Harvard Mark I
One of the earliest computers in history was the Harvard Mark I. The IBM Automatic Sequence Controlled Calculator (ASCC), called the Mark I by Harvard University, was an electro-mechanical computer. Its size took up a large part of a room and weighed about 5 tons.
The electromechanical Mark I was devised by Howard H. Aiken. He built the Mark I at IBM and shipped it to Harvard in February 1944. It began computations for the U.S. Navy Bureau of Ships in May. It was used for gunnery and ballistic calculations. Mark I was then officially presented to the university on August 7, 1944. It remained in operation until 1959. The Mark I was very reliable, much more so than early electronic computers. It has been described as "the beginning of the era of the modern computer" and "the real dawn of the computer age".
The Mark I was built from switches, relays, rotating shafts, and clutches. It used 765,000 components and hundreds of miles of wire, comprising a volume of 51 feet (16 m) in length, eight feet (2.4 m) in height, and two feet (~61 cm) deep. It had a weight of about 10,000 pounds (4500 kg). The basic calculating units had to be synchronized mechanically, so they were run by a 50-foot (~15.5 m) shaft driven by a five-horsepower (4 kW) electric motor. This made for a very loud computer when operating. This excerpt is from the IBM Archives:
The Automatic Sequence Controlled Calculator (Harvard Mark I) was the first operating machine that could execute long computations automatically. A project conceived by Harvard University's Dr. Howard Aiken, the Mark I was built by IBM engineers in Endicott, N.Y. A steel frame 51 feet (16 m) long and eight feet high held the calculator, which consisted of an interlocking panel of small gears, counters, switches and control circuits, all only a few inches in depth. The ASCC used 500 miles (800 km) of wire with three million connections, 3,500 multipole relays with 35,000 contacts, 2,225 counters, 1,464 tenpole switches and tiers of 72 adding machines, each with 23 significant numbers. It was the industry's largest electromechanical calculator.
The enclosure for the Mark I was designed by futuristic American industrial designer Norman Bel Geddes. Aiken considered the elaborate case to be a waste of resources, since computing power was in high demand during the war and the funds, $50,000 or more according to Grace Hopper, could have been used to build additional computer equipment.
The Mark I had 60 sets of 24 switches for manual data entry and could store 72 numbers, each 23 decimal digits long. It could do three additions or subtractions in a second. A multiplication took six seconds, a division took 15.3 seconds, and a logarithm or a trigonometric function took over one minute. It also had the ability to reference to previous results. All output was displayed on an electric typewriter.
The Mark I read its instructions from a 24 channel punched paper tape. It was divided into 3 fields - the data for input, the operation to be performed, and the result of the operation. It executed the current instruction and then read in the next one. It had no conditional branch instruction. This meant that complex programs had to be physically long. A loop was accomplished by joining the end of the paper tape containing the program back to the beginning of the tape, literally creating a loop. This separation of data and instructions is known as the Harvard architecture, although the exact nature of this separation that makes a machine Harvard, rather than Von Neumann, has been obscured with the passage of time. The first programmers of the Mark I were computing pioneers Richard Milton Block, Robert Campbell, and Grace Hopper.
The Mark I was followed by the Harvard Mark II, the Mark III/ADEC in September 1949, and then the Harvard Mark IV in1952 - all the work of Aiken. The Mark II was an improvement over the Mark I, but it also used electromechanical relays. The Mark III used mostly electronic components, using vacuum tubes and crystal diodes, and the Mark IV was all-electronic, using solid state components. The Mark III and Mark IV used magnetic drum memory and the Mark IV also had magnetic core memory. The Mark II and Mark III went to the US Navy base at Dahlgren, Virginia. The Mark IV was built for the US Air Force, but it stayed at Harvard.
The Mark I was eventually disassembled, although portions of it remain at Harvard in the Science Center. It is part of the Harvard Collection of Historical Scientific Instruments. To see where the Mark I falls in the history of computers check out The History Of Computers and Technology - A Comprehensive Timeline.