Night-Alarm Contacts. The night-alarm contact 4 of the drop shown diagrammatically in Figs. 274 and 275 would, if taken literally, indicate that the shutter itself actually forms one terminal of the circuit and the contact against which it falls, the other.  This has not been found to be a reliable way of closing the night-alarm contacts and this method is indicated in these figures and in other figures in this work merely as a convenient way of representing the matter diagrammatically.  As a matter of fact the night-alarm contacts are ordinarily closed by having the shutter fall against one spring, which is thereby pressed into engagement with another spring or contact, as shown in Fig. 249.  This method employs the shutter only as a means for mechanically causing the one spring to press against the other, the shutter itself forming no part of the circuit.  The reason why it is not a good plan to have the shutter itself act as one terminal of the circuit is that this necessitates the circuit connections being led to the shutter through the trunnions on which the shutter is pivoted.  This is bad because, obviously, the shutter must be loosely supported on its trunnions in order to give it sufficiently free movement, and, as is well known, loose connections are not conducive to good electrical contacts.

Grounded-and Metallic-Circuit Lines.  When grounded circuits were the rule rather than the exception, many of the switchboards were particularly adapted for their use and could not be used with metallic-circuit lines.  These grounded-circuit switchboards provided but a single contact in the jack and a single contact on the plug, the cords having but a single strand reaching from one plug to the other.  The ringing keys and listening keys were likewise single-contact keys rather than double.  The clearing-out drop and the operator’s talking circuit and the ringing generator were connected between the single strand of the cord and the ground as was required.

The grounded-circuit switchboard has practically passed out of existence, and while a few of them may be in use, they are not manufactured at present.  The reason for this is that while many grounded circuits are still in use, there are very few places where there are not some metallic-circuit lines, and while the grounded-circuit switchboard will not serve for 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]