74 On the Municipal Electric Telegraph, 
to the passage of the pin. Its arrangement in the figure shows 
its application to testing the continuity of one of the Signal 
Wires, represented in fig. 2. In its usual position, the testing key 
is held by a spring, in contact (which should be a sliding contact,) 
with the little anvil d. It will 8. | 
be seen that the wire &, con- 
circle, returning into itself 
through the screw-cups con- 
nected with d and the axis of 
the testing key. When moved 3 
by the revolution of the cyl- i @ 
inder, the key breaks the con- 4 
tact at d, and makes contact 
with the little anvil e, which 
is connected through the elec- 
tro-agnet f, with the platina 
le of the battery. Hence it 
completes the circuit, if the 
wire k is unbroken, and the 
bell A is struck by the ham- 
mer carried by the armature 
and lever g. 
The knob c, on the lower 
t of the testing key, makes 
it available for use, at any in- 
termediate time, by hand. The 
arrangement of this key also’ 
shows how a single wire of 
the Signal Battery can be 
brought into cireuit, for the 
purpose of communicating 
back to the Stations. It will be observed that one end of the 
wire & always remains connected with its battery, whether the 
key 6 is manipulated or not. Its function of signalizing fire 
therefore never interfered with. 
The testing key applied to the Alarm Circuit connects it mo 
mentarily with a battery, perhaps the Signal battery, too weas 
to set off any of the striking machinery, and yet strong enough 
to actuate the electro-magnetic alarm within the clock. In t 
Boston System, if the six wires of the Signal Cireuits and the 
three Alarm Circuits should be tested together, nine bells would 
be struck iu regular succession, at the time of the usual striking 
of the clock. These should be separated from each other 1 
tone by a musical interval, and bells representing different classe’ 
i he scale. If one 
of circuits should be in different parts of t @ 
