In a superconducting phase transition, the electric resistance changes with a jump, while the energy undergoes a continuous variation. One of the most important thermal characteristics of a substance, namely, the specific heat, or the amount of heat necessary to affect its temperature, also changes with a jump. There is a simple rule. In order that 1 g of water can be heated by 1 C at room temperature, 1 calorie of heat is needed (1 calorie is a little more than 4 J; one Joule (J) is the work done by a force of 1 Newton for a distance of 1 m). This rule means that the specific heat of water at room temperature is equal to unity. When a substance is cooled, its specific heat typically decreases; at the moment of a superconducting transition, however, it increases in a jump by approximately a factor of 2.5 3 (see Fig. 10).

Fig. 10: Temperature dependence of heat capacity near the superconducting transition. The blue line indicates the run of heat capacity of a normal metal as it might be in the absence of a superconducting transition.

For comparison, the heat capacities of several substances at room temperature are given in Table 4.