In all the previous descriptions of "field telephones" signaling between phones has used a DC voltage. While simple to design and use, alas it isn't employed except in very specialized applications like E&M signaling which is some times used between commercial PBX's. Also it it incompatible with most all commercial and military equipment you are likely to encounter.
That brings us to AC signaling which come in two principal flavors. 1) high voltage direct signaling: IE a 70-100 volt 20-30 Hz signal wich directly activates the the ringer ( or clacker) in a phone, and 2) low voltage audio tones (not unlike the familiar "touch tones" we all know) which are detected in the phone and in turn activates the alerting device ( speaker, or handset element) in the phone.
All commercial phones fall into the first category (except for not having a built in ringing generator), as do many military sets like the EE8, TA43, TA312, and the TA1040 ( from Sweden, and those German units that periodically become available).
Units that fall in to the second category are the TA341, TA838 and the TA1042 ( incidentally these set will also work on the civilian net work as well, when configured for it)
Field phones in the first category incorporate a built in ringing generator, usually hand operated, though the TA1040 has an electronic ring generator for signaling other phones or a switch board.
( see the first illustration)
Field Phones in the second group are some times multifunctional, in that they can be used for point to point communications using internal tone generators and detectors, or they can be switched over for use on a "analog" type military PBX or used on the civilian network.
( see the second illustration)
It should be noted that the second category using internal tone detection and generation are on at all times as that the tone detector is always waiting to receive a signal indicating an incoming call, this constant battery drain may affect battery life. Also note that this type of phone has a some what more complicated internal circuit, and that this mode of operation is not compatible with civilian equipment when used in the "point to point" mode.
You may encounter some other military type sets such as the TA954, and the TA1035, these are "Digital Field Phones" and can only be used on special "Digital" Military PBX's and are not useful for our purposes
(nice to look at but expensive paper weights).
The advantages of AC signaling over DC is practicably improved range ( The EE8 could communicate 26 miles under ideal conditions) and better compatibility with civilian equipment. For the purposed of this blog we will focus principally on direct AC signaling.
In the near future I will describe a DC powered (3-6v) ringing generator you can build, largely out of 1 off the shelf module and a few other discreet components, as well as several "voice networks" that can be built from of the shelf components as well.