The Germans have made effective use of land mines in every sector where
they have been engaged. They have not neglected the development of the mine
detector. One German mine detector, the Frankfurt 42 (see Tactical and
Technical Trends, No. 24, p. 14), is a very elaborate piece of apparatus, no economies
of any sort having been made in the design. This instrument consists of a search
coil attached to a 7-foot arm together with a detector set housed in a metal case
and carried on the operator's back. The complete equipment together with spares
can be packed into a wooden case with a carrying handle, with a total weight of
(1) Search Coil Head
The search coil head is shown at (1) in the accompanying sketch. It is
oblong in shape, weighs 3 pounds 6 ounces and measures 10 1/2 by 4 by 2 1/2
inches folded and 1 foot 8 inches by 2 1/2 by 2 1/2 inches unfolded. The short
handle (2) is provided with a spring stud and is shaped to fit into the female end
of an extension piece; it folds over the coil as shown in the drawing for ease in
stowage. A porcelain plug (3), fitted onto the end of the handle (2) for stowage, is
provided with a lead which connects at (4) the search unit to the detector pack.
(2) Extension Pieces
The search coil is carried at the end of a 7-foot rod made up of 3 duralumin
extension tubes with male and female joints held together by spring-loaded studs.
The cable and plug which form the connection between the coil and detector set
are threaded down the center of the tubes. In one type of arm the top extension
piece carries a pivot knob believed to be a volume control (5). The exact method
of connection of the search coil to the detector set in this type is not definitely
known. When not in use, the extension pieces are packed in a cylindrical canvas
(3) Metal Pack
This is shown at (6); it measures 13 3/4 by 11 3/4 by 4 1/4 inches and
weighs about 15 pounds. At one end of the pack is a control panel which carries
the following components: sockets (7) for earphones, switch (8), indicator (possibly
ammeter) (9), a sensitivity control (10) having three positions. The control
panel is removable, and covers a small handle (11) which serves to pull the
detector set out of the pack after loosening the screws (12). At the other side of
the pack is the socket (4) for the search coil plug, low-voltage batteries (13)
(two, 4-volt, 28 amps, nickel-iron type), and a recess (14) in which the search
coil is stored when not in use.
(4) Detector Set
The detector set is shown at (15). It comprises a 2-valve oscillator and
2-valve amplifier assembly, together with a vibrator rectifier which supplies
high voltage power.
b. Method of Operation
The following is a translation of the German operating instructions
enclosed with the instrument:
(1) Open the lid which covers the battery and take out accessories.
(2) Run the cable through search-coil rod and place the rod on the search
(3) See that the cable plug is thoroughly dry. Insert it into the socket
(to left of the battery) and press home.
(4) Insert cable and close lid.
(5) Open the lid which covers the switch, plug in earphones and lead cable
out towards the left behind the hook.
(6) Take out control box, insert remote-control cable and replace box.
Then lead remote-control cable to the left behind the hook. (If it
is intended to work with only one predetermined range of sensitivity
the control box may remain in the case.)
(7) Put on lid and close.
(8) Attach back support from below.
(9) Place the apparatus in the back support and canvas carrier and sling
over the shoulder. Put on earphones. Fix the control box to the belt;
(10) Turn switch to the right to position "Ein" (one).
(11) Grasp search coil and hold it in the air; only a high pitched singing
note should be audible. The real lower pitched signal occurs only if
the search coil approaches a metallic body. If the signal is audible
beforehand, it can be eliminated by adjusting the screws on the search
coil by means of the key fixed to it. Keep the search coil well clear of
buckle and other metal parts. The greater the sensitivity used (it can
be adjusted in 3 steps on the control box) the more exact is the zero
adjustment; the more exact the zero adjustment the better the detection.
If the signal increases while approaching the ground (for instance, if
the ground contains iron) a smaller sensitivity range must be chosen.
(12) Search by slowly moving from side to side close to the ground. When
the search coil passes over metallic bodies the signal increases in