ity being reinforced by adjacent
braidings of tinsel. The structure of such a cord is well shown in
Fig. 265. Beginning at the right, the different elements shown are, in
the order named, a strand of lock stitch twine, a linen braiding, into
the strands of which are intermingled tinsel strands, the inner spiral
steel wrapping, a braiding of tussah silk, a linen braiding, a loose
tinsel braiding, the outer conductor of round spiral steel, a cotton
braid, and an outside linen or polished cotton braid. The inner tinsel
braiding and the inner spiral together form the tip conductor while
the outer braiding and spiral together form the sleeve conductor. The
cord is reinforced at the plug end for a length of about 14 inches by
another braiding of linen. The tinsel used is, in each case, for the
purpose of cutting down the resistance of the main steel conductor.
These wrappings of steel wire forming the tip and sleeve conductors
respectively, have the advantage of affording great flexibility, and
also of making it certain that whatever strain the cord is subjected
to will fall on the insulated braiding rather than on the spiral
steel which has in itself no power to resist tensile strains.

_Parallel Tinsel Conductors._ Another standard two-conductor
switchboard cord is manufactured as follows: One conductor is of very
heavy copper tinsel insulated with one wrapping of sea island cotton,
which prevents broken ends of the tinsel or knots from piercing
through and short-circuiting with the other conductor. Over this is
placed one braid of tussah silk and an outer braid of cotton. This
combines high insulation with considerable strength. The other
conductor is of copper tinsel, not insulated, and this is laid
parallel to the thrice insulated conductor already described. Around
these two conductors is placed an armor of spring brass wire in spiral
form, and over this a close, stout braid of glazed cotton. This like
the others is reinforced by an extra braid at the plug end.

Ringing and Listening Keys. The general principles of the ringing
key have already been referred to. Ringing keys are of two general
types, one having horizontal springs and the other vertical.

[Illustration: Fig. 266. Horizontal-Spring Listening and Ringing Key]

_Horizontal Spring Type._ Various Bell operating companies have
generally adhered to the horizontal spring type except in individual
and four-party-line keys. The construction of a Western Electric