Why copper conducts: from crystal lattice to 100% IACS

The metal that became the yardstick

If you want to know how well a metal conducts electricity, you compare it with copper. That is not a figure of speech. The international unit for electrical conductivity is called the International Annealed Copper Standard, IACS for short, and it is defined on a piece of annealed copper wire. Everything industry has measured since, from aluminium to superconductors, is expressed as a percentage of that little wire.

In short

• The standard is annealed copper wire one metre long weighing one gram, with a density of 8.89 grams per cubic centimetre and a resistance of 0.15328 ohms at 20 degrees. That is 100% IACS by definition.
• Modern refined copper now reaches about 101%: better than the standard it is itself.
• Of all metals only silver conducts better, 6.30 against 5.96 × 10⁷ siemens per metre. Aluminium stops at around 3.5.
• Annealed copper conducts better than cold-worked copper: measured at 87.5% against 84.1% IACS.
• That difference sits in the crystal lattice, and it is exactly why a coppersmith heats the work between rounds of hammering.

What happens inside the lattice

Copper crystallises into a cubic lattice in which every atom does not quite hold on to one of its electrons. Those loose electrons move through the whole piece of metal, and that is what conduction is: not something flowing through it like water through a pipe, but a whole cloud of electrons setting off at once.

The more orderly that lattice, the further an electron travels before it bumps into anything. Every break in that order, a dislocation in the trade, scatters electrons and therefore costs conductivity. Well-annealed copper holds about 10¹⁰ of those breaks per square metre. Heavily cold-worked copper runs up to 10¹⁵: a factor of a hundred thousand.

Why heating restores the lattice

Cold working, meaning rolling, drawing or hammering without heat, packs that lattice with dislocations. It makes the metal harder and stiffer, and it makes it conduct less well. Heat it above the recrystallisation temperature afterwards and those dislocations disappear: old crystal grains are replaced by new, stress-free ones, and the electrons get through again.

That is not theory but a measured difference. Cold-rolled copper was measured at 84.1% IACS in a study on electrolytic copper; after an hour of annealing it stood at 87.5%. Which is precisely why virtually every copper wire in the world is annealed after drawing: without that step it would be too stiff to bend and would conduct less well on top of that.

Why that matters to a handmade object

A copper jug or cup is not milled from a block but raised from a sheet: thousands of hammer blows drawing the metal into shape step by step. Every blow makes the copper harder, and at some point it would split if the smith carried on. So the smith heats the work, lets the lattice recover, and carries on.

The hammered pattern you see on a Kopero product therefore exists only because the copper went back into the glow again and again in between. The same heating that keeps the metal workable puts the lattice back into the order in which it conducts best. How the two finishes differ is in smooth or hammered copper.

Heat takes the same road

Those same free electrons carry heat as well, and there too copper comes second to silver. In practice you notice it straight away: pour hot tea into a copper mug and the wall has warmed with it within seconds, which is why you pick it up by the handle. Fill a bottle with cold water and that is exactly why it sits cool in your hand within a minute. What that means day to day is in coffee and tea from a copper mug.

Why the electrician drives copper into the ground

When a building has to be connected to the earth, it is done with copper. Earth rods and earthing tape are copper or copper-bonded steel, for two reasons at once: it conducts better than almost anything, and it keeps doing so in wet ground for years. Of all the copper the world uses each year, the largest share goes into electrical applications, from motor windings to the cable on your phone charger.

And the tradition built on top of it

There are traditions that pick up on exactly that property. The tensor ring of Slim Spurling and the antennas of electroculture are not made of copper by accident: it is the metal that plays the conductor in ordinary life, and that is what those traditions ascribe to it. Where those stories come from is in tensor rings and Slim Spurling and in electroculture. We attach no health claims to any of it; what may officially be said in the EU about copper is in the eight official EU claims.

Where you feel it

Conductivity is not an abstraction once the metal is in your hands. It is the mug that warms straight away, the bottle that stays cool, the antenna standing in the soil without power or battery. Have a look at the copper mug, the tensor ring and the electroculture antennas, or browse the full range.