# Coupled System of Dual-Axis Clinostat and Helmholtz Cage for Simulated Microgravity Experiments
## 1 Introduction  
Plant cultivation in special physical circumstances, especially in different magnetic and gravitational conditions, is a relatively new research trend related to the development of space technologies. It is necessary, however, since plants are known to be influenced by gravitropism, resulting in roots growing downwards while shoots grow upwards, led not only by light, but also by gravity [1; 2; 3]. Such research can be carried out in two different ways. The first one relies on sending samples into space [4] and placing them on board the International Space Station (ISS) [5; 6; 7]. This approach is relatively expensive and limited by the capabilities of today's space technology. The second method is to arrange experiments in conditions similar to those met in space. A common approach to simulate plant growth in space is given by a clinostat, which constantly changes the plant orientation, so that, on average, all directions are indistinguishable [8; 9]. Typically, rotational speeds of around 1 rpm are used for plant experiments [10], while rotations that are too slow may lead to helical plant growth [11]. Comparisons of experiments in a clinostat and in real microgravity often show similar results, as long as suitable rotational speeds are chosen [12; 13]. Nevertheless, another important impact on plant growth, besides gravity, is often disregarded: the Earth magnetic field with its induction of the order of 45 \(\upmu\)T = 0.045 mT. Magnetic fields with a broad range of magnitudes have been investigated with respect to their impact on plant growth. Static and alternating magnetic fields of 400 mT and 30 mT, respectively, were shown to positively impact the seed germination and chlorophyll content of soybean seeds, respectively [14]. Static magnetic fields between 420 mT and 580 mT were also found to increase the root growth of _Arabidopsis thaliana_[15]. By applying a static magnetic field of 110 mT, the growth of _Pelargonium gravelens_ could be improved regarding plant height, root length,chlorophyll content, and other parameters [16]. While such relatively large magnetic fields are mostly regarded as positive for plants, only a few experiments were found on the influence of switching off the Earth magnetic field, and those showed often inconsistent or irreproducible results [17; 18; 19; 20], underlining the importance of further investigating plant growth in a zero magnetic field for future space missions as well as the influence of \(\mu\)T fields on the biological species important for a living environment. Shielding the Earth magnetic field is possible with a Helmholtz cage [21], as is described in the next section. It should be mentioned that descriptions of a clinostat inside the homogeneous magnetic field of a Helmholtz cage are only scarcely found in the literature. [PERSON] et al. report a pair of Helmholtz coils at the ends of the moving stage of a clinostat, i.e., the possibility to influence the inner part of the clinostat chamber by a magnetic field, which is moved together with the sample chamber and thus not able to counteract the Earth magnetic field or to apply a magnetic field along a defined orientation with respect to the laboratory inertial system [22]. [PERSON] et al. mentioned the importance of taking the hypomagnetic field far away from Earth into account in future simulations of biological responses for outer space missions, but while explicitly suggesting shielding the Earth magnetic field in clinostat experiments, no corresponding instrument was reported [23]. Only one paper was found that described cancelling the Earth magnetic field as well as using a clinostat for simulated microgravity [24]; however, the tested biological samples were only exposed to one of these specific conditions. Usually, previous experimental studies concentrated on investigating the influence of either the modified magnetic field [25; 26; 27] or simulated microgravity only [28; 29; 30; 31].