ANALOGIES 



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to describe the switch on your reading lamp because there are only two 

 positions, "off" and "on." The former is recorded by the digit zero (0) and 

 the latter by the digit one (1). 



Figure 11-5. IBM's 7090 Digital Computer — A typical installation. A big, fast, transis- 

 torized machine, it can be used to simulate neural networks. To the right of the operator's 

 console are the card reader and printer; to the far left are the magnetic tape units. (Cour- 

 tesy of International Business Machines, Inc.). 



At the same time, the binary system of two digits can nicely represent in- 

 formation which is transmitted as pulses, because the information-carrying 

 equipment either is or is not delivering a pulse of energy at any particular 

 instant. If it is, it is described by the digit 1; if it is not, by 0. Remember 

 now that information is carried along the nerve in the form of electrochemi- 

 cal explosions. The nerve is either firing (1) or it isn't (0). Therefore, the 

 all-or-none law is basically a physical manifestation of the binary number 

 system. 



In summary, digital computers built of mechanical or electric binary ele- 

 ments (e.g., relays) not only compute, but also provide a prototype or model 

 for the study of nerve transmission and neural switching. 



The Analog Computer 



This second general class of computational machines is built around the 

 fact that useful electrical or mechanical analogies can often be made of phys- 



