e rear end of the plug shell is usually bored out
just about the size of the outer covering of the switchboard cord, and
it is provided with a coarse internal screw thread, as shown. The cord
is attached by screwing it tightly into this screw-threaded chamber,
the screw threads in the brass being sufficiently coarse and of
sufficiently small internal diameter to afford a very secure
mechanical connection between the outer braiding of the cord and the
plug. The connection between the tip conductor of the cord and the tip
of the plug is made by a small machine screw connection as shown,
while the connection between the sleeve conductor of the plug and the
sleeve conductor of the cord is made by bending back the latter over
the outer braiding of the cord before it is screwed into the shank of
the plug. This results in the close electrical contact between the
sleeve conductor of the cord and the inner metal surface of the shank
of the plug.

Switchboard Cords. A great deal of ingenuity has been exerted toward
the end of producing a reliable and durable switchboard cord. While
great improvement has resulted, the fact remains that the cords of
manual switchboards are today probably the most troublesome element,
and they need constant attention and repairs. While no two
manufacturers build their cords exactly alike, descriptions of a few
commonly used and successful cords may be here given.

_Concentric Conductors._ In one the core is made from a double strand
of strong lock stitch twine, over which is placed a linen braid. Then
the tip conductor, which is of stranded copper tinsel, is braided on.
This is then covered with two layers of tussah silk, laid in reverse
wrappings, then there is a heavy cotton braid, and over the latter a
linen braid. The sleeve conductor, which is also of copper tinsel, is
then braided over the structure so formed, after which two reverse
wrappings of tussah silk are served on, and this is covered by a cotton
braid and this in turn by a heavy linen or polished cotton braid. The
plug end of the cord is reinforced for a length of from 12 to 18 inches
by another braiding of linen or polished cotton, and the whole cord is
treated with melted beeswax to make it moisture-proof and durable.

[Illustration: Fig. 265. Switchboard Cord]

_Steel Spiral Conductors._ In another cord that has found much favor
the two conductors are formed mainly by two concentric spiral
wrappings of steel wire, the conductiv