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When the AT
units were in original development, part of the development was done by the
firm Electroacoustic, Kiel, and part by Dr. Ing. Rudolf Hell, Berlin. The
Electro-acoustic (ElAc) development was finally accepted, and is the type
described in the pre-ceding paragraphs. Dr. Hell's subsonic detecting device
is of some interest, however. The magnetophone used in the AT units is the
work of Dr. Gerloff of SVK, Kiel, and required a vacuum-tube amplifier. Dr.
Hell, however, concentrated his efforts on producing a de-tecting device
which would have high enough output to operate relay systems directly, as in
the normal sonic acoustic system. The experimental detector produced by the
Hell firm was a carbon-button device which Dr. Hell claims had a very sharp
resonance peak at approximately 20 05 cps. This device, known as the "Hell
Doppelschwinger", the name having been changed because of unsatisfactory
results with an earlier device knwon as the "Einfachschwinger", was
constructed as shown in figure 218, which is modified
slightly for clarity. There are two weights, each mounted on two suspension
leaf springs, one above and one below. The upper weihgts, each mounted on
two suspension leaf springs, one above and one below. The upper weight and
spring system is adjusted to be resonant to 20.1 cps. These two weight
system have resonance curves approximating those shown in
figure 216. When the frequency is lower than the resonant frequency of
the weight system, the weights oscillate together with the mass of the mine
case to which the detectol is fixed. When the frequency is higher than the
resonant frequency of the weight system, the weights oscillate approximately
180° out of the phase with the mine case. When the frequency lies between
20.0 and 20.1 cps, the two weight systems are 180° apart, in phase. A
carbon-button microphone element is connected to the two eoght systems, and
measures the relative motion between them. Thus, the output of the
carbon-button reaches a peak when the frequency lies halfway between the
resonant frequencies of the two systems (20 05 cps.). The earlier type
("Einfachschwinger") made use of only one resonant weight system, and the
other part of the carbon-button was connected to a fixed part of the
assembly. In this system the movement of the one system only is measured and
there is no phasing effect. The "Einfachschwinger" had a high subsonic
output, but its response was too broad to be desirable for use in an AT
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