The finished glass pieces are illuminated by either a neon sign transformer or a switched-mode power supply, usually running at voltages ranging between 2–15 kV and currents between 18 and 30 mA (higher currents available on special order.) These power supplies operate as constant-current sources (a high voltage supply with a very high internal impedance), since the tube has a negative characteristic electrical impedance. Standard tube tables established in the early days of neon are still used that specify the gas fill pressures, in either Ne or Hg/Ar, as a function of tube length in feet, tube diameter and transformer voltage.