In wirewound resistors, another selection factor is the temperature characteristic of the wire.

The resistance of all materials changes as their temperature changes. If the temperature is lowered, resistance (typically) declines. In fact, if cooled sufficiently, the material becomes a “superconductor” with no significant resistance. Increasing the temperature (typically) increases resistance.

The temperature coefficient of resistance (TCR) of wire or a resistor relates the change in resistance to the change in temperature. It is usually expressed as “parts per million per degree Centigrade” (TCR = ppm/°C.) The temperature coefficient of resistance, then, tells you how much the resistance changes (ppm) if the temperature changes one degree Centigrade. (Sometimes, we measure temperature in degrees Fahrenheit. But today, degrees Centigrade is more common and acceptable.)

Special wire alloys are formulated to have special temperature coefficients. For example, “Evenohm” (a trade name for a low TCR wire alloy) is formulated to have a small TCR of 5 to 10 ppm/°C. Pure nickel has a much larger TCR of 6700 ppm/°C. Copper has a TCR of 3900 ppm/°C. These, and other alloys allow us to “tailor” the resistor to desired characteristics in applications where temperatures change.

As a practical example, a resistor with a resistance of 1000 ohms, made from pure nickel wire, would have a new resistance of 1670 ohms if we increase its temperature from 20°C to 120°C. In the same application, a resistor made with Evenohm wire would increase to only 1001 ohms.