h _3_
adapted to be manually operated is connected in the circuit with the
battery and the buzzer so as to open this circuit when the night alarm
is not needed, thus making it inoperative. During the portions of the
day when the operator is needed constantly at the board it is
customary to leave this switch _3_ open, but during the night period
when she is not required constantly at the board this switch is closed
so that an audible signal will be given whenever a drop falls. The
night-alarm contact _4_ on each of the drops will be closed whenever a
shutter falls, and as the two members of this contact, in the case of
each drop, are connected respectively with the two sides of the
night-alarm circuit, any one shutter falling will complete the
necessary conditions for causing the buzzer to sound, assuming of
course that the switch _3_ is closed.

_Night Alarm with Relay._ A good deal of trouble has been caused in
the past by uncertainty in the closure of the night-alarm circuit at
the drop contact. Some of the companies have employed the form of
circuit shown in Fig. 275 to overcome this. Instead of the night-alarm
buzzer being placed directly in the circuit that is closed by the
drop, a relay _5_ and a high-voltage battery _6_ are placed in this
circuit. The buzzer and the battery for operating it are placed in a
local circuit controlled by this relay. It will be seen by reference
to Fig. 275 that when the shutter falls, it will, by closing the
contact _4_, complete the circuit from the battery _6_ through the
relay _5_--assuming switch _3_ to be closed--and thus cause the
operation of the relay. The relay, in turn, by pulling up its
armature, will close the circuit of the buzzer _2_ through the battery
_7_ and cause the buzzer to sound.

[Illustration: Fig. 275. Night-Alarm Circuit with Relay]

The advantage of this method over the direct method of operating the
buzzer is that any imperfection in the night-alarm contact at the drop
is much less likely to prevent the flow of current of the high-voltage
battery _6_ than of the low-voltage battery _1_, shown in connection
with Fig. 274. This is because the higher voltage is much more likely
to break down any very thin bit of insulation, such as might be caused
by a minute particle of dust or oxide between contacts that are
supposed to be closed by the falling of the shutter. It has been
common to employ for battery _6_ a dry-cell battery giving about 20
or 24 volts, a