First generation computers, often referred to as mainframes, were large, costly to buy, expensive to power, abd often unreliable because of their delicacy and complexity. They were slow compared with today's computers, and their internal storage capacity was limited Vacuum tubes controlled the internal operations. The ENIAC contained more than 18,000 of these tubes and is rumored to have dimmed the lights of Philadelphia when it was turned on. Vacum tubes took up a lot of space and generated considerable heat. Special air conditioning had to be installed to cool the surroundings. Vacuum tubes could switch on and off thousands of times per second, but one tune would fail about every fitteen minutes. Too much time was wasted looking for the burned out tubes.