gh the metallic circuit of the line, the
two windings exert a differential effect. As soon, however, as the
calling subscriber hangs up his receiver, this differential action
ceases, due to the grounding of both sides of the line at the
subscriber's station. This relay, therefore, operates and cuts off
battery from the called battery supply relay and this, in turn, releases
its armature and thus changes the connection of the rotary side of the
calling line from ground to live side of the battery. The normal
condition of the battery connection now being restored, both the
vertical and the rotary relays at the connector become operated, due to
the ground on both sides of the line at the subscriber's station, and
this, as we have seen, is the condition which brings about the operation
of the connector release magnet, and the relaying back of the disconnect
impulse successively through the selectors to the line switch.

=Multi-Office System.= In exchanges involving more than one office, the
same general principles and mode of operation already outlined apply. If
the total number of subscribers in the multi-office exchange is to be
less than ten thousand, then four digit numbers suffice, and the first
movement of the dial may be made to select the office into which the
connection is to go, the subscribers' lines being so numbered with
respect to the offices that each office will contain only certain
thousands. The choosing of the thousand by the calling subscriber,
therefore, takes care in itself of the choice of offices. Where,
however, a multi-office exchange is to provide for connections among a
greater number of lines than ten thousand and less than one hundred
thousand, then it will take five movements of the dial to make the
selection--the five movements corresponding either to the five digits in
a number or to the name of an office, as indicated on the dial, and the
four digits of a smaller number. The lines may all carry five digit
numbers or, what is considered better practice, may be designated by an
office name followed by a four digit number. In this latter case the
numbers of the subscribers' lines will in each case be contained in one
or more of the tens of thousands groups, no number having more than four
digits. And the first movement of the dial, whether the name or number
plan be adopted, will select an office; or, looking at it another way,
will select a group of ten thousand and this being done, the next f