A highly sensitive absorption-type nuclear magnetic resonance (NMR) magnetometer capable of measuring low magnetic fields below 1 mT with a NMR sample volume of 3 cm3 is described. It will be used for calibration purposes in an accredited calibration laboratory. The magnetometer uses a direct digital synthesizer for generation of the radio frequency signal, a sampling detector for demodulation of the radio frequency signal and a digital signal processor with an analogue-to-digital converter and a digital-to-analogue converter for signal processing using a lock-in technique with a digital phase sensitive detector. Frequency traceability of the NMR magnetometer reference oscillator is established by means of the long-wave transmitter DCF77.
The geomagnetic field (GMF) varies over Earth's surface and changes over time, but it is generally not considered as a factor that could influence plant growth. The effects of reduced and enhanced GMFs and a strong static magnetic field on growth and chlorophyll a (Chl a) fluorescence of Lemna minor plants were investigated under controlled conditions. A standard 7 day test was conducted in extreme geomagnetic environments of 4 µT and 100 µT as well as in a strong static magnetic field environment of 150 mT. Specific growth rates as well as slow and fast Chl a fluorescence kinetics were measured after 7 days incubation. The results, compared to those of controls, showed that the reduced GMF significantly stimulated growth rate of the total frond area in the magnetically treated plants. However, the enhanced GMF pointed towards inhibition of growth rate in exposed plants in comparison to control, but the difference was not statistically significant. This trend was not observed in the case of treatments with strong static magnetic fields. Our measurements suggest that the efficiency of photosystem II is not affected by variations in GMF. In contrast, the strong static magnetic field seems to have the potential to increase initial Chl a fluorescence and energy dissipation in Lemna minor plants.
Buds of chestnut (Castanea sativa, Mill.) were grown on modified Heller's solid material medium with the addition of 0.1 mg/mL 6-benzylaminopurine (BAP). They were divided monthly and used for experiments with homogeneous sinusoidal magnetic fields of 5.9, 3.2, and 1.2 mT for 1 h/day, 6 days a week, for 28 weeks.The results are season-dependent; bud growth was significantly enhanced over controls in early summer (50% at 5.0 mT, 64% at 3.2 mT, and 62% at 1.2 mT). In late summer and fall, growth was inhibited slightly. In early winter, it was stimulated again. We also measured significant effects 1 month after cessation of stimulation.
Several methods for assuring a reference alternating (AC) magnetic field (densities up to few mT and frequencies up to 10 kHz) inside an air-cored coil are currently used (inductive pick-up coils, Hall magnetometer, fluxgate magnetometer). But irrespective of the method used, the measuring uncertainty of the reference AC magnetic field standard is always larger then the uncertainty of a reference direct (DC) magnetic field standard. The method proposed in this paper uses this fact. It employs a comparison between an unknown AC and a known DC magnetic field, generated by a Helmholtz coil, calibrated by means of a NMR (nuclear magnetic resonance) magnetometer. The basic idea of the method is to use a simple semiconductor magnetic sensor as a comparison medium. The purpose of this paper is the introduction of the common AC/DC transfer method into the field of magnetic flux density measurements, building an AC magnetic flux density standard, and evaluating its measuring uncertainty.
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