission before prospecting on private land and understand the rules for public or state-owned land. Get the Right Gear: Basic gear: gold pan, shovel, bucket, and sieve; Additional gear for desert prospecting or metal detecting: metal detector, pick, off-road vehicle, GPS, and safety gear. Choose the Right Time: Early morning or late evening is best. Spring and summer, particularly after rainfall, can be advantageous. Select the Right Area: Look for areas with a history of gold, high concentrations of gold, and relevant geological features.

Before pursuing your next gold prospective expedition, make sure you do the essentials. Ensure your equipment is ready and batteries are charged -- you wouldn't want your detector or headphones to stop working when things just get good! Prepare for different conditions to ensure your equipment is waterproof and suitable for the terrain. 

Other gold detectors: Entry-level: Fisher Gold Bug. Mid-level: Garrett AT Gold. High-end: XP Deus II, Makro Gold Kruzer, Standard Garrett ATX, Garrett ATK Waterproof Deepseeker Metal Detector Package.