Any thirsty soldier who has lugged an unwieldy, long-handled mine detector
over miles of dusty ground with the temperature at 100° plus, would have
appreciated one of the German "Pram" (perambulator) two-wheeled mine detectors. A
description of the apparatus is drawn from British sources.
* * *
The German designation of this equipment has not been learned, the only
marking being the figures 1138 painted on each of the component parts.
The detector consists of a folding, rectangular, tubular metal frame-work
mounted on two wheels, a search element housed in a wooden box, which is attached
to one end of the metal framework for operation, and the accessory electrical
equipment. This fits into a pack carried on the back of the operator.
(1) Framework of Pram
The frame is constructed of metal tubing 1 1/4 inches in diameter; one
half of this tubing is fixed to the wheels and the other half is hinged to the fixed
section and folds on top of it for transportation.
|Length folded|| ||3 ft|
|Length extended||6 ft|
|Width of folding section||11 in|
|Width of fixed section||8 in|
|Wheels||14-in diameter each fitted with two 1/2-in circular-section solid rubber tires.|
(3) Search Element Casing
This is a wooden box, 3 feet x 2 1/4 inches x 3 inches, carried separately
and attached to the open end of the framework for use; a length of detachable
cable runs from the pack to the search element which is operated by means of two
small levers on the handle of the framework; these are connected to the casing
by a length of cable. Near one end of the casing is a search element adjuster.
This is a wooden box, 12 x 12 x 4 inches, fitting into an ordinary canvas
pack and attached to the equipment. Inside is housed the accessory electrical
equipment including the oscillator, amplifier, headphones, and high voltage and
low voltage supplies. The electric fittings are:
|Amplifier|| ||3 stage type with 3 Telefunken tubes, DAF 11|
|Oscillator||2 Telefunken tubes, DC 11|
|H.V.||90-volt dry battery|
|L.V.||One and one-half volt dry cell|
400 cps oscillator and amplifier, consisting of two tubes, feeds the iron-cored
coil in the center of the search coil assembly. Located at each end of this
assembly are two pick-up coils connected in opposition. The difference in the
AC voltage developed across these coils is fed via a screened lead into the grid
of the first stage of a three-tube amplifier employing a Telefunken tube, type
DAF 11. In addition, the grid of this first stage is fed with a very small voltage
direct from the oscillator stage. This voltage can be varied in amplitude and
its phase can be reversed by means of a differential condenser. This voltage is
used to set up the detector to give minimum output when there is no metal near
the search coil. Also two balance controls are fitted on the search coil:
(1) A short-circuited turn located close to one pick-up coil, which can
be rotated, and thus alters the flux in the search coil by varying the field due to
eddy currents in the short-circuited turn. This is controlled by a knob on the
search coil assembly.
(2) A piece of steel near the other pick-up coil varies the flux in the coil. This
is controlled from the handle of the Pram by means of a flexible shaft ("Bowden") cable.
d. Control Set-up
The controls should be set up as follows:
(1) Assemble the search coil into the Pram
(2) Connect the oscillator to the search coil by means of the six-pin plug and socket
(3) Plug the headphones into the socket provided on the oscillator-amplifier
and keep oscillator as far as possible away from the search coil
(4) Adjustment of the controls should be made in the following order for minimum sound:
a - Flexible-shaft cable operated control
b - Turn on search coil assembly, and
c - Differential condenser.
(5) Repeat (4) until minimum output is obtained.