it reduced in size in second, third, and fourth generations due to use of miniaturized components that consumed less power than those of the generation before. by reducing the power consumed it was also possible to reduce the size of the cooling system.
- tubes: between 1 to 5 devices per package, typically 1 inch diameter and 3 inches tall, each package might consume as much power as a 100 watt lightbulb and get just as hot.
- transistors: 1 device per package, typically 0.25 inch diameter and 0.25 inch tall, each package rarely consumed more than 1 watt of power typically much less.
- ICs: 10s to 100s of devices per package, typically 0.75 inch long and 0.25 inch long and 0.125 inch tall, each package might consume a few watts of power.
- microprocessors: 1000s to billions of devices per package, wide range of different sizes, each package consumes from a few watts to perhaps 60 watts.
as a minimal computer needs roughly 2000 devices let us estimate its size, power consumption, and cooling requirements for each generation.
- assuming an average tube contains 2 devices, consumes about 50 watts of power, and needs at least 1 inch of airspace on all sides for cooling: the computer will have 1000 tubes requiring several times more space than 10 cubic feet and consuming 50,000 watts of power. the cooling system must remove roughly 3000 btu every minute.
- assuming an average transistor consumes about 0.25 watt of power and needs only as much airspace around it as it occupies for cooling: the computer will have 2000 transistors requiring somewhat more space than 0.04 cubic feet and consuming 500 watts of power. the cooling system must remove roughly 30 btu every minute.
- assuming an average IC contains 40 devices, consumes 2 watts of power, and needs only as much airspace around it as it occupies for cooling: the computer will have 50 ICs requiring somewhat more space than 0.001 cubic feet and consuming 100 watts of power. the cooling system must remove roughly 6 btu every minute.
- assuming the microprocessor contains all 2000 devices of the computer, consumes 5 watts of power, occupies 1 inch by 1 inch by 0.2 inch, and needs only as much airspace around it as it occupies for cooling: the computer will have one microprocessor requiring somewhat more space than 0.0001 cubic feet and consuming 5 watts of power. the cooling system must remove roughly 0.3 btu every minute.
real computers involve far more than i considered above (e.g. cabinet, circuit modules, passive components) each increases the space needed for the computer. passive components also increase power consumption.