How Do Metal Detectors Actually Work?

You swing your detector across a field, hear a signal, look at the Target ID and start digging. A few inches down is a coin that may have been sitting there for hundreds of years. But how did your metal detector know it was there?

There’s a surprising amount happening in the fraction of a second between your search coil passing over a buried object and the detector producing that familiar beep.

Modern metal detectors use electromagnetic fields to locate metal beneath the ground. The detector transmits a changing electromagnetic field from the search coil. When that field encounters a metallic object, electrical currents are created within the target. Those currents then generate a secondary electromagnetic field, which the detector receives and analyses.

It sounds complicated, but the basic principle is actually quite straightforward.

The Search Coil Is Doing More Than You Think

The search coil at the bottom of your detector isn't simply "scanning" the ground.

Inside the coil are windings of wire responsible for transmitting and receiving electromagnetic signals. Depending on the coil design, separate windings may perform these jobs, while some technologies can use the same winding for both.

When the detector is switched on, electrical current passing through the transmit coil creates a changing electromagnetic field around the search coil.

As you sweep the detector, you're effectively moving this field through the ground.

Nothing needs to physically touch the target.

If there is metal within that electromagnetic field, that's when things get interesting.

What Happens When the Signal Reaches Metal?

Imagine your search coil passing over a buried hammered silver coin.

The detector's electromagnetic field reaches the coin and causes tiny electrical currents to circulate within the metal.

These are known as eddy currents.

Those eddy currents then produce an electromagnetic field of their own.

The search coil detects this incredibly small response and sends it to the detector's electronics for processing. The resulting signal may ultimately become an audible tone, Target ID number or another indication on the detector's display.

In very simplified terms:

Detector creates electromagnetic field → field reaches metal → currents form within the metal → metal creates its own electromagnetic response → detector receives the response → you hear a signal.

And all of that happens virtually instantly.

How Does a Metal Detector Know What You've Found?

This is where modern metal detectors become clever.

Finding metal is one thing.

Trying to determine what kind of metal target you've found is considerably more difficult.

Different targets respond differently to the detector's electromagnetic field. Their response is influenced by characteristics including:

• Electrical conductivity
• Size
• Shape
• Thickness
• Orientation in the ground
• Metal composition
• Depth
• Surrounding soil conditions

The detector analyses the return signal and uses that information to estimate what type of target might be underneath the coil.

On most modern detectors, part of that information is displayed as a Target ID number.

That doesn't mean your detector actually knows there's a silver sixpence beneath your feet.

It's making an educated electronic judgement based on the target's response.

This is why completely different objects can sometimes produce very similar Target IDs.

A piece of aluminium, gold ring, hammered coin or piece of lead may overlap depending on the detector, target and conditions.

That's also why experienced detectorists don't rely entirely on the number shown on the screen.

What Is Metal Detector Discrimination?

Once a detector can analyse differences between target responses, it can also attempt to separate targets you probably want from those you probably don't.

This is known as discrimination.

For example, you may choose to reject certain ferrous responses so that your detector doesn't produce the normal accepted-target tone every time it encounters an iron nail.

The detector hasn't stopped detecting the nail.

It has detected it, analysed the response and decided — based on your settings — not to give you the same indication it would for an accepted target.

However, discrimination isn't magic.

Set it too aggressively and there's always the possibility of rejecting desirable targets that happen to produce a similar response.

That's why you'll often hear detectorists say:

If in doubt, dig it.

Why Do Different Metals Produce Different Signals?

Metals have different electrical properties.

Silver and copper, for example, are highly conductive, while other targets produce substantially different electrical responses.

But metal type alone doesn't determine the signal.

A huge piece of aluminium and a tiny piece of aluminium won't necessarily behave the same way. Likewise, a thin hammered silver coin can respond very differently from a large modern silver object.

Shape and orientation matter too.

A coin lying flat can present a very different target to the detector than exactly the same coin sitting almost vertically.

Add corrosion, nearby iron and hundreds of years of soil disturbance into the equation and you begin to understand why Target IDs aren't foolproof.

Why Does Ground Affect a Metal Detector?

Unfortunately, your detector doesn't get to analyse the target in isolation.

The electromagnetic field is travelling through the ground, and the ground itself can produce a response.

Some soils contain minerals that can interfere with the relatively weak signal produced by a buried metallic target.

A detector therefore needs to separate the response produced by the ground from responses that may indicate an actual metallic object.

This is where ground balance comes in.

Ground balancing allows the detector to compensate for the ground's response so that genuine metallic targets are easier to identify.

We've covered this in much more detail in our Ground Balancing Explained guide.

What Does Detector Frequency Have to Do With It?

You'll often see metal detectors described as operating at frequencies measured in kHz.

This relates to how rapidly the detector's electromagnetic field is changing.

Different frequencies can behave differently around different types and sizes of targets.

Broadly speaking, higher frequencies tend to be particularly responsive to very small and lower-conductivity targets, while lower frequencies can offer advantages with larger or higher-conductivity targets.

That's one reason you'll find specialised high-frequency detectors designed for locating very small natural gold.

Modern detectors have taken this further with simultaneous multi-frequency technology, allowing the detector to transmit and analyse multiple frequencies rather than relying entirely on one operating frequency.

We'll save the rabbit hole of detector frequency for another guide because it deserves an article of its own.

How Deep Can the Electromagnetic Field Reach?

This is where things get slightly more complicated.

A search coil doesn't send out a perfectly defined beam that suddenly stops at a particular depth.

The detector's ability to identify a target depends on numerous factors, including:

• Coil size
• Target size
• Target orientation
• Metal type
• Detector settings
• Operating frequency
• Ground mineralisation
• Electrical interference
• Moisture levels
• Nearby metallic objects

Generally, larger metallic objects can be detected deeper than very small objects.

So when somebody asks, "How deep does this detector go?", there isn't a single reliable number.

A detector capable of finding a large object at considerable depth isn't necessarily going to detect a tiny hammered coin at the same depth.

We've gone into this separately in our guide How Deep Can a Metal Detector Really Detect?

Why Does the Detector Sometimes Beep When Nothing Is There?

Every beep isn't necessarily a buried object.

Metal detectors are incredibly sensitive instruments attempting to identify extremely weak electromagnetic responses.

They can also encounter interference from things such as:

• Power lines
• Electric fences
• Other metal detectors
• Mobile/electronic equipment
• Highly mineralised soil
• Incorrect ground balance
• Excessive sensitivity
• Movement of the coil cable
• Impacts against vegetation or the ground

This is generally referred to as electromagnetic interference (EMI) or falsing, depending on what's causing the problem.

It's also why maximum sensitivity doesn't automatically mean maximum performance.

A detector running slightly quieter can sometimes allow you to recognise weak genuine targets more effectively than one constantly producing unstable signals.

Can Metal Detectors Detect Every Type of Metal?

Generally, hobby metal detectors can detect both ferrous and non-ferrous metals, including common targets made from iron, copper, bronze, brass, lead, aluminium, silver and gold.

What they cannot do is magically detect anything valuable.

Diamonds, gemstones, pottery, glass and other non-metallic objects won't produce the metallic response your detector is looking for.

If there's metal associated with the object — for example, a gemstone mounted in a gold ring — then the detector can locate the metal ring, not the gemstone itself.

Does a Metal Detector Know What's Underground?

Not really — and that's part of the fun.

Even the most advanced hobby detector cannot look into the ground and tell you:

"Henry III penny, 7 inches deep, slightly clipped."

What it can do is gather an extraordinary amount of information from a tiny electromagnetic response and give you clues.

Depending on your detector, those clues might include:

Target ID + tone + depth estimate + ferrous/non-ferrous information + strength and consistency of the signal.

The detectorist then interprets all of that information.

And sometimes, despite everything the screen tells you, the only way to know what's underneath your coil is to dig it up.

The Science Behind Every Signal

So the next time your detector gives a crisp signal in the middle of a field, there's quite a lot happening before that sound reaches your headphones.

Your coil has transmitted an electromagnetic field.

That field has interacted with a buried metal object.

Tiny electrical currents have formed within the target.

The target has generated an electromagnetic response.

Your detector has received it, processed it, compared its characteristics and presented the result to you — all in a fraction of a second.

The technology can tell you that something is there.

Finding out exactly what it is?

That's still the exciting bit.