580 



Electronic Computers /3I :5 



in centimeters is the analog of the reaction time; however, this pro- 

 portionality constant is different from the one relating this same distance 

 to real (sweep) time units. 



The problem can be solved because electronic units are available to 

 integrate, multiply by a constant (that is, amplify), multiply two 

 variables, add, and subtract. Such units are standard equipment and 

 are diagrammed below, using this symbolic code 



Integrator 



Multiplier 

 Adder 



Subtractor 



Parametric x_ 



Multiplier 



O 



■©■ 



-*■ Ax 



y 



{x+y) 



Variable 



Multiplier y 



M 



xy 



-U-y) 



Using these types of elements, one can compute the behavior of the 

 reactants for any chosen set of rate constants, k + , k_, /+, and m + . A 

 block-diagram type of circuit is shown in Figure 4. Note that the feed- 

 back loops fix the size of x, p, p', and a so that they obey the differential 

 equations. The clamps C hold the voltage at ground (zero) until time 

 zero and then release it to take on the values dictated by the circuit. 



-kx 



5. Digital Computers 



Digital computers are far more versatile than analog computers. When 

 properly instructed, they can carry out very complex calculations. How- 

 ever, it is necessary to understand the language of the computer in order 

 to instruct it properly. The set of instructions given the computer is 

 called the program; the utility of a digital computer depends on the 

 abilities of the programmer. 



Basically, there are only a limited number of types of operations which 

 a digital computer can do (about 100 for an IBM 704). These include: 

 add, subtract, multiply, divide, round off, shift the decimal point, read 

 numbers in, read numbers out, choose the larger of two numbers, 

 choose the smaller of two numbers, and change the sign of a number. 

 These are all essentially arithmetic operations. The computer also 

 must have a memory unit where it can store numbers, a control unit 

 which determines what it will do next, and a read-in and read-out unit. 



