This essay presents the timeline from the first to the last portable analog wireline local battery field telephones used in Germany, from the Imperial German Army to the Bundeswehr (Pictures are from instruments in my collection if not otherwise noted).
Electrical Telegraphy was used by the German Army from the mid 1850ies onwards. Initially part of the Pioneer Corps the "Telegraphentruppe" (Signal Corps) was established as independent branch in 1899 [31]. Portable telephones were first introduced in 1894 with the "Patrouillenapparat", a Handset with buzzer in the handle which requires an external battery box [21], let's call that one "Generation 0".
A distinction was made between a "Feldfernsprecher" (Field telephone) which was an integral set built into a box with a handset, a magneto, a ringer and a buzzer, and a "Armeefernsprecher" (Army telephone) which was equipped only for buzzer call, with the buzzer built directly into the handset. The first integral field telephone the "Feldfernsprecher" was introduced in 1904 [31], a main wood box holds the handset, a magneto, ringer, capacitor and the buzzer, the batteries are connected through an additional external wooden battery box. The buzzer coils are used as speech coils. The capacitor is switchable and allows to use the instrument either on local battery lines (no capacitor), local battery lines with end of call DC supervision (capacitor in speech circuit) or central battery lines (capacitor in ringer circuit). A fourth position where the capacitor is put into speech and ringer circuit is used to tap into telegraphy lines (where the capacitor filters the telegraphy DC pulses from the telephone) for simultaneous operation of telegraphy and telephony on the same wire [1] [21].
In the picture below a "Feldfernsprecher" (Siemens & Halske) connected to a battery box ("Sprechbatterie der Feldartillerie").
In 1905 an updated version of the Patrouillenapparat was introduced, initially called "Leichter Feldfernsprecher" and then later called "Armeefernsprecher" [31], an aluminium made handset with builtin buzzer, using an external battery box which also provides the line binding posts.
In the picture below an "Armeefernsprecher" model 1905 in use, connected to a "Sprechbatterie".
Bundesarchiv, Bild 146-1970-038-68 / CC-BY-SA 3.0, CC BY-SA 3.0 DE, via Wikimedia Commons.
In 1913 [34] the Armeefernsprecher and the Feldfernsprecher, were updated to use iron instead of aluminum and brass for the metallic parts, these updated models are called "Eiserner Armeefernsprecher" or "Eiserner Feldfernsprecher" [1] [1a] ("eisern" = made of iron). The "Eiserner Feldfernsprecher" also updates the functionality in that it supports patching of 2 wire setups (The old "Feldfernsprecher" only supported patching for 1 wire with earth) and it supports parallel buzzer and magneto operation (On the old "Feldfernsprecher" depending on which socket the local patch-cord was connected either the buzzer or the magneto circuit was connected).
In the picture below an "Eiserner Feldfernsprecher" (Zwietusch) with headset and battery box ("Feldsprechbatterie 16").
To be able to use the "Armeefernsprecher" with switchboards which did not support buzzer call a "Zusatzkasten" (extension box) containing a hook switch, a magneto and a ringer was introduced [1b] [2].
The batteries for all above models are external, with 3x 1,5V elements mounted into a wooden box with sockets for the "Feldfernsprecher" and "Armeefernsprecher" battery plugs and the "Armeefernsprecher" line plug. A distinction is made between the "Sprechbatterie" which includes a fix connected line patch-cord, and the "Armeesprechbatterie" without line patch-cord. Of both an "alte Art" (old version) and a "neue Art" (new version) exist. Both can be used to build a small "switchboard" using the built in line patch-cords on the "Sprechbatterie" or separate patch-cords on the "Armeesprechbatterie" and the built in patch sockets. The "alte Art" battery boxes only support single wire with earth line patching, whereas the "neue Art" support also two wire setups [3] [34]. The "Felsprechbatterie 16" (and the identical "Feldsprechbatterie 17") is functionally identical to the "Armeesprechbatterie neuer Art", but the line switching sockets are marked with "Anschlussklinke" instead of "A" and "Vermittlungsklinke" instead of "B" and their positions are crossed out [1a] [2].
In the picture below on the left an "Eiserner Armeefernsprecher" (C. Lorenz) with headset, battery box ("Feldsprechbatterie 16") and a "Zusatzkasten" (Magneto and ringer) and on the right an "Eiserner Armeefernsprecher" (Mix & Genest) in the pouch.
In 1916 the "Feldfernsprecher 16" combined all "Eiserner Feldsprecher" LB features (Magneto, Ringer, Buzzer) together with batteries and a slightly smaller handset into a single wooden box of the same size than the former "Feldfernsprecher". Initially old "Feldfernsprecher" were refurbished for that use (these were apparently designated "Type A" [34]), the final new product was then called "Type B" [36]. The "Feldfernsprecher 16" does not include a capacitor for connection to CB or SB ("Selbsttätiges Schlusszeichen", end of call DC supervision) networks.
In the picture below a "Feldfernsprecher 16" (Siemens) made from a refurbished "Feldfernsprecher (1904)", the handset got the talking side replaced, on the box where the capacitor switch was a perforated metal cover is attached and the "Feldfernsprecher 16" diagram is mounted to the lid.
In the picture below a "Feldfernsprecher 16 Type B" (Zwietusch).
The much lighter and smaller "Feldfernsprecher 17" with included magneto, ringer, battery and optional/pluggable buzzer was still developed in the war years but when introduced to the troops the war was practically over [31]. The "Feldfernsprecher 17" replaced the former "Feldfernsprecher" models (but without capacitor), to replace the "Armeefernsprecher" a "kleiner Feldfernsprecher 17" was developed using the same "Armeefernsprecher" concept of a Handset and a battery box [21], inside the battery box the same pluggable buzzer "Summerzusatz 17" can be plugged. To connect the "Grosser Feldfernsprecher 17" to a CB network there was an external hook-switch adapter "Hakenumschalter" [4]. The "Feldfernsprecher 17" defined the main form and setup for all subsequent german field telephones up to the latest post WWII models, with it's form factor, the pluggable handset and the magneto position parallel to the long side of the instrument and hence the magneto handle mounting on the short side.
After WWI Germany had to downsize the armed forces to 100'000 men based on the Treaty of Versailles. The treaty allowed for one signal detachment of ~300 men per the 7 infantry divisions, resulting in a max. total of ~2000 signal corps personnel (after the mobilisation 1914 ~25000, by the end of WWI 1918 ~180000). The German army turned over more than 200'000 field telephones and field telephone accessories for destruction. Per signal detachment only 120 field telephones were assigned, for a total of <1000 [33].
In 1926 a new "Feldfernsprecher" was introduced [3]. It is an updated version of the "Feldfernsprecher 17" with a lighter handset (identical to the postal handset), an integrated foldable hookswitch and reintroducing the original "Feldfernsprecher" option to connect directly to central battery lines based on a built on capacitor. The pluggable "Summerzusatz 17" is still supported by it. To distinguish from the later "Feldfernsprecher 33" it was later called "Feldfernsprecher 26" [4]. The "Feldfernsprecher 26" was produced until 1935 [35], by then production of the "Feldfernsprecher 33" was already under way. To connect the "Feldfernsprecher 26" to an automatic network there was an external dial adapter "Nummernscheibenkästchen" [4].
In the picture below a "Feldfernsprecher 26" (C. Lorenz 1935)", Feldfernsprecher 17" in the back and a "kleiner Feldfernsprecher 17" (both Zwietusch) in the front. In front of the "Feldfernsprecher 26" an open "Summerzusatz 17".
With the seizure of control of the Nazis the German Army was rearmed (in violation of the Treaty of Versailles). A number of new field line telephony equipment was introduced in the early 30ies, like the "kleiner Klappenschrank" with "Amtszusatz", "Amtsanschliesser 33", and amongst that the "Feldfernsprecher 33".
The basic form and setup is taken from the "Feldfernsprecher 17". The additional "Feldfernsprecher 26" features like hookswitch and direct connection to CB networks were dropped, also no buzzer was planned anymore. The anti-sidetone transmission coil setup, introduced on civil side in the late 20ies, was used for the first time in a german military field telephone. Instead of a wooden construction a metal chassis inside a bakelite box is used. All in all a "no frills" but reliable setup designed for mass production. To connect the "Feldfernsprecher 33" to a SB (LB line with DC call supervision), CB or automatic line the "Amtsanschliesser 33" can be used (which can also work as a standalone dial telephone), then there was also an "SB-Zusatz 33" for connection to a SB (LB line with DC call supervision) line [39].
About in 1939 the electrical setup was changed from an anti-sidetone setup with the RX circuit galvanically separated from the line to a more common setup with the TX and battery circuit galvanically separated for an improved line impedance matching [5]. Devices with the new setup were marked by a green line on the line marking label and the microphone capsules were marked by a green cross [39].
In the "Nachrichtenfibel" of ~1936 the "Feldfernsprecher 26" is still mentioned as the "am häufigsten vorkommende Feldfernsprecher" (the most common field telephone) [4]. My assumption is that mass production of the "Feldfernsprecher 33" started after 1935 (after the reinstatement of conscription and transformation of the 10 Div. "Reichswehr" to the 100 Div. "Wehrmacht" [33]) and increased by 1939 based on the model with improved line impedance matching [5].
Over the course of production of the "Feldfernsprecher 33" it also received mechanical changes to make production cheaper. The metallic chassis was made of iron instead of light-alloy (Some devices were specifically marke "E" (for "E"isen = iron), on later models the chassis lids were not added anymore, the battery lid used a simpler construction, the line binding posts were made of plastic instead of metal, the magneto handle was made of sheet metal instead of cast iron [39].
In the picture below a group of "Feldfernsprecher 33", on the left a very late version (Components 1944, Magneto stamped "30. Jan. 1945", No mfg. indication, Missing battery cup) in the middle an early version (F. Schuchhardt 1937), on the right a "green line" version (F. Merk Telefonbau 1940).
There is also a special version for the navy ("Marine") with an adapter fixed to the bakelite box to mount an adapter for the navy headset [39], the navy version also uses a different circuit setup similar to the early version setup, and is marked with a yellow stripe.
The "Feldfernsprecher 33" has still been used after WWII by the early "Bundeswehr" [10] and the German Post office [9], and others like the Norwegian army, and even some have even been still manufactured up to the 60ies.
In the picture below a device adapted by the Norwegian army with a Norwegian spelling table (originally made by R. Bosse, 1944), a navy version (note the adapter plate and the yellow stripe, made by Siemens Apparatebau, 1940), and one made in 1964 (no mgf. indication), in a metallic box, no patch-cord sockets, adapted for use with a D-Cell.
In 1943 the "OB Fernsprecher 43 für Ortsfesten Betrieb" was added to the lineup [39]. Based on the general "Feldfernsprecher 33" setup, size and form but highly economised on features and built quality, as it was intended to be used only at fix locations no sturdiness was required.
In the picture below a "OB Fernsprecher 43" (Mix & Genest).
During WWII also instruments from occupied countries were used, like e.g. the Czech "VZ 23" and "VZ 35", the Austrian "M 35" and the Norwegian "Modell 1930".
In the picture below an Austrian "M 35" (Kapsch & Söhne), a Czech "VZ 23" (VTD, Vojenské telegrafní dílny) and a Norwegian "Modell 1930" with german spelling table (Elektrisk Bureau).
After WWII both Germanys (West and East) did not have an official regular army again until 1955. Though on both sides predecessor organisations were activated from 1950 onwards, in West Germany the "Bundesgrenzschutz" (Frontier Protection) and in East Germany the "Kasernierte Volkspolizei" (Militarised Police) which both were transformed into regular armies in 1955 ("Bundeswehr" in West Germany and "Nationale Volksarmee" in East Germany. Also on both sides initially earlier field telephony equipment like the "Feldfernsprecher 33" was still used [10] [25] as well as on the West German side US equipment like the EE-8 [11] and on the East German side Russian devices like the TAI-43 and later the TA-57 [25].
The "Feldfernsprecher 53" (East Germany) is a LB only field telephone, electrically and mechanically similar to the "Feldfernsprecher 33", the Handset and handset plug are nearly identical. A remarkable feature of the "Feldfernsprecher 53" is the "long distance" switch which can activate the "Ruftransformator" (Calling transformer), and can switch the 2x 1,5V batteries from normally parallel to serial (3V) [23]. A second generation instrument called "Funkfernsprecher 53a" added the possibility to remotely operate radio equipment (by adding a handset contact which activates a DC current through the line). It also got rid of the "long distance" support requirement by using a better ringer, a better RX module, and an improved anti-sidetone coil setup [12]. To connect the "Feldfernsprecher 53" to a SB (LB line with DC call supervision), CB or automatic line the "Amtszusatz OB 52" can be used [23].
In the picture below a "Feldfernsprecher 53" and a "Funkfernsprecher 53a".
The "Feldfernsprecher OB/ZB" (West Germany, sometimes also designated "Feldfernsprecher 54" although it was not delivered before 1957 [13]) is an major improvement of the "Feldfernsprecher 33" setup, and also brings back the foldable hook cradle and the possibility to directly connect to a CB network known form the "Feldfernsprecher 26". In case of the "Feldfernsprecher OB/ZB" the position of the hook cradle defines the operation mode: folded in = LB, folded out = CB. Remarkable features of the "Feldfernsprecher OB/ZB" are the possibility to move the batteries out of the device to keep them near the operators body to keep them warm for winter operation and the handset "P.T.T" lever setup which disconnects the RX side when "hung up": "When the handset is properly hung up, the receiver capsule is single-pole disconnected from the line in all operating modes in the idle state and the station is therefore protected against eavesdropping to a limited extent." [13]. To connect the "Feldfernsprecher OB/ZB" to an automatic line the "Wählzusatz" can be used.
The "Feldfernsprecher 63" (East Germany) is the last German developed analog local battery field telephone. It is mechanically and electrically a new design. Still in a bakelite box, but when closed everything is sealed under the main lid, it is IP54 dust and splash protected. The concept of disconnecting the RX side when hung up is taken from the West German "Feldfernsprecher OB/ZB". On the TX side the setup is completely different as it uses an active microphone amplifier and hence a dynamic microphone capsule (The same capsule is used on the RX side) and not a carbon one. The microphone amp. requires a supply voltage of 6V, achieved by a special 6V accu or an adaptor for 5x AAA elements. The "Feldfernsprecher 63" was updated twice, first to the "FF 63M" (~1970) and then to the "FF 63S" (~1980, built until the end of the 80ies), the updates mainly improved and updated on the microphone amp [14] [15] [16]. To connect the "Feldfernsprecher 63" to an automatic line the "Amtsanschliesser AS 60" can be used.
In the picture below a "Feldfernsprecher 63S" with "Amtsanschliesser AS 60" and a "Feldfernsprecher OB/ZB" with "Wählzusatz" (both SEL).
Below table gives an overview of all above described instruments. The "generation" is my classification. The years of introduction are based on sources listed at the page end. All the instruments are also listed on the Global Field Telephones Comparison List.
| Generation | Name | Year | Weight [kg] | Battery | [B]uzzer or/and [M]agneto call |
|---|---|---|---|---|---|
| 0 | Patrouillenapparat | 1894 | 5 | 3x 1.5V "Feldelement" (External) | B |
| 1 | Feldfernsprecher | 1904 | 11 | 3x 1.5V "Feldelement" (External) | B/M |
| Armeefernsprecher | 1905 | 5 | 3x 1.5V "Feldelement" (External) | B/(M external) | |
| Eiserner Armeefernsprecher | 1913 | 5 | 3x 1.5V "Feldelement" (External) | B/(M external) | |
| Eiserner Feldfernsprecher | 1913 | 11 | 3x 1.5V "Feldelement" (External) | B/M | |
| Feldfernsprecher 16 (Types A, B) | 1916 | 9 | 2x 1.5V "Feldelement" | B/M | |
| 2 | Feldfernsprecher 17 | 1917 | 7 | 2x 1.5V "Feldelement" | B (optional)/M |
| Kleiner Feldfernsprecher 17 | 1917 | 2x 1.5V "Feldelement" | B | ||
| Feldfernsprecher 26 | 1926 | 6.5 | 2x 1.5V "Feldelement" | B (optional)/M | |
| 3 | Feldfernsprecher 33 | 1933 | 5.5 | 1x 1.5V "Feldelement" | M |
| OB Fernsprecher 43 | 1943 | 4.5 | 1x 1.5V "Feldelement" | M | |
| 4 | Feldfernsprecher 53 (East Germany) | 1953 | 5.2 | 2x 1.5V D-Cell | M |
| Feldfernsprecher OB/ZB (West Germany) | 1954 | 4.3 | 2x 1.5V D-Cell | M | |
| Feldfernsprecher 63 (East Germany) | 1963 | 4.2 | 5x 1.5V AAA | M |
I did not find any documentation for production numbers on any of the German field telephones. Most instruments have serial numbers, but these seem all to be manufacturer specific and not army inventory numbers. Also the "Feldfernsprecher 33" often have a serial number, also only relevant per manufacturer and not an overall inventory number, late production models (1944/45) often do not have a serial number. Some sources mention a production number for the "Feldfernsprecher 33" of more than 1.6M [37] [42].
For "first generation" equipment major manufacturers were Siemens & Halske, Mix & Genest, Zwietusch and C. Lorenz, but also a lot of smaller ones were involved. For the "Feldfernsprecher 26" which was built under the Treaty of Versailles conditions it seems that only C. Lorenz was allowed to produce it, but it did subcontract also to other companies [42]. For the "Feldfernsprecher 33" a good overview of all manufacturers (more than 30 different) and examples with pictures from a lot of different manufacturers can be found on the excellent German WWII communications specific webpage der-Fernmelder.de. The "4th generation" East German models were produced by RFT, the brand for telecommunications equipment of a consortium of state owned manufacturers ("VEB"). The "Feldfernsprecher OB/ZB" seems to have been developed and primarily manufactured by "SEL" (Standard Elektrik Lorenz, which included the former C. Lorenz and Mix & Genest, amongst others, and was then owned by ITT) though I have seen some made by DFG (Deutsche Fernsprecher Gesellschaft, Magdeburg).
Buzzer signalling was included on all instruments up to the "FF 26" (for "FF 17" and "FF 26" only optional). From the "FF 33" onwards only magneto signalling was used.
Spelling tables were added to all german field telephones (for the "Armeefernsprecher" type on the battery box). The original spelling alphabet was adapted by the Nazis, apparently to get rid names with jewish heritage (like eg. David). On later East German instruments the "nazi-fied" version was still used, on West German instruments the NATO alphabet was used.
Up to the "Feldfernsprecher 33" the "Feldelement" was used as power source. A WWI "Feldelement" was a primary element of size 57x57x120mm [1] (also called Size "Normaltyp No. 3" [21]), with a nominal voltage of 1,5V. The first generation "Feldfernsprecher" and "Armeefernsprecher" used Batteries made of 3x 1,5V elements, the "Feldfernsprecher 16/17/26" used 2x 1,5V elements and the "Feldfernsprecher 33" used 1x 1,5V element.
Originally the "Feldelement" was always of type "Füllelement" (fill-element), an inert type of element where the electrolyte can be activated by filling in water. Later this type of "Feldelement" was designated "Element c (F30)", where "F" stands for "Füllelement" and 30 for the capacity of 30Ah, and an new type of "Feldelement" was designated "Element d (T30)", where "T" stands for "Trocken" (dry) as it could be activated by air [7]. "c" and "d" seem to be a continuous type designation, there was a smaller "Feldelement a (F5)", and I assume also a "Feldelement b (XY)" but I've never seen one mentioned.
The new "Element d (T30)" was an air activated Zinc-air element, also called a "Lufsauerstoff-Trockenelement" which seems to have been the most widely used field telephony element during WWII. These WWII elements were made and marked based on DIN and VDE standards ("Deutsche Industrie Norm", "Verband Deutscher Elektrotechniker"). DIN VDE 1210 and VDE 0807 defined electrical parameters and DIN 40850 sizes and marking. The size was defined slightly leaner than the older version now defined as 55x55x125mm, often the elements are marked with the reference to the above DIN VDE standards and the respective DIN code "ELF" or "ELL". Where the first letter defines if it's an Element (and not a Batterie), the second letter defines the size of the used cell (for the normal field telephone element it is always "L") and the last letter defines the type: "F" for "Füllelement" and "L" for "Luftsauerstoffelement" [22].
The post WWII "Feldfernsprecher 53" and "Feldfernsprecher OB/ZB" used 2x 1,5V D-Cells. The "Feldfernsprecher 63" required a higher voltage power source for it's microphone amp., it operated either on a special 6V accu or on 5x 1,5V AAA batteries.
In below picture a WWI type "Feldelement" (Not original), two "Element d (T30)" (Original, Pertrix 1942, rvx 1945), a Bundeswehr "NBA-3030" (Nato Alkaline D-Cell, not Original) and a 5x AAA Adapter for the east-german "Feldfernsprecher 63".
The first generation instruments "Armeefernsprecher" (and "eiserner Armeefernsprecher") Feldfernsprecher (and eiserner Feldfernsprecher) had no builtin battery compartments and used one of the below listed battery cases [1a] [2] [34] [41].
| Name | Year | Size [cm] | Weight [kg] (with 3x "Feldelement") | Line type (Single/Dual) | Switching sockets |
|---|---|---|---|---|---|
| Sprechbatterie | 1905 | 31,5x8,5x15 | 3,3 | S | 6 |
| Armeesprechbatterie | <1910 | 21,5x8,5x15 | 2,6 | S | 6 |
| Sprechbatterie der Feldartillerie | <1910 | 21,5x8,5x15 | 2,6 | S | 6 |
| Sprechbatterie für Eisenbahntruppen | <1910 | 22,5x8,7x15 | 2,5 | - | - |
| Sprechbatterie neuer Art | 1913 | 31,5x8,5x15 | 3,3 | D | 6 |
| Armeesprechbatterie neuer Art | 1913 | 21,5x8,5x15 | 2,6 | D | 2 |
| Feldsprechbatterie 16 | 1916 | 21,5x8,5x15 | 2,5 | D | 2 |
| Feldsprechbatterie 17 | 1917 | 21,5x8,5x15 | 2,5 | D | 2 |
In below picture in front a "Sprechbatterie der Feldartillerie" (Simson & Co.), behind it a "Feldsprechbatterie 16" (Siemens & Halske).
All electromagnetic receivers can also be used as "sound powered" transmitters. All different iterations of "Kopffernhörer" (Headsets) were prepared with plugs combined with binding posts which also allow to connect directly to wires.
The "first generation" "Kopffernhörer" (1905, made of Aluminium) and "Eiserner Kopffernhörer" (<1915) can be plugged on all "first generation" field telephones, or can be used as "sound powered" small telephone e.g. when building or repairing a line. To signal a "Ruftrompete" (Signalling trumpet, made of brass) or a "Ruftrompete neuer Art" (New kind, made of horn), which does fit into the "Kopffernhörer" opening, can be used [1].
The "Kopffernhörer 17", "Kopffernhörer 26" and the "Kopffernhörer 33" all use the identical plug and fit the "second generation" field telephones and the "Feldfernsprecher 33", and also include binding posts on the plugs to fix them directly to wires.
In 1942 Siemens & Halske delivered a prototype for a sound powered "Feldfernsprecher b", slightly smaller than the "Feldfernsprecher 33". I've never seen one and I could only find one image of it here on the webpage of the Wehrtechnikmuseum.
In below picture an "Eiserner Kopffernhörer" (Paul Hardegen & Co.) with a "Ruftrompete".
The "Tischfernsprecher OB 05" was the desk telephone set of choice up to the early forties. It is a civilian LB device, but capacitors were added when requiring to connect to a SB or CB network [5]. Alone during WWI about 100'000 "Tischfernsprecher OB 05" were in Army use [31].
In 1938 the "Tischfernsprecher 38" was introduced as replacement for the "Tischfernsprecher OB 05". It is also an LB device, but has capacitors already built in to also support SB and CB connections. The "Feldfernsprecher 33" handset is used, the "Tischfernsprecher 38" has a socket under the hood to plug in the "Feldfernsprecher 33" handset [5].
In below picture a "Tischfernsprecher 38" (Siemens & Halske 1942) and a "Tischfernsprecher OB 05" (Wilhelm Dauernheim 1923)
For dial connections I assume all "incarnations" of the German Signal Corps had civil sets from the respective era in use. But there were also a bunch of dial devices for field use. From the "Feldfernsprecher 33" era the "Amtsanschliesser 33" which could be used as standalone dial set or also as dial module or as SB module (LB line with DC call supervision) for the "Feldfernsprecher 33", and using the handset of the "Feldfernsprecher 33" (or also on other instruments like e.g. the "kleiner Klappenschrank zu 10 Leitungen") [4].
In the 70ies/80ies the "Bundeswehr" introduced the "Wählfernsprecher, WF" (Made by Krone) for use on the AUTOKO [46] and the "Kryptowählfernsprecher, KWF" (made by ANT [47]). The "WF" is a standard dial set (it supports earth signalling and connecting a modem), the "KWF" includes a special 16kHz transmitter to signal the activation/deactivation of an external crypto modul [47].
In below picture an "Amtsanschliesser 33" (NTT 1937), a "Wählfernsprecher" (WF, Krone 1980) and a "Kryptowählfernsprecher" (KWF, ANT 1987)
The postal "Streckenfernsprecher R.T.V." has been used by the WWI German Signal Corps as "Streckenfernsprecher M15" (as it was much more compact and lighter than the Signal Corps material) [34]. There was also a WWI "Feldfernsprecher für Eisenbahntruppen" (Railyway troups field telephone, a Siemens & Halske device similar to the Swiss Central Telephon [34]) and I assume that the railway device "Streckenfernsprecher OB 33 Tragbar" had it's military use as well. On the other hand also the german post used at least the "Feldfernsprecher 33" [46] and the "Feldfernsprecher OB/ZB".
In below picture a "Streckenfernsprecher R.T.V." (Mix & Genest 1923) and a "Streckenfernsprecher OB 33 Tragbar" (T&N 1941).
Earth communication: During WWII "ETEL" ("Erdtelegrafie" - earth telegraphy) instruments were used to continue communication on defective lines, high attenuation lines or no lines at all. "ETEL" communication was based on buzzer senders and highly sensitive tube amplified receivers, this receivers could also be used as tapping devices [21] [41]. Beginning of WWII the "Erdsprechgerät" (Earth talking device) was introduced. It enabled communication over interrupted or high attenuation lines by using a high power microphone which was driven by a manually operated DC generator [6].
Untappable: To prevent tapping (eg. by listening on earth currents, see "ETEL" above) untapable communication was achieved by using very low DC currents with "flattened" edges [48]. The most famous implementation is the British "Fullerphone", the German WWI implementation was called "UTEL" which used a microphone mounted on a buzzer to receive the DC pulses, the WWII version was the "Sutel 40" which used an electronic tube oscillator [21] [48].
In below picture an "UTEL Modell 1917" with an "Armeefernsprecher 1905" connected (both Siemens & Halske).
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