Most electrofusion welding power supplies are constant voltage machines. Constant current machines would provide more consistent energy input due to the smaller fluctuations in current applied to the coils during welding. However, this additional consistency is generally not worth the higher cost of these machines. When a constant voltage machine is used, the value of the applied current slowly decreases throughout the welding process. This effect is due to the increasing resistance of the coils as energy is applied. As heat is generated in the coils, their temperature increases, leading to a higher electrical resistance in the coils. This increased electrical resistance causes a smaller current to be generated from the same voltage level as the process progresses. The extent of the current decrease is determined by the material used for the coil. The energy input per unit area can be calculated and used to monitor the process. Typical values for this range from 2–13 J/mm, with a value of 3.9 J/mm having been found to produce the strongest joints.