One of the special developments carried on in mine-firing devices by the Germans was in the distortion caused by target vessels to high-frequency alternating-current fields. The only experimental product of this development is the so called "Wellensonde", (figure 258), a development of Dr. Hell. The "Wellensonde" is an experimental device and never reached advanced development or trials.

No specimens of, or documents relating to, the "Wellensonde" were captured; and all information contained herein has been obtained through interrogation of prisoners of war.

The "Wellensonde" (wave-sounder) creates an alternating electro-magnetic field around the mine case. When this field is distorted by the transit of a ship or other body, the mine fires. The firing unit devised consists of a transmitter, a receiver, antennae for both transmitter and receiver, and a firing circuit. The transmitter has an output of five watts at 5000 cps. Each antenna is rectangular, made of metal tubing, and 50 by 80 cm in size. The two antennae are mounted in the LMB IV mine-case and cast into the explo-sive. Since the antennae are mounted at right angles to each other, there is little or no mutal induction. The mine-case is made of plasticized pressed paper (pressstoff) for mi-nimum interference with the field pattern. If the symmetry of the field produced by the transmitter is disturbed by the metal mass of a ship passing overhead, a small amount of alternating current appears in the receiver's antenna, and the firing circuit of the recei-ver output fires the mine.

Characteristics. The distortion effect at the receiving antenna through field distur-bance varies inversely with the 5th to 6th power of the distance to the disturbing force. Thus, the sensitivity is greatly dependent upon the depth of water. A change in depth of water from 65 to 72 feet will rsult in reduction of sensitivity by 50 per cent.

The high battery consumption of such a system makes it necessary to use a trigger-ing system which will allow the "Wellensonde" to stabilie before the fring impulse arises. It was intended to use a simple acoustic triggering system. A few seconds are necessary for the "Wellensonde" to come to equilibrium. A high geometric and electrical symmetry of the antennae and amplifiers is necessary to make the system operate properly. However, after a long period these characteristics then to vary somewhat, adn spontaneous firing may occur after the "Wellensonde" has been repeatedly switched on. In order to avoid this, a rate-of-change circuit is used in the firing circuit to allow firing only when sudden surges occur. This principle, however, was not adopted, because of the great variation in sensitivity with depth, and was considered of no operational value in a mine.

Figure 258a – Wellensonde Unit

Figure 258b – Unit in LMB