A telephone exchange central office contains means for connecting lines at will in that useful way.  The least complicated machine for that purpose is a switchboard to be operated by hand, having some way of letting the operator know that a connection is wished and a way of making it.  The customary way of connecting the lines always has been by means of flexible conductors fitted with plugs to be inserted in sockets.  If the switchboard be small enough so that all the lines are within arm’s reach of the operator, the whole process is individual, and may be said to be at its best and simplest.  There are but few communities, however, in which the number of lines to be served and calls to be answered is small enough so that the entire traffic of the exchange can be handled by a single person.  An obvious way, therefore, is to provide as many operators in a central office as may be required by the number of calls to be answered, and to terminate before each of the operators enough of the lines to bring enough work to keep that operator economically occupied.  This presents the additional problem, how to connect a line terminating before one operator to a line normally terminating before another operator.  The obvious answer is to provide lines from each operator’s place of work to each other operator’s place, connecting a calling line to some one of these lines which are local within the central office, and, in turn, connecting that chosen local line to the line which is called.

Such lines between operators have come to be known as trunk lines, because of the obvious analogy to trunk lines of railways between common centers, and such a system of telephone lines may be called a trunking system.  Very good service has been given and can be given by such an arrangement of local trunks, but the growth in lines and in traffic has developed in most instances certain weaknesses which make it advisable to find speedier, more accurate, and more reliable means.

For the serving of a large traffic from a large number of lines, as is required in practically every city of the world, a very great contribution to the practical art was made by the development of the multiple switchboard.  Such a switchboard is merely such a device as has been described for the simpler cases, with the further refinement that within reach of each operator in the central office appears every line which enters that office, and this without regard to what point in the switchboard the lines may terminate for the answering of calls.  In other words, while each operator answers a certain subordinate group of the total number of lines, each operator may reach, for calling purposes, every line which enters that office.  It is probable that the invention and development of the multiple switchboard was the first great impetus toward the wide-spread use of telephone service.

Coincident with the development of the multiple switchboard for manually operated, central-office mechanisms was the beginning of the development of automatic apparatus under the control of the calling subscriber for finding and connecting with a called line.  It is interesting to note the general trend of the early development of automatic apparatus in comparison with the development, to that time, of manual telephone apparatus.