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 the face of the switchboard, but it is more usual to mount them in strips of five or ten.  A strip of five drops, as manufactured by the Kellogg Switchboard and Supply Company, is shown in Fig. 248.  The front strip on which these drops are mounted is usually of brass or steel, copper plated, and is sufficiently heavy to provide a rigid support for the entire group of drops that are mounted on it.  This construction greatly facilitates the assembling of the switchboard and also serves to economize space—­obviously,