Originally Posted by J_Player
Hi Morgan,
If this is the standard method to put a sample of material in the coil circuit, then there are two known electrical effects that can come from changing to different materials. The sample is placed inside a magnetic field from the wire that is wound around the ring. This means the small coil will feel some impedance from the silver ring in the center which is caused by eddy currents. There will also be a phase change to the small copper coil due to the time constant for the silver material.
This effect will also be present for any other coils that are in the area where they can be influenced by a magnetic field, (like the larger coils), which may even see more impedance from the ring in the center, depending on how much current passes in these coils. And the larger coils can be expected to actually induce a current traveling in the circle direction of the ring, which we can expect to be stronger current than if it was simply a small ball of silver. For this reason, we can expect the larger coils which follow the same direction of the ring will also see the effect of the solder joint resistance.
A third thing to consider is this is probably not pure silver. Unless you are using commercially pure silver, it is an alloy which contains copper and maybe some other metals, same as most gold contains copper, silver and maybe other metals in the alloy. This means the resistance of the metal will be different than pure silver or pure gold, depending on what alloy, and the eddy currents will change along with the impedance and phase shift to the zahori coils. Then there is also the solder junction where the ends of the wire were soldered together with higher resistance than silver.
To get an idea how this can influence the circuit, look at the VLF metal detectors that have a dial which shows the different coins and metals it detects. The change in eddy current strength can be determined by measuring electronic changes in the RX coil when different metals are placed in the magnetic field of these coils. We know silver is the most conductive common metal which we find at one end of the scale, with copper near to it. Then we see less conductive metals like gold and aluminum farther down on the dial, then weak conductors are near the bottom where we finally get a reverse effect from iron materials and hot rocks.
Still, this says nothing to help show how putting a metal sample into the coil field will help find a buried sample of similar material. For a normal VLF metal detector, any metal sample you put at the coil will only make the coil less sensitive to buried metal you are hunting for, because the coil must now find a second piece of metal in addition to the piece that is put in the center of the coil. This is the reason why most metal detectors use non-metallic shafts and plastic hardware near the coil, to avoid having metal there which will make the coil less sensitive.
In the case of the zahori, we are not searching for buried metal eddy currents. We are searching for changes in the charge that the circuit can measure in the air. So we do not need to be concerned that we have added some metal to create impedance to those coils or a phase shift. More important, in the zahori circuits, there is no VLF tuned circuits I can see. These are simple coils connected to a high impedance amplifier. This means any eddy current effect will be working on changes in charge sensed which happen rapidly. I would expect any pulses that you might detect could be from a static discharge or even VLF impulses or natural frequencies from a few KHz up to the GHz region (or up to the limits of your transistors). The impulse signals you might measure from charge pulses with time durations in these ranges could possibly be influenced by the effect of the sample metal in the coil fields. Considering the single loop coils, and the 10 turn coil, I might expect it could be more sensitive to frequencies and pulses in the VHF range if it is sensitive to frequencies at all. This would depend on the circuit capacitance at these coils. The sample material may even be helping to tune the coils, or acting as a filter, considering the different metals have different time constants.
It might be interesting if someone connected an oscilloscope to see how the signal looks at the first transistor input with the sample in place and with the sample removed. Especially interesting what the signal looks like when you make a battery sparks nearby with the sample in place and with the sample removed.
Best wishes,
J_P
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