Cold Metal
Your hand isn't always a good thermometer. When you touch a variety of materials, some will seem warmer or colder than others, even when they're at the same temperature.
- Various materials (metal, wood, Styrofoam, glass, plastic, cardboard, and anything else that comes to mind) with a flat surface larger than the size of your hand
- An infrared thermometer (the inexpensive kind works fine)
None needed. Be sure that you have many different surfaces and that they are large enough for you to touch easily. Allow an hour or so for all the materials to come to room temperature before you begin.
Place your palms flat on the various surfaces and compare how cold they feel. Arrange the materials in order from cold to warm. Then use the infrared thermometer to measure each surface. Notice that all the materials are at the same temperature.
Note: Materials will warm to above room temperature after just a few rounds of being touched. The surfaces should be allowed to cool for a few moments after each person's turn. It might be useful to have multiple metal samples. While you are using one sample, the extras have time to cool back to room temperature.
The temperature-sensitive nerve endings in your skin detect the difference between your inside body temperature and your outside skin temperature. When your skin cools down, your temperature-sensitive nerves tell you that the object you are touching is cold. An object that feels cold must be colder than your hand, and it must carry your body heat away so that your skin cools down.
Styrofoam (or plastic foam) and metal are two materials that work well for this activity. They both start at room temperature and are both colder than your hand. They do not feel equally cold because they carry heat away from your hand at different rates.
Styrofoam is an insulator—a very poor conductor of heat. When your hand touches the Styrofoam, heat flows from your hand to the Styrofoam and warms the Styrofoam surface. Because this heat is not conducted away quickly, the surface of the Styrofoam soon becomes as warm as your hand, so little or no additional heat leaves your hand. There is no difference in temperature between the inside of your body and the outside of your skin, so the temperature-sensitive nerves detect no difference in temperature. The Styrofoam feels warm.
The metal, in contrast, is a good conductor of heat—it carries heat away quickly. Heat flows from your hand into the metal and then is conducted rapidly away into the bulk of the metal, leaving the metal surface and your skin surface relatively cool. That’s why metal feels cool.
You probably have experienced this many times without thinking about it very much. Walk around barefoot in your house and see how warm or cold different surfaces feel, even though they’re all at room temperature. Put one bare foot on a carpet or wooden floor and the other on the tile floor of your bathroom, for instance, to feel the difference.
