|MadSci Network: Engineering|
This is a good idea. Generally electromagnetic fields change with both time and frequency. In this case, I think you can use time-harmonic electromagnetic fields, that is, electromagnetic fields whose dependence on time is sinusoidal function. If you use some device to watch the field at any point, you'll find the waveform is a periodic and oscillatory sinusoidal curve. If you want to study effects of electromagnetic fields with different frequencies, you need a signal generator which can generate electromagnetic fields of different frequencies. If the signal is too weak, you may need an amplifier. Then you need some transmission structure to guide the electromagnetic fields to the seeds. The complexity and price of the devices you need depend drastically on the frequency range you plan to investigate. If you only plan to study the static electromagnetic field, that's very simple. In this case the electric and magnetic fields can exist independently. You can fill a box with soil, then put two metal plates at the top and bottom of the box. Then you connect the two plates to batteries or DC power and a static electric field is generated between the two plates. Actually it's just a parallel-plate capacitor. The strength of the field is proportional to the voltage so you can easily control it. You can build several such capacitors and observe the effects of different field strength. Accurately measure or calculate the value of the field strength is relatively difficult. If the capacitor is almost full of soil, you can measure its capacitance first, and then calculate the electric field strength. To generate a static magnetic field you need a coil. Put the soil and the seeds in the coil. The magnetic field is proportional to the currents through the coil so you can control it by controlling the voltages. If you can measuer its inductance and the coil is almost full of soil, you can also calculate the magnetic field strength in it. For frequencies lower than megahertz you can still use the above methods. Only in this case you'll connect the parallel plates or the coil to a signal generator. For higher frequencies the radiation and interaction between electric and magnetic fields become significant. You can not simply use wires to connect the parallel plates or the coil. The electromagnetic fields will radiate into space and cannot reach them. Guiding structures such as coaxials or waveguides are needed, and you have to put the soil and seeds in a close metal box to prevent leakage. I think some books on electric circuits and electromagnetics may help, especially those chapters on the fields in a parallel-plate capacitor and a coil inductor.
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