In a large city, where several inter - connected exchanges have to be built and thousands of subscribers are put into communication with each other, the service is at once more costly and more valuable than in a small town with a few hundred subscribers accommodated in one exchange.
An exchange is a central station to which wires are brought from the various subscribers in its neighbourhood, any two of whom can be put in telephonic communication with each other when the proper pairs of wires are joined together in the exchange.
When the subscribers in a local area exceed a certain number, or when for some other reason it is not convenient or economical to connect all the subscribers in the area to one exchange, it is usual to divide the area into a number of districts in each of which an exchange is placed, and to connect these district exchanges together by means of " junction circuits."
These inter-area or long-distance lines, called trunk circuits in England, terminate at one exchange in each local area, and between that exchange and the various district exchanges junction circuits are provided for the purpose of connecting subscribers to the trunk lines.
This board was equipped with spring-jacks and annunciators (calling-drops) for the subscribers' lines, and with flexible cords terminating in plugs for connecting purposes.