Non-contact methods for measuring voltage in conductive materials

[fluidistic]

Consider a solid material such as a conductor. I would like to know if there are ways to determine the voltage at specific spots on the material, without plugging a metallic wire and connect it to a voltmeter. I know voltage is relative and I'm interested in checking voltage differences across different specific spots on a conductive material without making any physical contact that could disturb the potential difference. A high Tc superconductor would do the job, but only for temperatures too low for my interest.

I was thinking to use EM waves such as a laser and check the reflectivity for instance, but my knowledge is lacking and I do not know whether the reflectivity or other properties can depend on the local voltage.

 

[anorlunda]

If the material is a conductor, wouldn't the voltage be substantially the same everywhere? Or perhaps there is a large current flowing that you didn't mention.

At very high voltages we have Corona. https://en.m.wikipedia.org/wiki/Corona_discharge

 

[fluidistic]

If the material is a conductor, wouldn't the voltage be substantially the same everywhere? Or perhaps there is a large current flowing that you didn't mention.

Not necessarily. If the material lacks a uniform temperature, a Seebeck effect is going to take place, so there will be a potential difference between spots at different temperature even though there's no current.

 

[f95toli]

I am not aware of any such method for DC voltages and I am pretty sure I would know about it if one existed (I work at an institute where we do a LOT of electrical metrology).
You would have to find an effect which allowed you to calculate the potential based on a measurement of some other property (e.g. some type of electrooptical effects); but then you would need to rely on some model and there is no guarantee that that measurement would not disturb the sample as well (lasers can cause a lot of local heating etc).
There are nearly always ways of getting round the fact that you will "disturb" the sample via a measurement; in some cases all you need is simply a good enough voltmeter (with a high enough input impedance) and/or the right measurement configuration. Voltage in one of the things we are VERY good at measuring.

 

[fluidistic]

I am not aware of any such method for DC voltages and I am pretty sure I would know about it if one existed (I work at an institute where we do a LOT of electrical metrology).
You would have to find an effect which allowed you to calculate the potential based on a measurement of some other property (e.g. some type of electrooptical effects); but then you would need to rely on some model and there is no guarantee that that measurement would not disturb the sample as well (lasers can cause a lot of local heating etc).
There are nearly always ways of getting round the fact that you will "disturb" the sample via a measurement; in some cases all you need is simply a good enough voltmeter (with a high enough input impedance) and/or the right measurement configuration. Voltage in one of the things we are VERY good at measuring.

Thanks. Indeed, I didn't realize it would perturb the system, which could be troublesome if the sample is small. I can imagine a sort of probe consisting of a capacitor with a voltmeter connected to it, measuring the induced charge when that capacitor passes nearby the sample. But it would modify the voltage of my sample, I think. Well, it's an electrostatics problem to solve.

 

[Henryk]

I know voltage is relative and I'm interested in checking voltage differences across different specific spots on a conductive material without making any physical contact that could disturb the potential difference

I have some good news for you.
First, do you really need to have a non-contact measurements or are you only interested in not disturbing the potential?
If you just don't want to disturb the potential at the spot, you can use a potentiometric method. It is quite simple. You do have to attach a contact to the spot you want to measure potential at. Next, connect a current meter between your contact and a variable voltage source (the other side of the voltage source has to be connected to a reference point). Adjust the voltage source until you read zero current. That gives you the true voltage at that point.
There is also a way to measure voltage without making a contact by using a vibrating electrode. You have a small electrode vibrating close to the surface of you conductor. If there is any potential difference between your electrode and the surface, vibration will induce AC current synchronous with the vibration. Normally, bias the electrode to null the AC current and you can read the true voltage. The voltage is averaged over a certain area depending on the size of the vibrating electrode and the distance from the conductor. Furthermore, it actually measures contact potential difference and that is different than the voltage measured by the potentiometric method.

 

[ZapperZ]

I have some good news for you.
First, do you really need to have a non-contact measurements or are you only interested in not disturbing the potential?
If you just don't want to disturb the potential at the spot, you can use a potentiometric method. It is quite simple. You do have to attach a contact to the spot you want to measure potential at. Next, connect a current meter between your contact and a variable voltage source (the other side of the voltage source has to be connected to a reference point). Adjust the voltage source until you read zero current. That gives you the true voltage at that point.
There is also a way to measure voltage without making a contact by using a vibrating electrode. You have a small electrode vibrating close to the surface of you conductor. If there is any potential difference between your electrode and the surface, vibration will induce AC current synchronous with the vibration. Normally, bias the electrode to null the AC current and you can read the true voltage. The voltage is averaged over a certain area depending on the size of the vibrating electrode and the distance from the conductor. Furthermore, it actually measures contact potential difference and that is different than the voltage measured by the potentiometric method.

I think what you are describing is the Kelvin probe method. It is used to measure the "contact" potential difference. I put "contact" in parenthesis because in the KP method, the tip doesn't actually touch the surface. The surface resolution of this method depends on the size of the probe's tip. It can vary from the order of mm, all the way down to sub-microns in the case of KPFM (Kelvin probe forced microscopy).

The problem I see is that this method is typically used to measure the variation in the chemical potential, and consequently, the work function (if you know the potential of the tip of the probe). It requires that the sample is well-grounded. I am not sure if this is what the OP wants.

Zz.

 

[f95toli]

The problem I see is that this method is typically used to measure the variation in the chemical potential, and consequently, the work function (if you know the potential of the tip of the probe). It requires that the sample is well-grounded. I am not sure if this is what the OP wants.

I thought of KP when answering the question; but I am not sure it is what the OP is after. KP (as well as many other SPM methods) are used quite heavily by some of my colleagues to study e.g. graphene Hall devices; but in all (I think) of their experiments they also have electrical connections to the sample so that they can selectively ground different electrodes.
Some other form of scanning gate spectroscopy might work; but it seems a bit too involved and I am not sure I would describe it as a "DC measurement" of voltage.