Copper as a shield: from the MRI room to the lightning rod

The metal engineers reach for when something has to be shielded

When a hospital installs an MRI scanner, the entire room around it gets lined with copper. When an electrician bonds a building to earth, they do it with copper wire. And when a lightning rod goes on a roof, copper tops the list in the standard. The same metal every time, for three very different problems. This article is about why that is, and about the point where the engineering turns into tradition.

In short

• An MRI room is lined with copper sheet as standard, to keep outside radio waves out.
• Fine copper mesh attenuates around 60 decibels across the range from 10 megahertz to 1 gigahertz, which is 99.9 percent of the signal.
• The European lightning protection standard EN-IEC 62305 lists copper as a conductor and as an earth electrode, and a copper conductor in it has a cross-section of at least 50 square millimetres.
• The antistatic wrist strap in electronics factories is copper-bearing textile against the skin, with a 1 megaohm resistor, laid down in IEC 61340-5-1.
• Hindu temples have carried a copper kalash on the spire for centuries, in the tradition both a lightning rod and an antenna.
• Underneath all of it sits a single property: copper takes charge exactly where you want it to go.

The copper box around the MRI scanner

An MRI scanner listens for radio signals coming out of your body, and those signals are faint. So faint that a two-way radio outside, a dimmer switch in the corridor or a phone in the car park would already disturb the image. The solution is as simple as it is radical: you put the scanner in a box made of copper.

A room like that is called an RF-shielded room. The walls, the ceiling, the floor, the door and even the window are lined with thin copper sheet or copper mesh. Radio waves cannot get through, because they induce small currents in the copper that immediately cancel the field on the outside. The higher the frequency, the thinner the copper layer needs to be: at the frequencies an MRI works with, a few tenths of a millimetre is enough.

The idea is almost two hundred years old. In 1836 Michael Faraday showed that a conductive enclosure keeps the field outside, by sitting inside a foil-lined room himself while heavy discharges went off outside. Inside, he measured nothing. Ever since, such an enclosure has been called a Faraday cage.

How much difference it makes, in numbers

For copper mesh those numbers are simply measurable. Finely woven copper mesh achieves around 60 decibels of attenuation across the range from 10 megahertz to 1 gigahertz. Sixty decibels sounds abstract, but it means that for every thousand units of signal, one gets through: 99.9 percent stays outside. That range holds radio, the baby monitor, the DECT phone and a large part of wifi.

Which is exactly why shielding is an ordinary trade with ordinary standards. Why copper is so good at it, and what the crystal lattice has to do with it, is in why copper conducts.

Bringing the lightning down

Shielding is one half of the story. The other half is exactly the opposite: letting charge through on purpose, all the way to the earth.

The European lightning protection standard, EN-IEC 62305, lists copper for that job, both for the conductors and for the earth electrode in the ground. Under that standard a copper conductor has a cross-section of at least 50 square millimetres, about as thick as a pencil. A lightning strike delivers tens of thousands of amperes in a fraction of a second, and the metal carrying that away has to come through unscathed. Copper has been doing that for as long as lightning rods have existed.

Benjamin Franklin worked out the principle in 1752, and remarkably little has changed since: a point on top, a thick copper conductor running down, and an earth electrode in the ground. What we call earthing is literally a copper connection between a building and the soil beneath it.

Copper against the skin, in the electronics factory

There is one more place where copper carries charge away, and it is closer to home than you would expect: around the wrist of everyone who works with chips.

An antistatic wrist strap is made of copper-bearing textile that sits against the skin, with a lead to an earthing point and a 1 megaohm resistor in between. That resistor makes the charge drain away calmly rather than in one jolt. The whole arrangement is set out in an international standard, IEC 61340-5-1, because a single unnoticed static discharge costs a chip its life.

So there is an industrial standard in which copper sits against human skin all day long to conduct charge. What copper does with skin beyond that, and what has been measured, is in copper on your skin.

The copper pot on the temple spire

Move from the engineering to the tradition, and you meet the same metal in the same place: right at the top.

On the spire of Hindu temples stands a kalash, a copper pot, often gilded. In the building tradition that object has two functions at once. The first is ordinary physics: the highest point of the building is metal and is connected to the ground through the structure, exactly what a lightning rod does. The second belongs to the tradition itself, in which the kalash counts as an antenna for what people call cosmic or divine frequencies, caught at the highest point and passed on to the space below.

We attach no health claim to that. What does stand out: the builders of those temples and the drafters of EN-IEC 62305 reached the same conclusion about where you put copper and why.

Earthing: the tradition and the metal

That same thought, direct contact with the earth through a conductor, sits underneath the earthing tradition you now hear called grounding. Research has been done on it: the review article by Chevalier and colleagues in the Journal of Environmental and Public Health from 2012 gathers what had been published on it up to then. It is a live and debated field of research, and here too we attach no health claim to it.

What the tradition did see clearly is the choice of material. Your body runs on electricity itself: an ECG measures nothing other than the electrical activity of a muscle, and a nerve impulse is literally a wave of voltage. Copper is the metal electrical engineering uses to steer all those currents. That it also became the metal of choice in this corner is therefore less of a coincidence than it looks.

Why our copper is solid

All of these applications have one thing in common: the copper is really there, with nothing in between. It is no different with us. Everything we make is solid, uncoated copper: a tensor ring you drop into water, electroculture antennas you push into the garden, a bracelet that rests against your skin all day.

What the tradition around those products actually says, you can read in electroculture and in the tensor ring. Want to see the metal itself first? Browse the whole collection.