Experiments have shown that the value of L, while roughly constant, is not exactly the same for all materials. Kittel gives some values of L ranging from L = 2.23×10 V K for copper at 0 °C to L = 3.2×10 V K for tungsten at 100 °C. Rosenberg notes that the Wiedemann–Franz law is generally valid for high temperatures and for low (i.e., a few Kelvins) temperatures, but may not hold at intermediate temperatures.