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I'm working on a project using an ultrasound method for identifying objects in water. For example, such a device could be used to locate bodies on the floor of an ocean bed after a plane crash. Our technique relies on moving charge between two wireless, independent amplifiers, each powered by their own LiPos.

I have a set-up like this: bucket ultrasound device

On the left side there's a circuit powered by a DC single-supply battery. On the right side there's a second circuit powered by a DC single-supply battery.

The two circuits are oscillators. A signal communicated wirelessly from a host device (not pictured) to each amplifier is non-invertingly amplified at A, while the signal is invertingly amplified at B. When there's a high voltage relative to the first LiPo's ground at point A, there's a low voltage relative to the second LiPo's ground at point B.

Each circuit has a "contact point" against a central load, R, which is a big body of water. The (theoretically) injected charge makes it easier for our product to locate bodies in the water. The two circuits aren't connected aside from a single contact point on each side.

When I use my oscilloscope, there's no voltage difference between points A and B, even when point A has an objectively positive charge, and B may have an objectively negative charge relative to A. When I use my ammeter, no current is running over the load. (Why is this?)

Does this have something to do with the output impedance of the op-amp amplifiers that I am using to amplify the charge? Are the op-amps unable to take in current and move it to ground? Is there a specific op-amp or amplifier that might allow this?

Does it have to do with the way the oscilloscope or amplifier measures these values? Is current actually moving, just not measurable by these tools?

I've noticed when I add two more contact points, C and D, the oscilloscope/ammeter produces more expected readings. However, I don't know whether current is running downwards (R1, R3), or across the load (R2, R4). Because battery life (and prior investment in design considerations) has been accounted for current only running across the load (and only two contact points), I would really like to find a way to not have to add contact points C and D. This would also ensure current was running solely across the load, and not down along its sides.

bucket ultrasound with two extra contacts

First of all, why doesn't current run when there are only two contact points? Would I need to connect the circuits with a wire to complete the movement of charge?

Secondly, how can I test whether current is running downwards (R1 to R3) or across the load? I need current to move across the body of water.

I would really like to build two auxiliary wireless devices that don't need a wire connecting them. But I also need current to move across the body of water, not downward along its sides, ideally with two contact points.

Is it similar to the way charge doesn't move when batteries are connected in series? Is there any workaround?

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    \$\begingroup\$ You need a return path to pass current. Not a brain teaser at all. \$\endgroup\$ – Andy aka Feb 6 '18 at 17:34
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Current flows in complete loops.

Charge is the time integral of current. If a current was flowing, the charge of one device would be increasing and the other would be decreasing, creating an electric field to oppose that current.

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You can try using the capacitance to ground/self capacitance as the return path but beware a tank of water has a much higher self capacitance than your device can, so very little of the current will flow through the other device.

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This is similar to how capacitive touchscreens work.

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