Speer carbon composition resistors, in particular the 470 Ω and 220 Ω 1/2 W grade 1002 resistors, have been used as secondary thermometers at temperatures below 4 K for many years. Their zero field resistances have been measured between 300 K and 4 K using a "dip" probe. Above 10 K, the resistance behavior can be explained using a simple temperature power law, R(T) ≈ R0/T 0.16. The resistance measurements have been extended to 0.02 K using dilution refrigerators. Between 4 K and 0.3 K, the resistances exhibited activated laws having hopping exponents y ≈ 0.5. Below 0.3 K, the 470 Ω resistors exhibit a crossover to a weaker activated law. Crossover resistance expressions suggest that the resistances follow a Mott variable-range hopping (VRH) law below 0.05 K. The low temperature magnetoresistance (MR) data showed changes of less than ±12 % of the zero field resistance values in fields up to 10 T. Fits using the wave function shrinkage and the forward interference models gave only fair agreement with the MR data.
- Activated and variable-range hopping (VRH) resistance laws
- Low temperature thermometry
- Speer carbon resistors