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.

Yeah, but where's the circle in the screwdriver light live cable detector?

 

Yeah, but where's the circle in the screwdriver light live cable detector?

From AC supply source, screwdriver end tip, human body connected to metal clip on screwdriver (neon in between), through whatever standing on to earth ground, through earth back to AC grounded neutral source.
Max.

 

There can be a small current from the mains voltage through the body capacitance to ground.
Anytime current flows there's always a complete circuit, although sometimes it's not apparent where the path is.

 

But I said "what if you wear rubber shoes or professional electrician boots?"
I thought that would open the circuit?

 

But I said "what if you wear rubber shoes or professional electrician boots?"
I thought that would open the circuit?

Yes rubber shoes or plastic ladders will be an Insulator and the Neon will not light up...

As for the multimeter probes you need two to make a circuit, one serioulsy won't work!!

 

"what if you wear rubber shoes or professional electrician boots?"
I thought that would open the circuit?

It opens the resistive circuit but not the capacitive.

 

It opens the resistive circuit but not the capacitive.

I'm kind of losing it right now. I know that capacitors actually don't touch each others plate (open circuit one could say), nevertheless, current can go through (jump through air???) if there's a change in voltage over time (AKA V is not constant). But I don't know how to apply that to this specific example xd

 

I know that capacitors actually don't touch each others plate (open circuit one could say), nevertheless, current can go through (jump through air???)

Not the current, but the electric field across the capacitor air/dielectric gap, which causes charges on one plate to move in response to the charges on the other plate.
Thus an AC current can move "through" a capacitor, even though there is no DC resistive connection.
It's magic.

 

Yes rubber shoes or plastic ladders will be an Insulator and the Neon will not light up...

It will in America.

ak

 

OK, let me try to remember what I wanted to talk about.

The input Z of a modern multimeter is 10 M ohms usually. There is the ohms/volt spec which comes from a DC current meter movement with it's own internal resistance and an external resistor. Fancy expensive meters might have a different and even higher input Z depending on the range. e.g. 1 G ohm Prior to the DVM of today, there was the VTVM or vacuum tube voltmeter. I owned a FET TVM.

The oscilloscope has USUALLY 1 M || 22 pf or a 50 ohm input Z, sometimes switchable. Let's ignore the 50 ohm one.

When you add a x10 probe to a scope with 1M || 22 pf and you properly compensate the scope probe, the scopes input becomes 10 MEG OHMS purely resistive. 22 pf is a common value. A square wave is use as the input for compensation, you adjust a capacitor on the probe till the wave is square. So, the probes have to be matched to the scope somewhat.

Everything has frequency limitations.

"parasitics" or unwanted capacitance, inductances and resistances can and will haunt you. Watch temperature and frequency dependence.

A 1x probe, you can't compensate.

 

I'm kind of losing it right now. I know that capacitors actually don't touch each others plate (open circuit one could say), nevertheless, current can go through (jump through air???) if there's a change in voltage over time (AKA V is not constant). But I don't know how to apply that to this specific example xd

Not the current, but the electric field across the capacitor air/dielectric gap, which causes charges on one plate to move in response to the charges on the other plate.
Thus an AC current can move "through" a capacitor, even though there is no DC resistive connection.
It's magic.

Perhaps taking the problem from another angle.

Consider that a Neon lightbulb has a tremendously high impedance (there is a graph of current versus voltage at this wikipedia page) thus, if the body resistance+capacitance (impedance) is small or relatively small in comparison, a voltage applied will almost be entirely available at the lightbulb's terminals.

 

Thank you for the painting, hahahaha. I'm actually taller than that, but it's OK, how would you know.

Anyways, the problem of your drawing, the one I see, is that the circuit is open for my eyes. I completely understand what you draw, but I don't understand how can you close a circuit with your feet. There shouldn't be electrons moving? Where?
If the light bulb lights up, that means electrons are moving, but certainly not through your body, hence open circuit???

I hope you understand where's my problem. I clearly got it wrong because the light lights up, but I don't know where I'm wrong. I mean, if I believe you just like that, then I'd say air also closes the path between ground and live wire.

 

but I don't understand how can you close a circuit with your feet. There shouldn't be electrons moving? Where?
If the light bulb lights up, that means electrons are moving, but certainly not through your body, hence open circuit???

.

Do you understand how current can flow via capacitance alone?
i.e. no physical contact from one plate to the other.
Actually it is through your body!
If you don't touch the screwdriver pocket clip it does not light.
Max.

 

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

(Unintentional potentially offensive remark removed by possibly overly sensitive moderator. Please consider how your comment might be interpreted by others.)

Now if he would have said hold it under a running faucet of water, I might have had to ponder it for a minute!

What are you missing ?
If what you were talking about worked..... We wouldn't need "ground" wires.
We would all be electrocuted when standing by power lines
Do I need to keep going.

Brzrkr

 

An NE2 requires about 100K resistor if operated at 120 VAC. It also needs about 70 V to conduct. It turns off at a lower voltage. Once the gas conducts, you have a low R path, hence the series resistor.

If you put an NE2 near high voltage it will light. No leads required.

 

Think of a voltmeter as a 10Mohm resistor, if it is an open circuit no current can flow (even in the nano amp range).

 

Thank you for the painting, hahahaha. I'm actually taller than that, but it's OK, how would you know.

Anyways, the problem of your drawing, the one I see, is that the circuit is open for my eyes. I completely understand what you draw, but I don't understand how can you close a circuit with your feet. There shouldn't be electrons moving? Where?
If the light bulb lights up, that means electrons are moving, but certainly not through your body, hence open circuit???

I hope you understand where's my problem. I clearly got it wrong because the light lights up, but I don't know where I'm wrong. I mean, if I believe you just like that, then I'd say air also closes the path between ground and live wire.

So the issue seems to be about how large is Zbody (or even infinite) right? I will try to explain below.

Consider that Zbody is comprised of Rbody (resistance of skin from the tip of the finger to the foot inside the boot) plus a Cboot (a capacitor formed with the gap between the foot and the ground, with the sole acting as the dielectric).

(Note: there are simplifications there, given the resistance of a human body would also have capacitive behaviour effects, as well as a high variability depending on the voltage applied. Also, the capacitance of the boot will also have a resistance. There could also be an Lbody caused by inductive effects, but the sky is the limit - I suggest searching for HBM or Human Body Model).

Taking also into account there is no infinite impedance on Earth (perhaps on absolute vacuum), there will always be some current flow across any body ( Zbody in the case above) - even if it is infinitesimally small.

In the case above, the Cboot would have a resistive component (Rboot), but the resistance is so enormously high that the predominant effect is the space between the conductor bodies. The difference between the amount of charge of each conductor body (caused by VAC above) becomes predominant in relationship with the resistance (Rboot). This difference of charge among each conductor is also called potential difference or commonly "voltage".

Here's the part that used to confuse me a lot when I studied this for the first time. This difference causes disturbances on each conductor (via an electromagnetic field), which causes electric charges to be displaced on each conductor circuit (the foot and the ground on the example). If the disturbance plateaus (DC), the two conductors will settle with an electric field between them. If the disturbance is continuously variable, the conductors will always play "catch" and follow each other. I visualized this concept of charge disturbance across a gap after learning and building some fundamental gizmos when I was a kid: an electrophorus and a Leyden jar.

Please note that ideally (for an infinite impedance across the boot) no material exchange happens between the two conductors: the number of electrons on each side of the capacitor remains exactly the same (there is no leap across the insulator) - again, it is only the effect of their movement (electric charge) that causes the displacement of charges on each side of the arrangement (the foot and the ground on the example).

This displacement of charges is in essence the electric current. If you displace enough charges by increasing the difference of potential (voltage) or by speeding up its variations (also called frequency), you can induce enough current across the foot-ground interface to lit up your NE-2 lamp.

A similar principle can be extended for air or a gas-filled container, the resistance is so enormously high that the only measurable thing left is the space between the conductors - that is where the effects of the difference between the amount of charge of each conductor is predominant.

The variance in air resistance can be perceived if you live in a place where there are drought seasons: you get more frequently zapped by electrostatic discharges during the "dry season" when compared to the "wet season", given the charge buildup on certain places (caused by friction with air or other materials) does not dissipate due to the extremely high resistance of dry air.

 

Here's a Q, why don't u see BNC DMM probes like a for scope, and with just 1 cable ??

 

Here's a Q, why don't u see BNC DMM probes like a for scope, and with just 1 cable ??

There are BNC adapters for just about any possible set of probe inputs.
https://www.digikey.com/en/product-highlight/p/pomona-electronics/bnc-between-series-adapters

BNC connectors are typically not totally insulated (or high voltage rated like MHV connectors) so there could be exposed shield metal at a possible high potential when the common (black) lead is floating.

 

Here's a Q, why don't u see BNC DMM probes like a for scope, and with just 1 cable ??

The reason that there are not BNC connectors on a multimeter, digital or analog, is similar to why there are no wheels on a power boat: They would not serve any useful purpose. An oscilloscope is intended to display waveforms, and so the connection must be shielded to keep noise out. A meter is intended to display voltage or current as a numerical value. That is quite different. The uses are different and the applications are different, and most important, the information displayed is different. An adequate study of basic circuit theory should provide you with an adequate understanding, and only require an hour of your time.