note3 - CSCI 120 Introduction to Computation History of...

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Unformatted text preview: CSCI 120 Introduction to Computation History of computing (cont.) (draft) Saad Mneimneh Visiting Professor Hunter College of CUNY 1 The electrical age The technology in the years 1600 - 1900 was unable to produce the complex gear driven machines of Pascal, Leibniz, and Babbade in an economically fea- sible manner. But with the advances of electronics in the early 1900s, this barrier was overcome. It was clear at that point in time that what is needed is a programmable machine that uses electrical signals instead of punched cards to create programs. With electronics, however, the only way to simulate holes in punched cards is through the presence or absence of electric current. Therefore, the concept of a switch emerged. A number of switches is used to configure and program the machine. Needless to say, those switches must be operated by the machine itself and not by a human being. At first, the switches were electro-mechanical. For example, an electromechanical machine was completed by George Stibitz at Bell Laboratories in 1940. It was designed to perform calculations of complex numbers, and it used the binary system described by Leibniz. Similarly, Mark I was completed in 1944 at Harvard University by Howard Aiken and a group of IBM engineers. This was a general purpose com- puter influenced by Babbage’s Analytical Engine (and hence used the decimal system). Mark I was followed by Mark II. The following figure shows Mark I. Figure 1: Mark I These machines made heavy use of electronically controlled mechanical re- lays. The relays act as on-off switches to control various operations of the machine. In some sense, they encode the logic or the “smartness” of the com- puter. For instance, one could say if A OR B then C . Here’s a schematic for this statement using relays (electric current flows when either switch is closed). B A C ~ Figure 2: OR logic A concept of an electromechanical relay is show below: ~ Figure 3: Electromechanical relay The metal piece is held in place by means of a spring. The electric current creates a magnetic field through the coil that attracts the metal piece forward against the spring tension, causing it to be in contact with the upper piece. When the current ceases to exist, the spring brings back the piece to its original position. Therefore, by the time these machines were built, they became obsolete, since other researchers were applying the technology of vacuum tubes to replace the mechanical relays and to construct totally electronic computers! So what is a vacuum tube? A vacuum tube is an pure electronic relay as shown below (influenced by the work of Thomas Edison): Figure 4: Vacuum tube Vacuum tubes resemble light bulbs in that they have a filament sealed in a glass envelope which has been evacuated of all air. When hot, the filament releases electrons into the vacuum. This results in a negatively charged cloud of electrons. These electrons will be drawn to a metal ”plate” inside the envelope if the plate (also called the anode) is positively charged relative to the filament...
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This note was uploaded on 03/27/2010 for the course CSCI 120 taught by Professor Saadmneimneh during the Spring '09 term at CUNY Hunter.

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note3 - CSCI 120 Introduction to Computation History of...

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