multimeter curiousity

[the guy]

When I set my multimeter to the DCV 200mV setting and then move the probes around through the air, I get fluctuating readings. It is only when one or both of the probes are in motion.

 

I'm sure there is a perfectly simple explanation but, pray tell, what is it?

[Greg H.]

I am going to go with static electricity from the moving probes, but I'm not an expert.

 

And I found this from an owners manual for one of Radio Shack's multimeters:

 

Notes: The display might show a phantom reading in some DC and AC voltage ranges when the test leads are not connected to a circuit. This is normal. The high input sensitivity produces a "wandering" effect. When you connect the test leads to a circuit, a real measurement occurs.

 

http://support.radioshack.com/support_meters/doc46/46988.htm

[WHR]

There are perfectly good explanations for that there is a potential voltage between every point in space. The larger the potential voltage, the more likely for a current or a spark to occur. Nonetheless the potentials exist.

 

If you take a resistance measurement between any two given points (let's say on the surface of your skin) you will not read infinity. You will read Megohms of resistance, thus there is also a voltage present. Resistance does not exits without potential. This is also why you shouldn't handle sensitive components like CMOS devices without taking static precautions.

 

This basic principle applies with DC meters, with AC meters you will often see a mV reading and if your meter has a Hertz scale, you might find that your probes in midair will read so many mV at 60 (or 50 in some countries) Hertz. You are reading the EM field generated by various things like the ballasts in lamps, the transformer near your home or building, all the electronic devices in your vicinity, and even the wiring in your home or building. Again, as long as there is potential, one lead is picking up a stronger EM force than the other, and as long as that field is alternating, you will see an AC reading and it will be measurable in cycles per second...related to the predominant source in the vicinity, the power grid!

[ewmon]

WHR, I agree with ensuring static dissipation when handling sensitive devices, but I find myself unable to agree with the remainder of your post. My most obvious disagreement is about resistance unable to exist without a potential. Perhaps if you could explain more clearly.

[WHR]

WHR, I agree with ensuring static dissipation when handling sensitive devices, but I find myself unable to agree with the remainder of your post. My most obvious disagreement is about resistance unable to exist without a potential. Perhaps if you could explain more clearly.

Sure. Resistance is defined in electrical terms by a component's characteristic of hindering electrical current flow. Electrical current cannot flow without potential energy, and potential energy cannot exist without a differential. That differential being resistance. They are 3 necessary elements of the same phenomenon. You cannot measure a voltage without a voltage drop...and a voltage drop requires resistance. Conversely, you cannot measure resistance without voltage and current.

 

Use basic physics as a tool to see this. A rock resting on a hill has potential energy. The hill itself resists said potential energy. The rock rolling down the hill is the realization of said potential. Take any one of the ingredients away and you do not have anything to discuss. A boulder or rock in space would have no potential. A hill by itself offers no resistance without something resting upon it to resist. And obviously, without the boulder there to roll, the release of potential will never be realized in the act of rolling.

 

This is all demonstrated in the formula e=I x R

 

Or R=E/I

There you see? Mathematically expressed. Resistance Is voltage (electromotive potential) divided by I (which represents current or amperes). This is the mathematical expression of the reality. Without the potential and the current, there is no resistance. This is called Ohm's law.

 

A multimeter works off this principal. The battery is used as a voltage source for resistance measurements. Voltage and current measurements are made with a resistance bridge or various other methods with digital circuits. But all 3 elements are required for the meter to function.

 

Impedance is a whole topic in itself, requiring an understanding of reactance. Impedance is a function of frequency in an AC circuit, so a simple meter can't measure the impedance of a coil or capacitor without special add-on circuits and a working knowledge of the circuit that the component is functioning in.

[John Cuthber]

I guess it's a philosophical point. There's a 1000 ohm resistor on my bench at the moment.

Does it have a resistance, even though it has no applied voltage?

 

If I put 2 volts across it I get 2 mA of current. One volt gives me 1mA and half a volt gives 0.5mA.

In each case I get a current of a thousandth of the voltage so the resistance is 1000 Ohms.

Why would that suddenly stop being true at zero volts?

I couldn't measure the resistance at zero volts, but that doesn't mean it isn't there.

 

Perhaps more importantly, there's never exactly zero potential across a resistor for any finite interval of time.

http://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise

 

 

Anyway, back at the original question.

Part of the "voltage" seen by the meter may be due to changes in the capacitance of the leads and leakage current from the DVM.

A bit like this

http://en.wikipedia.org/wiki/Electrophorus

[WHR]

In pure technical terms, nothing "has resistance". Things "offer resistance". Therefore, technically, no. A resistor doesn't offer resistance until it is applied to a circuit. If not in a circuit, it might as well be a Christmas ornament. We can know what the value of the resistance would or should be when and if voltage is applied to it, to a margin of tolerance. We call it a "resistor" because we know what it would or should do when applied to a circuit. However, it isn't a resistance until placed in a circuit and doing what is predicted. Just as a light bulb isn't a light or lighting something until it is energized.

[John Cuthber]

"In pure technical terms, nothing "has resistance". Things "offer resistance". "

Says who?

 

From

http://dictionary.reference.com/browse/resistance

 

 

"Also called ohmic resistance. a property of a conductor by virtue of which the passage of current is opposed, causing electric energy to be transformed into heat: equal to the voltage across the conductor divided by the current flowing in the conductor: usually measured in ohms."

 

Since it's a property ,a resistor "has" (rather than "offers") it.

[WHR]

By which the passage of current *is opposed* ---which indicates a state of being energized. If the definition said, "by which the passage of current can be opposed" then I would agree.

 

Let's see this from another angle. A resistor is also a conductor. Anything that resists also conducts. One is the inverse of the other. Conductance is measured in Mhos.

 

You would not say that a long piece of wire is conducting unless it is part of a circuit. it can be "used as" a conductor. We know this because of the accepted electrical properties of copper, gold, or aluminum. You and I can also be used as conductors, but really we are just the atoms of our constituents and nothing more. We can predict what the conductance of a particular gauge piece of wire of a particular length will be, and inversely what it's resistance will be. But I say "will be", because without potential and current applied, it is just a lump of atoms of metal with properties that can be predicted.

 

I stick to my guns. If mathematics is the language of science, then Ohms law tells us what resistance is. Using an online dictionary or even a website tutorial is not ideal or absolute because spoken English is not the language of science.

 

And besides, this thread started in context of the use of a measuremeny tool, a DVM, which by definition can only function with voltages and currents produced by the battery in the case of resistance measurements, or produced by a circuit in the case of voltage and current measurement. Therefore, the voltages detected by a DVM on the DC scale are real. They may in fact be a function of capacitance of the leads. That is something I neglected to agree with but also, there are in fact potential differences between every object in space which are in a constant state of flux. Voltage itself is a very relative term. A positive battery post on a 9 volt battery has +9 volts of potential with respect to its negative terminal, but it has -3 volts of potential with the positive terminal of a 12 volt battery. An electronic device designed to run off of 9 volts can easily operate within specifications when connected to a 1009 volt source at the positive and a 1000 volt sorce at the negative.

[ewmon]

This reminds me of: If a tree falls in a forest and no one is around to hear it, does it make a sound? And the answer is: Yes. So a resistor has resistance, even when it is not part of a circuit. And yet, here we are at a point in astronomy, where the common belief is that the universe is mostly "dark matter" that we can't measure directly.

 

I also disagree that I can sit at home or in my office and measure a palpable amount of electric field from the electric company's transformer on the utility pole at the street. That's like saying that, from Earth, I can measure the gravitational field produced by Mars. Sure, such fields exist, but they're so infinitesimal that no instrument exists that is sensitive enough to measure it.

Edited August 6, 2012 by ewmon

[WHR]

This reminds me of: If a tree falls in a forest and no one is around to hear it, does it make a sound? And the answer is: Yes. So a resistor has resistance, even when it is not part of a circuit. And yet, here we are at a point in astronomy, where the common belief is that the universe is mostly "dark matter" that we can't measure directly.

 

I also disagree that I can sit at home or in my office and measure a palpable amount of electric field from the electric company's transformer on the utility pole at the street. That's like saying that, from Earth, I can measure the gravitational field produced by Mars. Sure, such fields exist, but they're so infinitesimal that no instrument exists that is sensitive enough to measure it.

 

Absolutely not true. A simple antenna and radio can detect the EM field of that transformer, your meter at the utility entrance, even the clock circuit of a cheap computer or the switching transistor of a switch mode power supply will produce detectable EMI (electromagnetic interference). I can purchase a cheap tool from Fluke that can detect AC voltage in an insulated conductor and will light up within the E-Field when held close to it. This is a cheap instrument with cheap simple circuitry. Unless your house is very large and your utility pole is quite a bit distant, the inverse square law would not allow the loss of the field to be so great from a short distance.

 

Another fact. I have an amateur radio license. I can generate an EM field with a simple dipole antenna, using only 5 watts of power, that can be detected on the 20 meter band on the other side of the globe. That EM field is quite tiny.

 

We have detectors that are picking up extremely low power EM waves from the Voyager probe that is now past Pluto.

 

Yet, you are claiming that I wouldn't be able to sense an EM field produced by a transformer only a few hundred feet away at most? I have very cheap instruments at home that refute that.

 

Edit to add:

 

I know we have went round and round...I think a better argument would be that we're counting how many angels can dance on the head of a pin.

 

But for the sake of argument, I don't think the tree falling analogy supports your argument either. During the process of the tree falling, we could measure a lot of things. Air resistance, friction, Force, momentum, heat generated, sound waves, etc. an entire host of things. Once the tree is on the ground very few measurements can be taken. This is because many of these measurements are a function of time. So, for instance, one Ampere is one coulomb of charge moving past a given point per second. For current to flow, it must have a potential. For potential to exist, there must be resistance. Mathematically this is all a function of time, an event with a duration. When you have a resistor laying on your bench (fallen tree), you make an assumption that the resistance will be X when energized in a circuit. However, sometimes a lead junction is bad, a failure had occurred with no visible signs, a defect in manufacture, etc. so until it is measured or placed in an operating circuit with predictable function, there is uncertainty. Uncertainty remains each time you remeasure it or re-energize it because a failure could have occurred. Certainty about the resistance value of the component is only 100% when under measurement and/or an energized state. This is why I check my meter leads prior to and after a measurement for continuity and against a known voltage source, because I have no certainty in the function of the meter or the continuity of the leads. And round and round we go

 

Here is a good tutorial on EMI

http://www.signaltransformer.com/sites/all/pdf/Controlling%20EMI%20in%20Transformers%20and%20Switch-Mode%20Supplies.pdf

[John Cuthber]

"You would not say that a long piece of wire is conducting unless it is part of a circuit. "

No, I wouldn't.

But I would say "copper is a conductor" and I would say "a copper wire is a conductor".

For what it's worth I have a carefully calibrated bit of copper wire that has (as it sits on a bench down in the basement) a conductance of 1000 mohs.

 

One describes the way in which the copper wire is acting at this moment (it is carrying or not carrying a current).

The other describes a property of the copper wire; it has (whether you like it or not) a resistance of 0.001 ohms.

 

I asked this before , and you haven't answered it.

Why does a resistor suddenly not have a resistance when the voltage across it drops to zero?

 

Also, I can, in principle, actually measure the resistance without an applied voltage /current, but I will let you work out how.

[WHR]

"You would not say that a long piece of wire is conducting unless it is part of a circuit. "

No, I wouldn't.

But I would say "copper is a conductor" and I would say "a copper wire is a conductor".

For what it's worth I have a carefully calibrated bit of copper wire that has (as it sits on a bench down in the basement) a conductance of 1000 mohs.

 

One describes the way in which the copper wire is acting at this moment (it is carrying or not carrying a current).

The other describes a property of the copper wire; it has (whether you like it or not) a resistance of 0.001 ohms.

 

I asked this before , and you haven't answered it.

Why does a resistor suddenly not have a resistance when the voltage across it drops to zero?

 

Also, I can, in principle, actually measure the resistance without an applied voltage /current, but I will let you work out how.

I can in principle CALCULATE the resistance of a copper wire based on the length of the conductor and it's gauge and the known properties of copper at a certain temperature. Assuming a certain purity level in the copper I might add. I cannot however measure it directly, anything else would be an inference. And unless you are a magician, I do not know of another method that doesn't require a certain amount of voltage, even with a high impedance digital meter.

 

The other stuff is in effect semantics. Again verbage and not science. Math tells the complete story.

 

Last example on the falling tree. Let's assume we wanted to know the resistance of the air underneath the tree as it falls. One could calculate the density of the air within the arc of the fall and the surface area of the tree. This is similar to knowing the length and gauge of the wire. I can then infer what the resistance of the event would be. Without the event there will be no resistance. Only air density and tree surface area. The action of falling must be mated to this

Other variables to establish resistance.

 

Let's take the word resistance and use it in a sentence, since we insist on analyzing this with language. "Resistance creates heat". I'm certain that this is a statement that you can agree with. I didn't even add a qualifier about the circuit conditions. Yet, the circuit conditions are not debatable. There has to be power applied to the resistor for heat to occur and be detected. A deenergized resistor will not self heat. So if resistance is truly a term that is independent of circuit conditions, then the term "resistance creates heat" is false.

Language trips you up. Yes, I'm fully aware that in the tree mind experiment that the variables can all be figured up, written down, and a prediction can be made. But again they are all independent variables until the action brings them all together.

 

Wind resistance of an aerodynamic object can be surmised by known variables, but there is no resistance until there is something (wind) to resist.

 

If you push against me, it is the act of pushing back and countering with resistance that defines the resistances. Or I could choose not to resist. If I am unable to resist an ice cream cone with all the fixings, it would follow that I actually consumed it. If I did resist the urge, I didn't consume it. Either way, there is an action, a decision tree, and a course. If I never saw the cone, then I didn't resist anything. A much looser metaphor but it fits with the error of language.

 

BTW I have a very nice photo of my DVM measuring 60 Hz of E-Field in my office with the leads in midair, with another photo showing zero Hz with the leads shorted. But this app will not at this time let me upload. I think I need to do it from home. There is also a voltage being picked up, just a few milliivolts but it drops to zero as well when the leads short.

[John Cuthber]

I can in principle CALCULATE the resistance of a copper wire based on the length of the conductor and it's gauge and the known properties of copper at a certain temperature. Assuming a certain purity level in the copper I might add. I cannot however measure it directly, anything else would be an inference. And unless you are a magician, I do not know of another method that doesn't require a certain amount of voltage, even with a high impedance digital meter.

 

Let's take the word resistance and use it in a sentence, since we insist on analyzing this with language. "Resistance creates heat".

 

I gave you a hint earlier about how to measure the resistance with no current flowing; it certainly isn't magic.

Why not take a sentence that isn't clearly false?

 

It's like saying "resistance creates breakfast cereal", and then trying to deduce something from that sentence.

[WHR]

You can not deduce anything from such a rediculous statement. But I can come close.

 

How about this sentence. A resistor creates heat. This sentence is absolutely false. You MUST add the qualifier that power be applied for the sentence to not be false. As I said, a resistor in a deenergized state will produce no heat. Whereas, you do not need the qualifier when saying, resistance creates heat. At least not with anyone trained in electronics. They understand that the application of power is implied by the statement. They MAY take the statement that a resistor creates heat without analyzing it and agree. But if you put the two sentences side by side and asked them to identify the false statement, they would realize that the resistOR statement has to be false because they could point to a shop drawer full of resistors at room temperature. However, when they visualize that "resistance" is a part of ohms law and that the power must be part of the equation they will see the reality.

 

Impedance creates heat. This is also a statement that can be agreed upon. Impedance is the sister of resistance. I would have said cousin but I think their relationship is closer it is absolutely 100% certain that impedance can only occur in an energized circuit, and not only this, but Since impedance is frequency specific we must also know that part of the equation. There must be a collapsing and building magnetic field generated around an inductor or a capacitor building an e_field as charges collect on the plates and then discharge. Otherwise, a coil is just a shorted piece of copper and a capacitor is an open circuit. That is why impedance cant be measured with a DC source.

Again, the impedance can be predicted, just like I can predict that if I mix certain portions of eggs, flour, sugar, milk, and chocolate flavoring in a bowl and stick it in the oven, and bake for X amount of time at Y temperature, that I'll have a chocolate cake. But I would not call the batter or the ingredients chocolate cake.

.

 

As far as the measurement technique you eluded to, the Johnson-nyquist noise, this is still I'm sorry to say an inference. It may be an accurate one, just as its accurate to calculate known variables about copper wire, transformer cores, lengths, temperatures, etc and predict that a coil will behave a certain way. If this were not possible, then manufacturing would be a fruitless endeavor. However, assigning this value assumes that the coil will be in a circuit. It assumes certain conditions of the circuit. All of this is prediction based on measurement of other variables, not the variable itself that we wish to measure.

[John Cuthber]

"Impedance creates heat. This is also a statement that can be agreed upon"

Nope, not in a purely reactive circuit.

Admittedly they aren't practically realisable, but that's not really the point.

 

BTW, this

"Whereas, you do not need the qualifier when saying, resistance creates heat. " is begging the question.

It is only true if you assume that a resistor has no resistance when there's no applied voltage.

 

If you make that assumption, you can't then use it to show that a resistor has no resistance when there's no applied voltage.

 

And I'm still waiting for you to answer my question.

 

While I'm at it this

" All of this is prediction based on measurement of other variables, not the variable itself that we wish to measure." is troublesome too.

If I measure the current and the voltage in a resistor then calculate the resistance from Ohm's law are you going to say that I'm measuring voltages and current not resistance which is the variable that we wish to measure?

Edited August 6, 2012 by John Cuthber

[WHR]

I've posed many questions and made many assertions that haven't been refuted or answered.

[John Cuthber]

You have made many assertions that were refuted and, as far as I can tell you asked just 2 questions (I got Chrome to look for question marks so if you forgot the "?" at the end of a question I will have missed it)

The first was "There you see?"

I thought that it was a rhetorical question but I will answer it if you like.

Yes, I see that you do not think that resistance is an intrinsic quality of an object- such as a resistor- but that you think it is a mathematical abstraction calculated from the ratio of the voltage to the current.

However as I pointed out, the definition of the word doesn't agree with your supposition.

On that basis you are simply wrong.

 

The second was "Yet, you are claiming that I wouldn't be able to sense an EM field produced by a transformer only a few hundred feet away at most? "

Now the direct relevance of that is questionable but I think the answer is "it depends". If the transformer were in a nice empty pasture somewhere then I have little doubt that you could detect the em field from it.

However if you were sat in a room with lots of other electrical equipment also generating 60Hz em fields I think it would be difficult ( or, at best, impractical) to assess the particular contribution to the net field made by that from the transformer.

 

So, for the record, that's another assertion (I've posed many questions) refuted.

 

On the other hand, you don't seem to have really answered any questions at all.

Do you plan to?

Edited August 6, 2012 by John Cuthber

[WHR]

Impedance is reactance plus resistance of a component. Yes impedance creates heat. Period. Ideal reactance would theoretically not add heat. (aka loss). Impedance necessarily does...and as was pointed out, impedance Isn't just "there" in a reactive component. The circuit must be energized for it to occur.

 

We are treating resistance like it is a thing and not an electrical function. "the resistor has resistance" like the "the wheel has turn". Wheels turn, resistors resist. LOL!!

 

You most certainly did not refute the part about the transformer. That's like saying if I draw a black dot, then draw a bigger black dot over it, the original black dot isn't there. I stated that all of these things can be picked up by even a simple device such as an AM diode receiver. I could take any one away and see the other. They contribute to the noise floor. It's like harmonics. Just because a square wave is a square wave, that doesn't mean you ignore the harmonics that it creates. This all goes back to the original problem with the DVM and you specifically claiming that no sensor could detect the field of a transformer, which is clearly wrong.

[John Cuthber]

"We are treating resistance like it is a thing and not an electrical function. "the resistor has resistance" like the "the wheel has turn". Wheels turn, resistors resist. LOL!! "

No.

The resistor has a resistance like the wheel has a moment of inertia.

 

What's the impedance of a 1µF capacitor (assumed to be ideal for the sake of simplicity) at 50Hz?

 

Here's a hint

http://en.wikipedia.org/wiki/Electrical_impedance#Capacitor

 

How much power does it dissipate?

And, BTW, I didn't claim to have refuted your claim about the transformer (though I still doubt that you can do it)

I refuted your claim that you had asked many questions.

I thought that saying "So, for the record, that's another assertion (I've posed many questions) refuted." made that clear.

 

Two is not "many"

Edited August 6, 2012 by John Cuthber

[WHR]

Editing to add that I'm going to respectfully agree to disagree with you on this topic. It clearly is a circular argument. I come from the standpoint that resistance is not something I can store in a jar, then go pick up an empty component, open some little compartment, and pour resistance in. I can certainly fabricate a resistor that will have a predictable resistance in a circuit. But outside of the circuit, this is meaningless. Outside of e meter circuit (which is just another active circuit) it is meaningless.

 

But it is clear that we both have a strong background in electronics theory. Even if our interpretations of that theory are not exactly the same. Perhaps it is a philosophical point and it's very hard to debate those.

 

Two may not be many, but I do not think it is clear that you have refuted any of my statements either. But I'm not arguing to be petty. I think some fundamental aspects of electronics slip if someone doesn't solidly understand the mathematics behind it. Down to the atomic level. For instance many people think that current is little electrons moving down the wire, when in actuality it is the AVERAGE of electrons and the difference between the ones that happen to not move at all or move in the opposite direction. And that the electrons do not actually flow, they effectively push the existing electrons in the conductor. And some bounce back and go the other way. It's the net movement that is measured.

[John Cuthber]

" I come from the standpoint that resistance is not something I can store in a jar, then go pick up an empty component, open some little compartment, and pour resistance in. "

Strawman.

Nobody said that.

You can't pour diameter into a car tyre- but it still has one.

 

You have repeatedly failed to answer my questions and it seems that you are now refusing to do so.

 

I'm sure others here will form their own opinions on "I do not think it is clear that you have refuted any of my statements either."

They may well base those opinions on the web pages I have cited.

[ACUV]

I'll assume that John Cuthber and WHR both have electrical knowledge and it is quite interesting that the meaning of a fundamental such as resistance is being questioned. Is a term such as Potential Resistance / Static Resistance useful as opposed to Actual Resistance / Dynamic Resistance. Invent a compromise!

[WHR]

I only cited one page I'd I remember but it pretty clearly testifies to my assertions. Even a small transformer inside a device is enough to generate EMI.

 

The rest is about semantics regarding resistance as a word. It is impossible to cite a website that goes into distinguishing between the mathematics and the verbiage. It's not an argument that I'm accustomed to. But none of the sites you linked to make that distinction either. For instance the Johnson-Nyquist link. I also have a chemistry background. I could go into detail about gas chromatography and how compounds are seperated and analyzed...I could also describe how many other accurate things can be infered from that analysis. We could infer the solid point, flash point etc of a chemical compound from its constituents and their volumes. However we aren't actually measuring those phenomenon and it is often inadequate to do so from inferences when precision and accuracy are required. But again I'm going to defer the debate and agree to disagree.

 

Haha ACUV thanks for the comment. I am sincerely not looking for a trophy. I just strongly believe in emphasis on the mathematics because deeper understanding lies in that area. Until my studies got deep into the mathematics, I was very lost and couldn't see many other things.

 

I'll assume that John Cuthber and WHR both have electrical knowledge and it is quite interesting that the meaning of a fundamental such as resistance is being questioned. Is a term such as Potential Resistance / Static Resistance useful as opposed to Actual Resistance / Dynamic Resistance. Invent a compromise!

 

Since I didn't properly quote you earlier ACUV, I reckon I should you actually touched on part of the issue that confronts circuit design or repair. Many components may have an initial resistance VALUE as intended in a circuit, but interacting with the other aspects (voltage and current) as well as the changes in demand of a load, threshold voltages, breakdown voltages, dielectric effects, coupling and isolation, etc etc etc (very long laundry list of considerations) all of these things can cause an initial resistance to become DYNAMIC. However I will not create a term, I think this just needs to be recognized.

 

I did manage to get my simple dvm experiment photos uploaded

Edited August 6, 2012 by WHR

[ACUV]

It is extremely important for clarity that the meaning of resistance is understood in it's true meaning as either inside of or outside of a circuit or both , whichever case is applicable.