The Feldmeßkätschen 18 was a voltresistance meter used by Nachrichtentruppen to test field telephone lines and batteries. One of its uses was to check the resistances of field telephone lines to detect any abnormal conditions that would adversely affect communications. Let’s see how this was done.
Think of the field telephone circuit as a loop with resistors in series. We have to consider the La wire to be one resistance in the field telephone circuit. The field telephone itself is another resistance in the circuit. Lastly, the Lb wire or ground path is another resistance in the circuit. Since all three resistances are in series, they add together.
We’ll have to use some math here. To begin with, lets define some terms:
R = Total Measured Resistance (ohms).
L = Distance of Field Telephone (km).
RT = Internal Resistance of Field Telephone (ohms).
RW = Restance of Wire (ohms/km).
RE = Resistance of Ground (ohms/km).
For two wire field telephone installations:
R = RW x L + RT + RW x L
For example, suppose a field telephone is located 5 km away from headquarters. The field telephone uses two 5 km wires connected to its La and Lb/E terminals. If the field telephone has an internal resistance of 600 ohms, and the wire has a resistance of 6 ohms/km, then what should the normal total measured resistance be on this field telephone line?
R = RW x L + RT + RW x L
R = (6 ohms/km) x 5 km + 600 ohms + (6 ohms/km) x 5 km
R = 660 ohms
Since the resolution of the meter is not that great, the field telephone line is normal if the resistance is between 600 and 700 ohms.
For one wire/ground field telephone installations:
R = RW x L + RT + RE x L
For example… A single wire line is run to a field telephone 5 km away. The return is through ground stakes. The field telephone has an internal reistance of 600 ohms. The resistance of the wire is 6 ohms/km. If a meter measures the total resistance to be 5000 ohms, what is the resistance of the ground return?
R = RW x L + RT + RE x L
5000 ohms = (6 ohms/km) x (5 km) + 600 ohms + RE x 5 km
RE = 874 ohms
Finding how far out a short circuit is in a field telephone line:
A meter measures 400 ohms on a two wire field telephone line that normally runs for 35 km. The field telephone at the end of the line doesn’t work, and it is suspected that there is a short circuit somewhere along the line. How far along the line is the short?
R = RW x L + RT + RW x L
Since the telephone is outside the short circuit, RT is taken out of the equation, and it becomes:
R = RW x L + RW x L = 2 x (RW x L) = 2 x RW x L
400 ohms = 2 x (6 ohms/km) x L
L = 33.3 km
So, the short circuit is about 33 km from the Feldmeßkätschen 18.
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WW2 German Field Telephone Equipment… Checking Field Telephone Lines

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Re: WW2 German Field Telephone Equipment… Checking Field Telephone Lines
In the US such a device is generally called, today, an "Ohmmeter" or "Megohmeter," often referred to as a "Megger." It generates a higher voltage to send through the wire / circuit (today 600 to 1200 VDC). I would assume that the one pictured is used with a hand generator to create the voltage and measure the line resistance.