nd for the operation of the buzzer itself, a similar
battery of about two cells giving approximately 3 volts.

_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