metallic-circuit
lines, the metallic-circuit switchboard will serve equally well for
either metallic-circuit or grounded lines, and will interconnect them
with equal facility. This fact will be made clear by a consideration
of Figs. 276, 277, and 278.

[Illustration: Fig. 276. Connection Between Metallic Lines]

[Illustration: Fig. 277. Connection Between Grounded Lines]

_Connection between Two Similar Lines._ In Fig. 276 a common magneto
cord circuit is shown connecting two metallic-circuit lines; in Fig.
277 the same cord circuit is shown connecting two grounded lines. In
this case the line wire _1_ of the left-hand line is, when the plugs
are inserted, continued to the tip of the answering plug, thence
through the tip strand of the cord circuit to the tip of the calling
plug, then to the tip spring of the right-hand jack and out to the
single conductor of that line. The entire sleeve portion of the cord
circuit becomes grounded as soon as the plugs are inserted in the
jacks of such a line. Hence, we see that the sleeve contacts of the
plug and the sleeve conductor of the cord are connected to ground
through the permanent ground connection of the sleeve conductors of
the jack as soon as the plug is inserted into the jack. Thus, when the
cord circuit of a metallic-circuit switchboard is used to connect two
grounded circuits together, the tip strand of the cord is the
connecting link between the two conductors, while the sleeve strand of
the cord merely serves to ground one side of the clearing-out drop
and one side each of the operator's telephone set and the ringing
generator when their respective keys are operated.

_Connection between Dissimilar Lines._ Fig. 278 shows how the same
cord circuit and the same arrangement of line equipment may be used
for connecting a grounded line to a metallic-circuit line. The
metallic circuit line is shown on the left and the grounded line on
the right. When the two plugs are inserted into the respective jacks
of this figure, the right-hand conductor of the metallic circuit shown
on the left will be continued through the tip strand of the cord
circuit to the line conductor of the grounded line shown on the right.
The left-hand conductor of the metallic-circuit line will be connected
to ground because it will be continued through the sleeve strand of
the cord circuit to the sleeve contact of the calling plug and thence
to the sleeve contact of the jack of the grounded line, whic