HELP on TEMPERATURE COEFFICIENT of RESISTANCE (tcr) in ppm/░C
The electrical resistivity of a metal increases non-linearly with temperature (except for plutonium which decreases). The rate of change of resistivity with temperature is the temperature coefficient of resistance, measured in parts per million per degree centigrade (ppm/░C). The tcr varies with temperature, so is quoted at STP. In a metal, thermal agitation ensures that all the atoms are continually jiggling about, occasionally colliding with each other. The mean time before collision determines the resistivity of the metal, the longer the time, the greater the electrical resistance of the metal. Because the mean free time increases with temperature, so too does the electrical resistance. Thus the tcr of a metal is a positive value.
Things are different for some alloys. In a disordered solid solution alloy, there are many grain boundaries and crystal defects which increase the resistivity of the alloy well above that for either metal alone, and because these defects are fixed and invariant of temperature, the temperature dependence of the mean free path of electrons no longer dominates the resistivity change, and alloys therefore have a much lower temperature coefficient of temperature. Some, like manganin and constantan, because they have such high resistivities, have tcr's close to zero.
Nickel has the highest tcr at 6900ppm/░C. All the transition elements have tcr's of around 3000-7000. Generally, the lowest tcr's are exhibited by the rare earths (1000-4000) with Cerium having the lowest positive tcr of only 850ppm/░C. Platinum, which exhibits a more repeatable tcr (of 3927ppm/░C) is used as a thermometer in platinum resistance thermometry. The resistance of the tungsten element (tcr 4500ppm/░C) of incandescent light bulbs increases by ten-fold in going from cold to 2500░C. Plutonium is the only element to have a negative tcr (of -870ppm/░C), which is also very low.
