How Does A Metal Detector Work?

Metal detectors can show us things we never knew were there, things that are invisible to the naked eye until they are retrieved from somewhere under the ground. Metal detectors could lead us to treasures underfoot that we go around completely oblivious to.

But as much as they might seem to work like magic, they aren’t. There’s a lot of science and technology that goes into making a metal detector that will actually work, and in this article today, we’re going to debunk it for you, and explain all the science behind it.

And while we’re at it, we’ll also tackle some of your most frequently asked questions along the way. Do you dig metal detectors? Intrigued so far? Read on…

How Does A Metal Detector Work?

As you may already be aware, a typical metal detector consists of a handheld unit with a sensor probe that you sweep over the ground, emitting a beeping noise when it detects any metal underneath. But how does it do that, you may ask.

Electromagnetic Fields

The battery in a metal detector sends electricity to a large, coiled wire in the base. This coil of wire is wrapped around a circular head and is known as the transmitter coil. This transmitter coil transmits an electromagnetic field from the coil and into the ground underneath. 

Any metals that lie within this electromagnetic field in the ground will become energized and will generate an electromagnetic field of their own, otherwise known as eddy currents. This causes a signal to be sent up from the transmitter coil in the base right back up to the control box in the handle. This signal is basically a magnetic pulse.

So, only materials that are magnetic can be detected this way by a metal detector. And magnetic materials are always made of metal. However, not all metals are magnetic. More on that a little later.

Programming of the Control Box

The control box of the metal detector houses the gadget’s electronics. It houses the transmitter chip, which when the switch is turned on transmits the electricity that induces the electromagnetic field in the transmitter coil.

But the control box also features a receiving chip. When the receiving chip receives a signal from the eddy currents of any metals detected underground, the receiving chip will emit an audio signal, which is the beeping sound that you will hear.

The manufacturer will have programmed this receiving chip so that different metals that emit different eddy currents through the device will emit different audible sounds. That way you can gauge what kind of metal has been detected by the machine without having to necessarily dig anything up. 

This way, the metal detector can be set up to ignore unwanted targets, and generate less false positives of whatever type of metal it is that you are looking for.

Usually, valuable metals like gold or silver will make a high pitch beep on your metal detector, while low value metals like iron will make a dull beep. 

Unfortunately, it is beyond the scope of this article to go into detail about how the electronics in the control box work. But, by this point, you should now have a good general understanding of just how a metal detector works.

When Were Metal Detectors Invented?

The first ever metal detector was invented by famous inventor Alexander Graham Bell in 1881, and it’s a great story…

It is said that as then President James Garfield lay dying following an assassination attempt, Bell hurriedly put together a very basic metal detector in order to locate the bullet in the President’s body.

However, the first industrial metal detectors weren’t developed until much later, in the 1960s and their uses included detecting land mines, and the detection of weapons such as knives and guns. 

As time has gone on since then, they have come to be used widely in the field of archeology, and by treasure hunters and hobbyists.

What Metals Cannot Be Detected By A Metal Detector?

So, as we touched upon earlier, only materials that are magnetic can be detected by a metal detector. And while magnetic materials are always made of metal, not all metals are necessarily magnetic.

Iron for example is magnetic, so it will always be picked up by a metal detector. And consequently any metal that contains iron, such as steel, will also be picked up by a metal detector. 

Other items that are typically found with a metal detector include the likes of pins, staples, paper clips and screws.

Even magnetic items that have started to rust through exposure to air and water can be picked up by a metal detector, because they are still able to produce an eddy current and magnetic pulse.

There are metal detectors that can detect gold, but some metal detectors are more sensitive to it than others.

However, it is difficult for a metal detector to pick up stainless steel because, despite being made in part from iron, steel has quite poor electrical conductivity and won’t generate a strong magnetic pulse when in an electromagnetic field. 

Metals that are not magnetic can include the likes of copper, aluminum, brass and lead, so these will go completely undetected by a metal detector.

Sometimes a metal detector will pick up interference from magnetic minerals in the ground, however there is a Ground Balance feature on many metal detectors that will work to minimize magnetic interference from these minerals.

Also, a metal detector cannot usually pick up a signal from precious gemstones, as they are not usually magnetic, although there are some gems that contain iron, which may generate a weak signal.

Do Bullets Show Up On Metal Detectors?

Bullets are generally housed in a metal jacket, and this metal jacket is usually composed of an alloy of zinc and copper, or in some cases steel or silver. So, yes, a metal detector can bullets. However, they would have to be quite big to be sufficiently detectable.



Written by Gavin Bowler

Keen history and blogging enthusiast. I love metal detecting. Read more of Gavin's articles.