to do this and still leave it possible for any other
line to pick out and connect with any other idle line without
interference with the first connection. A number of parallel paths must
be provided for each possible connection. Groups of trunks are,
therefore, provided instead of single trunks between common points to be
connected. The subscriber who operates his instrument in making a call
knows nothing of this and it is, of course, impossible for him to give
any thought to the matter as to which one of the possible paths he shall
choose. It was by a realization of these facts that the failures of the
past were turned into the successes of the present. The subscriber by
setting his signal transmitter was made to govern the action of the
central-office apparatus in the selection of the proper _group_ of
trunks. The group being selected, the central-office apparatus was made
to act at once _automatically_ to pick out and connect with _the first
idle trunk of such group_. Thus, we may say _that the subscriber by the
act performed on his signal transmitter, voluntarily chooses the group
of trunks, and immediately thereafter the central-office apparatus
without the volition of the subscriber picks out the first idle one of
this group of trunks so chosen_. This fundamental idea, so far as we are
aware, underlies all of the successful automatic telephone-exchange
systems. It provides for the possibility of many simultaneous
connections through the switchboard, and it provides against the
simultaneous appropriation of the same path by two or more calling
subscribers and thus assures against interference in the choice of the
paths.

_Outline of Action._ In order to illustrate this point we may briefly
outline the action of the Strowger automatic system in the making of a
connection. Assume that the calling subscriber desires a connection with
a subscriber whose line bears the number 9,567. The subscriber in making
the call will, by the first movement of his dial, transmit nine impulses
over his line. This will cause the selective apparatus at the central
office, that is at the time associated with the calling subscriber's
line, to move its selecting fingers opposite a group of terminals
representing the ends of a group of trunk lines leading to apparatus
employed in connecting with the ninth thousand of the subscribers'
lines.

While the calling subscriber is getting ready to transmit the next
digit, the automatic apparatus,