Sorry for the title, I meant VOLTMETER, NOT MULTIMETER.

This might sound so stupid, but I was thinking... A voltmeter works by reading the potential difference between 2 points, so you want to connect it parallel to the device you want to measure.

My question is, if air is at 0V, why a voltmeter doesn't read the voltage if you put one probe in the 120V live wire and the other flying in the air?
Why are you required to put it into the ground wire hole to read those 120V?

This looks so stupid but I'm having my mind blown. Take me out of this stupidity!

 

Two questions for you.
1) Where did you get that air is at 0v
2) Is air not an insulator?
There has to be a physical connection to the meter to read voltage. Waving it around won't get you there.

 

Well, if you put one probe on ground and another on the air, you get 0V difference, which means they are at the same voltage, that's 0V, so they should be interchangeable, but they aren't, and I don't know why.

What the heck am I missing? I feel so stupid xd

 

Well, if you put one probe on ground and another on the air, you get 0V difference, which means they are at the same voltage,

No, it doesn't.

No voltage and zero voltage are not the same thing. And, air usually has an electrostatic potential that is above the local GND potential. But it cannot force enough electron flow to overcome the input impedance of a low cost meter.

I have a upper-end Keithley meter, and on the DC scale with the probes just dangling off the bench, the display changes significantly when someone walks into the room. Back in the 60's-70's, Popular electronics had a construction article about a passive "people detector" as an add on to an alarm system. Basically (working from memory) it was a dual-gate insulated-gate MOSFET with essentially a holy-crap input impedance, followed by a signal conditioner and output driver. It really worked.

ak

 

It's like water if you take two pipes pour water in one it runs out but the one that you did pour water in nothing happens
there is no easy way to ground air.

Now if you touch the hot side and stick a rod in the ground you will get a reading.
I have a video of just that.

 

Air molecules can most definitely hold charge, giving them a potential difference (voltage) relative the ground. A digital volt meter is an imperfect device, cheap ones tend to be worse than fancy ones. Just because it cannot measure voltage at the molecular level doesn't mean such voltage doesn't exist. Also as pointed out above, air is a very good insulator. When measuring voltage, some current flows through your imperfect volt meter. Because the air is such a good insulator (it inhibits the movement of charge), as the multi meter probe drains charge to the ground probe, no more charge can flow through the air to the probe, so essentially your probe is giving you a short to ground due to the lack of charge mobility in the air. Again some meters are better than others.

Think about a storm cloud. The term air is a bit ambiguous, but the cloud is essentially air. It builds more and more charge until the voltage surpasses the breakdown voltage (the voltage at which an insulator starts to flow current) of the air, and we get a big discharge called lightning.

 

Of course I'm not caring about those 100mV difference in the air. A voltmeter acts as an open circuit, so why you don't get 3V difference if you put one probe in a 3V battery positive pole and the other probe in ground?

There's my confusion, I don't understand why.

 

A voltmeter acts as an open circuit,

No, it doesn't.

For a typical digital DC voltmeter, the input appears as a large-value resistor. The meter indicates the voltage across that resistor. That voltage comes from the current through the resistor, per Ohm's Law. Granted the current is very small and the resistance is very high, but without electron flow (current), there is no reading. and that electron flow comes from a complete circuit of which the meter's input resistance is only one element. Waving a probe in the air does not complete the circuit. If you hold a battery in one hand and an LED in the other, the LED does not light. Same thing.

ak

 

why you don't get 3V difference if you put one probe in a 3V battery positive pole and the other probe in ground?

The voltmeter needs a complete (return) connection so that current can flow.
There's no connection between ground and the battery so no current can flow, and you read no voltage.
Always remember that all electrical circuits need a complete circuit to operate.
You can't just connect one terminal and expect anything to happen electrically.

 

The negative end of the 3V battery would have to be connected to the ground at the same point the meter probe is. The poles of a battery have no relation to “ground”.

Edit: whoops, crutschow got there first.

 

Oh, oh, ok, now I get it. A voltmeter doesn't act as an open circuit, but almost. OK, I got it when you said it acts as a large resistor. I was so used to say when analyzing circuits "I connect the voltmeter in parallel, acts as an open circuit, all the amps go through the other cable, no amps through the voltmeter, and I get the voltage".

So, there must always travel some amps (milliamps or microamps) through the voltmeter for it to correctly read something, right?

 

Probing a circuit disturbs a circuit. Its no longer the exact same circuit. How much disturbance are you willing to put up with?

At this stage of the game consider the disturbance a 10 M resistor your connecting with your probes in Volts mode.

 

Good advice I've heard is that "the meter is always part of the circuit".
It's then up to you to determine whether it's a significant part.

 

Yes, thanks you-all!

 

All points are correct.

Just adding to the interesting point mentioned by crutschow:

Good advice I've heard is that "the meter is always part of the circuit".
It's then up to you to determine whether it's a significant part.

In the older vacuum tube systems, the input impedance of voltmeters (and multimeters by extension) was relatively small for the power supplies used in electronics equipment. It was not uncommon to have 200, 300V on radios and TV sets, with 500V in higher power amplifiers. All that at currents of tens or sometimes hundreds of mA.

Also, the schematics used to show voltages at various points of the circuit to help the repairman and always mentioned the sensitivity of the voltmeter/multimeter used to get these voltages, specified in kΩ/V (one example is here). This sensitivity translates directly to the input impedance of the voltmeter/multimeter, which varies depending on the selected input range. A very common sensitivity was 20kΩ/V which, in the scale of 1000V, translated to an input impedance of 20MΩ (20k x 1000). Just for comparison, the modern DMMs have a typical impedance of about 10MΩ, thus using one to measure voltages on a vacuum tube circuit will render different readings due to the extra load caused by the DMM.

 

Oh, oh, ok, now I get it. A voltmeter doesn't act as an open circuit, but almost. OK, I got it when you said it acts as a large resistor.

A similar example is if you are familiar with the electricians neon screwdriver tester, this has a neon lamp in the handle and a high value resitor, when you hold the screwdriver handle and touch the screwdriver blade to any appreciable AC supply source, the neon lights, even though you may be apparently insulated by standing on for e.g. , a wooden crate. i.e. no apparent return path.
But when the service is earth grounded at the source, there is enough conduction through yourself to earth GND and complete the path to result in the neon conducting.
Max.

 

I wrote some stuff on my cell yesterday but lost it. I may come back and address what I was going to say.

Calibrating tube testers took into account the ohm/V of the meter. Calibrating with a modern DVM will mess up the calibration,

 

A similar example is if you are familiar with the electricians neon screwdriver tester, this has a neon lamp in the handle and a high value resitor, when you hold the screwdriver handle and touch the screwdriver blade to any appreciable AC supply source, the neon lights, even though you may be apparently insulated by standing on for e.g. , a wooden crate. i.e. no apparent return path.
But when the service is earth grounded at the source, there is enough conduction through yourself to earth GND and complete the path to result in the neon conducting.
Max.

Yes! And I never knew how they worked. So, mains AC's return path is ground, so you yourself, holding the screwdriver, are the one that closes the path between mains and ground?
That really sounds crazy!
But then, if you have electrician boots or shoes, with nice insulating material, this shouldn't work, right? No path return?

 

Consider also the word "circuit", which implies a closed circle of some size. If you are measuring a voltage between two points, then there needs to be a complete loop, to provide a complete connection. Read about circuits and basic electrical theory and principles and you will understand.

 

Yes! And I never knew how they worked. So, mains AC's return path is ground, so you yourself, holding the screwdriver, are the one that closes the path between mains and ground?
That really sounds crazy!
But then, if you have electrician boots or shoes, with nice insulating material, this shouldn't work, right? No path return?

No still works. Boots or bare feet!
I remember an example when working on a site in the UK where in a newly built brick house, a builder complained of getting a 'tingle' off of the outer brick wall, when the wall was touched with one of the screwdriver testers it glowed quite brightly!
One of the electricians had mounted an outside light and accidentally pierced the live conductor with a wall mounting screw.!
Max.