Figure 5-21.--Two-circuit, four-drop annunciator.

Figure 5-21 is a simple diagram for a two-circuit,

CIRCUIT EM - SOUND-POWERED

TELEPHONE SIGNAL

four-drop annunciator. Stations that have several

CIRCUIT

sound-powered telephones, each on a different circuit,

are provided with drop-type annunciators to identify the

circuit of the station that originates the call. Each drop

Circuit EM provides a means of signaling between

is embossed with the circuit letter and is held

sound-powered telephone stations, where more than six

mechanically in the nonindicating (normal) position.

stations are to be called by the calling station. Circuit

There is also an audible signaling device provided with

EM is divided into the same functional circuits as circuit

the annunciator.

E except that 1EM through 5EM is used. Circuit EM

uses type IC/D call signal stations, which require no

One side of each drop and one side of an

external power, and provides the operator with selective

audible-signal relay are connected together. When the

calling of up to 16 individual stations. The associated

circuit is energized by operating a switch at the calling

sound-powered telephone circuit is independent of the

station, the current flows through the drop and the relay.

signal circuit and provides the voice communication

The audible-signal relay closes its contacts to the

facilities between stations. Each sound-powered

audible signal and an electromagnet causes the proper

telephone circuit can accommodate only one

drop to fall to the indicating position. The audible signal

conversation over its facilities. The IC/D call signal

sounds only while the switch is operated. The drop is

station (fig. 5-22, views A and B), normally called a

returned to its normal position by a hand-operated reset

growler or a howler, uses a magneto generator to

button.

transmit a noninterrupted or interrupted signal to a

selected station.

The annunciator maybe equipped with one or more

audible-signal relays as required by the number of

associated circuits, but only one common audible signal

IC/D Noninterrupted Call Signal

device is used.

Station

The IC/D noninterrupted call signal station is made

of cast aluminum, with all of the equipment mounted on

the cover except for the terminal board and a

sound-powered jack outlet. Equipment mounted on the

cover include a 16-position rotary selector switch, an

index plate, a hand-operated magneto generator, a

howler unit, and an attenuator. The associated

sound-powered telephone circuit may be either the

string or the switchboard type.

To operate the station, you simply turn the rotary

selector switch to the station to be called and crank the

magneto generator handle. The howler (a modified

sound-powered telephone receiver unit) at the selected

station will produce a high distinctive howl. The howl

will continue for as long as the calling station generator

is cranked. The attenuator is used to control the volume

of the individual howler at its respective station. Each

Figure 5-21.--Two-circuit, four-drop annunciator.

EM circuit station is equipped with an IC/D call signal

5-23