Answer:
1st - Mechanical Relays, 2nd - Vacuum tubes, 3rd - Transistors
Explanation:
<em>From Tubes...</em>
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The three main components of a basic triode vacuum tube.
The type of tube used in early computers was called a triode and was invented by Lee De Forest in 1906. It consists of a cathode and a plate, separated by a control grid, suspended in a glass vacuum tube. The cathode is heated by a red-hot electric filament, which causes it to emit electrons that are attracted to the plate. The control grid in the middle can control this flow of electrons. By making it negative, you cause the electrons to be repelled back to the cathode; by making it positive, you cause them to be attracted toward the plate. Thus, by controlling the grid current, you can control the on/off output of the plate.
Unfortunately, the tube was inefficient as a switch. It consumed a great deal of electrical power and gave off enormous heat—a significant problem in the earlier systems. Primarily because of the heat they generated, tubes were notoriously unreliable—in larger systems, one failed every couple of hours or so.
<em>...To Transistors</em>
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The invention of the transistor was one of the most important developments leading to the personal computer revolution.The transistor was invented in 1947 and announced in 1948 by Bell Laboratory engineers John Bardeen and Walter Brattain. Bell associate William Shockley invented the junction transistor a few months later, and all three jointly shared the Nobel Prize in Physics in 1956 for inventing the transistor. The transistor, which essentially functions as a solid-state electronic switch, replaced the less-suitable vacuum tube. Because the transistor was so much smaller and consumed significantly less power, a computer system built with transistors was also much smaller, faster, and more efficient than a computer system built with vacuum tubes.
The conversion from tubes to transistors began the trend toward miniaturization that continues to this day. Today’s small laptop PC (or netbook, if you prefer) and even Tablet PC systems, which run on batteries, have more computing power than many earlier systems that filled rooms and consumed huge amounts of electrical power.
