Long before LED and Vacuum fluorescent displays were invented, tubes were used to display the numbers in early computers, calculators, and electronic test equipment. These unique tubes are called Nixie Tubes. Unlike tubes we use in audio circuits and power supplies, nixie tubes have a cold cathode. This means they do not need heat for their cathodes to function and rarely get much warmer than room temperature. As well, they are actually not vacuum tubes at all, but rather filled at low pressure with gas.