Notes on the use of 100 series loudspeakers:

These loudspeakers are just that.  They simply amplify the sound fed into 
them and present it to a loudspeaker.  The various models from the 100 
through the 107 all do exactly the same thing.  Other equipment would be 
required to use them in a two-way conversation such as a speakerphone. These 
fulfill exactly the same function as the speakers on your computer or your 
stereo.  They are common at test boards, particularly special service 
boards.  The office I worked in had one at each test bay where we accessed 
toll carrier channels for testing.  It was frequently necessary to wait 
until a conversation was over so that a circuit could be busied out for 
maintenance. In order to determine when the conversation was over, we would 
usually plug the speaker into the circuit so we could listen to the 
conversation while we did other duties. When we heard them end the 
conversation, we could then quickly hop over to the relevant bay and busy 
the circuit out.

Other uses include telephone circuits that have many phones over a great 
geographical area all connected to one circuit such as a railroad dispatcher 
circuit.  Many important sites would monitor the line on a speaker much like 
a two-way radio would monitor for a call and so that each site could hear 
the information exchanged and be aware of events transpiring up and down the 
railroad.  When the site would pick up their handset to talk, the speaker 
would mute preventing feedback.  These circuits were usually four-wire which 
works quite differently from your home phone which is two-wire.  Multi-point 
permanent conference circuits use bridge circuits and amplifiers to maintain 
volume throughout.

Another common useage is for a "junkyard" circuit.  Many automobile 
junkyards would all subscribe and be connected to a common telepone circuit.  
Each would listen on a loudspeaker.  Any yard needing a part could ask on 
the line for what they were seeking and every other subscriber could hear 
them and respond if they had the needed part. On-line voice bidding could 
take place if more than one yard had the part.  The circuits were four-wire 
and when the handset was picked up, the speaker would mute.

The FAA uses these speakers on circuits between airport control towers, 
flight service stations and air route traffic control centers.

They are used to monitor one-way radio remote receivers. In Alaska many 
agencies such as the department of fish and game, fish processing plants and 
the troopers use SSB radio on MF and HF bands.  Frequently the office is in 
a fairly urban environment subject to quite a lot of electrical interference 
and they have great difficulty hearing weak signals over the noise.  A radio 
receiver can be installed at a remote, fairly quiet, site and connected to 
the office with a telephone line.  At the office they will use a monitor 
speaker on the cable pair to listen to the remote radio receiver.

At the offices of the Voice of America in Washington DC where they program 
in many different languages simultaneously, there is a need in offices to 
monitor many different program channels.  They had modified type 100 
speakers with dials connected to Strowger switches so they  could select 
different things to monitor from any office in the complex.

2002 June 30
Joe Stevens