THE BLACK BOX 6/7 



connexions" is not unique. The radio set, for instance, has one 

 diagram of connexions if considered electrically and another if 

 considered mechanically. An insulator, in fact, is just such a 

 component as will give firm mechanical connexion while giving no 

 electrical connexion. Which pattern of connexions will be found 

 depends on which set of inputs and outputs is used. 



Even if the diagram of immediate effects is unique, it does not 

 indicate a unique pattern of connexions within the Box. Thus 

 suppose a Black Box has an output of two dials, x and y; and 

 suppose it has been found that .y dominates y. The diagram of 

 immediate effects is thus 



X 



(in which the two boxes are parts of the whole Box). This relation- 

 ship can be given by an infinity of possible internal mechanisms. A 

 particular example occurs in the case in which relays open or close 

 switches in order to give a particular network of connexions. It 

 has been shown by Shannon that any given behaviour can be pro- 



A p- 



B P- — X- 



C P^V- T. 



^n 



-X- 



Fig. 6/7/1 



l_f _1 



duced by an indefinitely large number of possible networks. Thus, 

 let X represent a contact that will be closed when the relay X is 

 energised, and let x represent one that will be opened. Suppose 

 similarly that another relay Y has similar contacts y and y. Suppose 

 that the network is to conduct from p to q when and only when 

 both X and Y are energised. The network A of Fig. 6/7/1, in 

 which X and y are connected in series, will show the required be- 

 haviour. So also will B, and C, and an indefinitely large number 

 of other networks. 



The behaviour does not specify the connexions uniquely. 



Ex. : (Ex. 6/5/4 continued.) Deduce the diagram of immediate effects when the 

 input is fixed at a. (Hint : S.4/ 1 3 .) 



93 



