BrightKidz Library
Subjects
Heat leaving a warm touch into a block of wood and into a block of metal Two blocks of the same size stand side by side on the same floor, and they are at the same temperature. The block on the left is wood, drawn with three long wavy grain lines inside it. The block on the right is metal, drawn plain with one straight shine line. Arrows come down from above into each block, standing for heat leaving a warm finger. Only two arrows reach the wood, and they stop just inside the top surface, because the wood cannot carry heat onward. Five arrows reach the metal and every one of them runs the whole depth of the block, because metal carries heat away quickly. The blocks are equally warm. What is different is how fast each one takes heat from whatever touches it.

Why Metal Feels Colder Than Wood

About 10 minutes

Find a table with metal legs and a wooden top. Put one hand flat on the wood and the other around a leg. The leg is cold. The top is not.

Now think about where they have been. Both have stood in the same room, in the same air, all night long. Nothing has warmed the wood and nothing has chilled the metal. They have had hours to settle at the same temperature as everything around them.

Tape a thermometer to each and wait. The two readings come out the same.

So the metal is not colder. Something else is going on, and it is happening in your hand rather than in the metal.

Your skin has no thermometer in it

This is the part most people have never been told. The nerve endings under your skin do not measure how hot a thing is. They cannot. All they can report is what is happening to your own skin: whether it is losing heat, and how fast.

Cold is not a thing you detect. Cold is what you call it when heat is leaving you quickly.

That changes the question completely. It is no longer "how warm is this object". It is "how fast does this object take heat off my fingers".

Wood fills up. Metal keeps taking

Put your finger on wood. Heat starts moving out of your fingertip into the thin layer of wood underneath it, because that is what heat does: it moves from the warmer thing to the cooler one.

But wood is very bad at passing heat along. The heat arrives in that top layer and mostly stays there. Within a few seconds the little patch of wood under your finger has warmed up to nearly your own temperature, and once it has, there is almost nothing left to flow. Your fingertip stays warm. Your nerves report nothing much.

Now put your finger on metal. The same heat starts moving out. But metal passes heat along beautifully, and the heat that arrives at the surface is carried straight down and sideways into the rest of the bar, away from your finger.

So the metal under your fingertip never warms up to meet you. There is always somewhere colder for the heat to run to. Your fingertip keeps losing heat, second after second, and your nerves keep reporting the same thing: heat is leaving, still leaving, still leaving.

You read that as cold.

Both spoons must sit in the same place for at least an hour first. Not in sunlight, not near a radiator, not fresh out of a drawer someone just opened. They need to reach the same temperature, because the whole point is that they are equally warm.

Hold the handle of the wooden spoon in one hand and the metal spoon in the other, and keep hold. Move on to the next stage only when you have actually felt it.

  1. The first moment. The metal feels cold and the wood does not. Both have been sitting in the same room for an hour. Whatever you are feeling, it is not a difference in temperature.
  2. After about five seconds. The wood no longer feels cool at all. The patch under your fingers has warmed up to nearly your own temperature, so the flow has almost stopped, and your nerves have nothing to report.
  3. After thirty seconds. The metal is still cool. It has not stopped taking heat, because every bit of warmth you put into it gets carried away down the handle. There is always somewhere colder for it to go.
  4. Let go and touch the same spot again straight away. The wood is warm where you were, and stays warm for a while. The metal is not. Your heat did go into it, and then it went somewhere else.

Notice what you have just proved. The wood changed and the metal did not. If the metal had really been colder to start with, holding it would have warmed it up, the way holding the wood did.

Which materials are fast, and why

The quick ones are the metals: copper, aluminium, steel, silver. Inside a metal, some of the electrons are not tied to any one atom. They drift through the whole piece, and they carry heat with them as they go. That loose crowd is also what makes metals carry electricity, which is why the same short list of materials keeps turning up for both jobs.

The slow ones are wood, plastic, wool, paper, and, best of all, still air. Air is a superb blocker of heat as long as you stop it swirling about. That is the trick behind everything warm you own: a woolly jumper, a duvet, a sleeping bag, the foam in a cool box. None of them makes heat. They are all just clever ways of holding still air in place so your own heat cannot get away.

You take a metal tray and a cardboard box out of the same freezer. Both have been in there overnight. Which feels colder, and why?

What to try next

Go round the house with the back of your hand and rank things from coldest-feeling to warmest-feeling. A radiator that is switched off. A window pane. A book. A tea towel. A door handle.

Then sort your list a second way, into things that are made of metal and things that are not. The two lists will look remarkably alike, and now you know it is not because the metal things have been chilled. It is because they are all in a hurry to take your heat.