circuit in Fig. 347, play no part in the
regular operation of connecting two local lines, as described above.
They are order-wire keys, and the circuits with which they connect lead
to the telephone sets of other operators at distant central offices, and
by pressing either one of these keys the operator is enabled to place
herself in communication over these so-called order-wire circuits with
such other operators. The function and mode of operation of these
order-wire circuits will be described in the next chapter, wherein
inter-office connections will be discussed.

_Wiring of Line Circuit._ The line circuits shown in Figs. 345 and 347
are, as stated, simplified to facilitate understanding, although the
connections shown are those which actually exist. The more complete
wiring of a single line circuit is shown in Fig. 348. The line wires are
shown entering at the left. They pass immediately, upon entering the
central office, through the main distributing frame, the functions and
construction of which will be considered in detail in a subsequent
chapter. The dotted portions of the circuit shown in connection with
this main distributing frame indicate the path from the terminals on one
side of the frame to those on the other through so-called jumper wires.
The two limbs of the line then pass to terminals _1_ and _2_ on one side
of the so-called intermediate distributing frame. Here the circuit of
each limb of the line divides, passing, on the one hand, to the tip and
sleeve springs of all the multiple jacks belonging to that line; and, on
the other hand, through the jumper wires indicated by dotted lines on
the intermediate distributing frame, and thence to the tip and ring
contacts of the answering jack. A consideration of this connection will
show that the actual electrical connections so far as already described
are exactly those of Figs. 345 and 347, although those figures omitted
the main and intermediate distributing frames. Only two limbs of the
line are involved in the main frame. In the intermediate frame the test
wire running through the multiple is also involved. This test wire, it
will be seen, leads from the test thimbles of all the multiple jacks to
the terminal _3_ on the intermediate frame, thence through the jumper
wire to the terminal _6_ of this frame, and to the test thimble of the
answering jack. Here again the electrical connections are exactly those
represented in Figs. 345 and 347, although those f