The ABC (Atanasoff-Berry Computer)
Although only recognized as such many years later, the ABC (Atanasoff-Berry Computer) was really the first electronic computer. You might think "electronic computer" is redundant, but as we just saw with the Harvard Mark I, there really were computers that had no electronic components, and instead used mechanical switches, variable toothed gears, relays, and hand cranks. The ABC, by contrast, did all of its computing using electronics, and thus represents a very important milestone for computing.
Although it was electronic, the computer's parts were very different than what is used today. In fact, transistors and integrated circuits are required just to have the same building blocks. They did not exist in 1939 when John Atanasoff received funding to build a prototype, so he used what was available at the time: vacuum tubes. Vacuum tubes could amplify signals and act as switches, so they could thus be used to create logic circuits. However, they used a lot of power, got very hot, and were very unreliable. These were tradeoffs he and others had to live with and were unfortunate characteristics of the computers built from them.
The logic circuits he created with the vacuum tubes were fast, and could do addition and subtraction calculations at 30 operations per second. While it would be normal today, it was rare for a computer to use a binary system, since it was not a number system with which many were familiar at that time. Another important technology was the use of capacitors for memory, and "jogging" them with electricity to keep their contents (similar to a DRAM refresh used today). Memory was not truly random though, as it was actually contained in a spinning drum that rotated fully once per second. Specific memory locations could only be read from when the section of the drum they were in was over the reader. This obviously had serious latency issues. Later, he added a punched-card machine (punched cards were very extensively used by businesses at that time to store records and perform computations on them) to hold data that could not fit in the drum memory.
In retrospect, this computer wasn't terribly useful. It wasn't even programmable. But it was, at least on a conceptual level, a very important milestone for computers, and a progenitor to computers of the future. While working on this machine, Mr. Atanasoff invited a man named John W. Mauchly to view his creation. Let's find out why that was significant.