number of series contacts in the jacks. This is not to be desired
in any case, but it was particularly objectionable in the early days
before jacks had been developed to their present high state of
perfection. A particle of dust or other insulating matter, lodging
between the tip spring and its anvil in any one of the jacks, would
leave the line open, thus disabling the line to incoming signals, and
also for conversation in case the break happened to occur between the
subscriber and the jack that was used in connecting with the line.
Another defect due to the same cause was that the line through the
switchboard was always unbalanced by the insertion of a plug, one limb
of the line always extending clear through the switchboard to the drop
and the other, when the plug was inserted, extending only part way
through the switchboard and being cut off at the jack where the
connection was made. The objection will be apparent when it is
remembered that the wires in the line circuit connecting the multiple
jacks are necessarily very closely bunched together and, therefore,
there is very likely to be cross-talk between two adjacent lines unless
the two limbs of each line are exactly balanced throughout their entire
length.

Again the busy-test conditions of this circuit were not ideal. The fact
that the test rings of the line were connected permanently with the
outside line circuit subjected these test rings to whatever potentials
might exist on the outside lines, due to any causes whatever, such as a
cross with some other wire; thus the test rings of an idle line might by
some exterior cause be raised to such a potential that the line would
test busy. It may be laid down as a fundamental principle in good
multiple switchboard practice that the busy-test condition should be
made independent of any conditions on the line circuit outside of the
central office, and such is not the case in this circuit just described.

[Illustration: CABLE RUN FROM INTERMEDIATE FRAME TO MULTIPLE Cortlandt
Office, New York Telephone Co.]

=Branch-Terminal Multiple Board.= The next important step in the
development of the magneto multiple switchboard was that which produced
the so-called branch-terminal board. This came into wide use in the
various Bell operating companies before the advent of the common-battery
systems. Its circuits and the principles of operation may be understood
in connection with Fig. 338. In the branch-terminal system there are