DIGITAL COMPUTERS — McCORMICK 285 



To obtain the very high speed and extreme reliability required for 

 accurately producing, controlling, and counting pidses, electronic 

 engineers use devices ^Yhich represent numbers not by a base of 10, 

 but by a base of 2, that is, a binary system using only and 1. Thus 

 numbers are represented by combinations of many individual elec- 

 tronic devices, each of Avliich are either on or off, or by pulses which 

 are either present or absent at any given time. Many of these elec- 

 tronic devices have small neon lamjis connected to them so that the 

 lamps either glow or not depending on whether that device is repre- 

 senting a 1 or a 0. These lamps flash on and off quite rapidly as a 

 computer operates and are frequently shown in movies or television 

 views of computers. The binary system of representing numbers will 

 be considered in detail later. 



Since the nmnbers inside a computer are represented by electrical 

 pulses sent from one part of the computer to another, the input and 

 output devices for computers must operate in much the same manner. 

 A common device for input and out]:)ut for a computer is quite similar 

 to a teletype machine. The numbers are represented by combinations 

 of holes punched in a paper tape. As this tape is moved over a 

 reading device, the presence or absence of holes in the tape produces 

 a series of electrical pulses Avhich the computer uses to represent the 

 numbers. Similarly, a series of pulses in a computer can cause an 

 output device to punch a series of holes in a paper tape. These 

 pulses can also cause an electric tj^pewriter to type the numbers in 

 the usual form. This is desirable as the numbers are difficult to read 

 as holes in a tape. Further, the typing of numbers on the keyboard 

 of an input device will produce a punched paper tape suitable for 

 input to the computer. 



Many computers use business-accounting type machines for input 

 and output. Numbers are represented by holes in the familiar 

 pimched card. However, the basic operation of these devices is essen- 

 tially the same as considered above. 



METHOD OF SOLVING PROBLEMS 



An analogy can be used to illustrate the method by which a com- 

 puter solves problems. Consider a room in which there is a large 

 nmnber of file cabinets, each file drawer marked by a number. Each 

 drawer contains a slip of paper which is either a number or an instruc- 

 tion for some action to be taken. In this room is a clerk who goes 

 to the first drawer and obeys the instruction he finds there. He will 

 then go to the second drawer and obey the instruction there, and so on. 

 The only exception to this sequence is when the instruction in a 

 drawer specifically states that the next instruction is to be taken 

 from some other storage location. The clerk, in obeying most of 



