sh _2_, will be connected to the
brush _5_ only while it is positively charged and with the brush _4_
only while it is negatively charged. As a result, brush _5_ has a
succession of positive impulses and brush _4_ a succession of negative
ones. Obviously, therefore, when key _K_^{1} is depressed only the
bell at Station A will be rung, and likewise the depression of key
_K_^{2} will result only in the ringing of the bell at Station B.

[Illustration: Fig. 171. Principle of Selection by Polarity]

_Four-Party Line._ From the two foregoing two-party line systems it is
evident that a four-party line system may be readily obtained, that
is, by employing two oppositely polarized biased bells on each side of
the metallic circuit. The selection of any of the four bells may be
obtained, choosing between the pairs connected, respectively, with the
two limbs of the line, by choosing the limb on which the current is to
be sent, and choosing between the two bells of the pair on that side
of the line by choosing which polarity of current to send.

Such a four-party line system is shown in Fig. 172. In this the
generators are not shown, but the wires leading from the four keys are
shown marked plus or minus, according to the terminal of the generator
to which they are supposed to be connected. Likewise the two bells
connected with the lower side of the line are marked positive and
negative, as are the two bells connected with the upper side of the
line. From the foregoing description of Figs. 170 and 171, it is clear
that if key _K_^{1} is pressed the bell at Station A will be rung, and
that bell only, since the bells at Station C and Station _D_ are not
in the circuit and the positive current sent over the lower side of
the line is not of the proper polarity to ring the bell at Station B.

The system shown in Fig. 172 is subject to one rather grave defect. In
subsequent chapters it will be pointed out that in common-battery
systems the display of the line signal at the central office is
affected by any one of the subscribers merely taking his receiver off
its hook and thus establishing a connection between the two limbs of
the metallic circuit. Such common-battery systems should have the two
limbs of the line, normally, entirely insulated from each other. It is
seen that this is not the case in the system just described, since
there is a conducting path from one limb of the line through the two
bells on that side to ground, and t