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Basically let’s say I have a sheet of paper and on that paper parts of it have aluminum sections/squares inside (not visible from outside) how can I map out which parts/sections inside contain aluminum using a device like a multimeter?

Can I set it to resistance then touch the leads to opposite sides of the paper to determine the resistance and if less make the assumption there is aluminum inside? Is there a better device than a multimeter for mapping out the sheet of paper to determine what parts have aluminum like a precise stud detector?

What about thermal conduction, is there a device that can measure heat conduction on opposite sides of the paper to determine if there is a sheet of aluminum inside? Can I use a thermal imaging camera to detect subtle temperature differences?

Ideally I would like to obtain an image of the aluminum sections hidden inside the paper/cardboard covering.

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  • $\begingroup$ paper is a flat insulator and partial conductor/reflector of IR light while foil is a conductor and good reflector of electrical field signals. Can you define the volume of paper and access to either side like printer, scanner access with modified sensors? What resolution accuracy is required? $\endgroup$ Commented Jul 29, 2022 at 0:53
  • $\begingroup$ Bombard the sheet with a stream of K-mesons and measure the scattering cross-section at each location. OK, srsly: what exactly are you testing; what's the relatieve size of the Al patches and the relative density of them? $\endgroup$ Commented Jul 29, 2022 at 12:27
  • $\begingroup$ You mean like a minesweeper game with needles on a multimeter test for shorts? sure $\endgroup$ Commented Jul 29, 2022 at 13:03
  • $\begingroup$ I'd hold the paper sheets up in front of a very bright light. I'd envision you could see the shape of the aluminum items $\endgroup$
    – tckosvic
    Commented Jul 29, 2022 at 16:50
  • $\begingroup$ If you touch the multimeter leads to opposite sides of the paper, the current will pass through layers of paper in series with the Al layer, and there may not be any appreciable difference in resistance associated with the Al being there or not. Suspect you'd do better to touch both probes to the same side of the paper for conduction through paper and aluminium in parallel. $\endgroup$ Commented Aug 2, 2022 at 12:01

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You can also sense the aluminum inside the paper using a miniature metal detector, which contains an electric coil that is carrying an AC current. when anything made of metal comes near the coil, it changes the coil's inductance which changes the current, which is detected by a sensitive circuit connected to the coil. This method is used by traffic light controllers to detect the presence of a car sitting on top of the coil, which is buried in the pavement.

There is an inexpensive electrical device called a stud finder which allows a carpenter to find the positions of wooden beams hidden inside a finished house wall. The carpenter sweeps a wand containing the circuitry back and forth across the wall surface, and watches for indicator lights in the wand to light up. For metal beams, the magnetic method as outlined above is used, and for wooden beams the capacitive circuit described in the answer above is used.

You can also do this trick with a device called an eddy current crack detector which you can read about on the web.

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Some theory for the starting point of experiments:

Let's have 2 separate wires connected to an AC voltage source. There's some current through the capacitance between the wires. If the free wire ends happen to be near each other the capacitance between the wire ends can be the one which defines a substantial portion of the AC current. If there's some metal close the wire ends, substantially more capacitive current can occur than without the metal. That's because the metal shortens the free air distance between the wire ends.

We can well assume that the wire ends are on the same side of the paper. See the next image:

enter image description here

The red output wires of the voltage source are really thin to keep their capacitance low. The wire ends have horizontal metal plates (=blue). If there's the same piece of metal (=grey) under both blue plates the AC current can be substantially higher than when the blue plates are not above the same piece of metal.

The idea is not mine, it has been used a long time in capacitive push-buttons and proximity sensors.

If you scan over the interesting area of the paper you may find the areas where there's metal inside the paper.

This principle is useless if the paper is wet or it's conductive for some other reasons.

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  • $\begingroup$ How much would it cost to produce a device that measures the electric current flowing through it to scan the paper out into an image showing the exact parts containing aluminum? Can I create an electric grid/mesh and pass current through it and use a special device on the other side to measure it and create an image? Can I buy something like that which is already being sold? $\endgroup$
    – Max
    Commented Jul 28, 2022 at 23:39
  • $\begingroup$ The engineering is your job. I give only qualitative advice. I know nothing of your actual paper, dimensions, the environment, possible disturbing fields nor the needed resolution. By having a piece of the paper with realistic aluminium implants I could start the experiments to find some numbers for a proper design. You may be able to use existing capacitive sensing ICs after getting some numbers to be compared with the specs. If you are not a competent electronics designer, you must find a local one. $\endgroup$
    – user33233
    Commented Jul 29, 2022 at 10:41

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