d-Style Drop]

Liability to Cross-Talk:--This type of drop is suitable for use only
on small switchboards where space is not an important consideration,
and even then only when the drop is entirely cut out of the circuit
during conversation. The reason for this latter requirement will be
obvious when it is considered that there is no magnetic shield around
the winding of the magnet and no means for preventing the stray field
set up by the talking currents in one of the magnets from affecting by
induction the windings of adjacent magnets contained in other talking
circuits. Unless the drops are entirely cut out of the talking
circuit, therefore, they are very likely to produce cross-talk between
adjacent circuits. Furthermore, such form of drop is obviously not
economical of space, two coils placed side by side consuming
practically twice as much room as in the case of later drops wherein
single magnet coils have been made to answer the purpose.

_Tubular Drops._ In the case of line drops, which usually can readily
be cut out of the circuit during conversation, this cross-talk feature
is not serious, but sometimes the line drops, and always the
clearing-out drops must be left in connection with the talking circuit.
On account of economy in space and also on account of this cross-talk
feature, there has come into existence the so-called tubular or
iron-clad drop, one of which is shown in section in Fig. 247. This was
developed a good many years ago by Mr. E.P. Warner of the Western
Electric Company, and has since, with modifications, become standard
with practically all the manufacturing companies. In this there is but
a single bobbin, and this is enclosed in a shell of soft Norway iron,
which is closed at its front end and joined to the end of the core as
indicated, so as to form a complete return magnetic path for the lines
of force generated in the coil. The rear end of the shell and core are
both cut off in the same plane and the armature is made in such form as
to practically close this end of the shell. The armature carries a
latch rod extending the entire length of the shell to the front portion
of the structure, where it engages the upper edge of the pivoted
shutter; this, when released by the latch upon the attraction of the
armature, falls so as to display a target behind it.

[Illustration: Fig. 247. Tubular Drop]

[Illustration: Fig. 248. Strip of Tubular Drops]

These drops may be mounted individually on