Contradictory? I don’t believe so, with the information I have.

As mentioned earlier, The OP will end up using spheres or points to measure this.

And I am assuming that the pressure is not changing.

Therefore the most predominant factor of the voltage required, will be the distance.

I doubt he could see any difference with temp or humidity.

I gave a link for setups and formulas. This is modern science’s explanation.

Has anyone here ever connected a car battery?

Has anyone here ever done any welding?

It’s like I said.

It’s the distance.

 

Contradictory? I don’t believe so, with the information I have.

As mentioned earlier, The OP will end up using spheres or points to measure this.

And I am assuming that the pressure is not changing.

Therefore the most predominant factor of the voltage required, will be the distance.

I doubt he could see any difference with temp or humidity.

I gave a link for setups and formulas. This is modern science’s explanation.

Has anyone here ever connected a car battery?

Has anyone here ever done any welding?

It’s like I said.

It’s the distance.

Since no distance has been given, how can you possibly claim that the breakdown voltage is 200 V to 300 V or even less? In many parts of the world household electrical voltage is 240 VAC, which means that the peak voltage is about 340 V. According to you, that is well in excess of what it takes to breakdown air, so why aren't things arcing continuously? Overhead transmission lines can operate at voltages well in excess of half a million volts. So, again, where does this 200 V to 300 V figure come from? Do you understand the difference between voltage and electric field strength?

As for welding, the arc is almost always struck initially through direct and transient contact and not via dielectric breakdown (though in theory the breakdown occurs just prior to contact, but since the distance at which this happens would be less than the thickness of a sheet of paper it is immaterial).

 

.....As for welding, the arc is almost always struck initially through direct and transient contact and not via dielectric breakdown................

In MIG and stick welding you are correct, the arc is initiated by shorting the welding wire to the metal. Although, In modern TIG welding, the arc is initiated using a high frequency, high voltage between the electrode and metal. No contact is required to start the arc. The older TIG machines, without high frequency start, required the welder to "scratch" start the arc, which is rough on the electrode.

 

In MIG and stick welding you are correct, the arc is initiated by shorting the welding wire to the metal. Although, In modern TIG welding, the arc is initiated using a high frequency, high voltage between the electrode and metal. No contact is required to start the arc. The older TIG machines, without high frequency start, required the welder to "scratch" start the arc, which is rough on the electrode.

I think I have heard of that, but if so had completely forgotten it. Thanks.

 

Around about the time of WW2, they started getting aircraft up to altitudes where the ignition spark could happen pretty much anywhere *EXCEPT* the plug gap.

First time I've heard of this phenomenon. Mind elaborating?

 

Dense air is more 'electronegative' as a system than a partial pressure atmosphere. Once you begin to reach vacuum the breakdown voltage increases but there is a zone in-between that has a much lower breakdown voltage than either.
https://en.wikipedia.org/wiki/Paschen's_law

 

The definitive answer is here:

<http://hypertextbook.com/facts/2000/AliceHong.shtml>

Hodgman, Charles D. & Norbert A. Lange. Handbook of Chemistry and Physics 10th Edition. Cleveland, OH: Chemical Rubber Publishing Co, 1925: 547. "Spark length (cm), .10; Point electrodes, 3720; Ball electrodes, 1 cm diameter: Steady potential, 4560; Alternating potential, 4400" 3.7–4.5 × 106 V/m

The breakdown voltage does indeed depend on temperature and humidity (air density), but primarily on air density (altitude) and the shape of the electrodes as well as the rate of increase of voltage. Note above: spark length 1mm, ball electrodes 4560V, but pointed it drops to 3720V. AC is slightly lower than DC. Basically, an ignition-type spark jumping a 1cm gap is about 40kV. Ever get a shock from static discharge??
Note the date of the handbook - 1925!

Air can be ionised, so potential is less than in vacuum. High pressure (e.g. combustion chamber) needs much higher voltage to jump the small plug gap.

 

The link produces a 404 error.

But, aside from that, on what basis are you claiming that this is the definitive answer? What about it is defining?

 

The link produces a 404 error.

But, aside from that, on what basis are you claiming that this is the definitive answer? What about it is defining?

I was thinking the same thing. It's hardly definitive, especially considering it was written 90 years ago....

 

The link produces a 404 error.

But, aside from that, on what basis are you claiming that this is the definitive answer? What about it is defining?

Because it's the friggin' CRC Handbook of Chemistry and Physics!

About 2000 pages of physical data on everything from motor oil viscosity to steam tables. Refractive index, boiling point, melting point, molecular mass and solubility of 10,000 organic molecules and almost every physical constant you will ever need. It has been the Wikipedia of reference books for the last 100 years. If there is an error in the answer, then the measurement method has changed and the correct values will be in the newer editions.

This is basic information. Look in Wikipedia and various other sources. The details are there. The values are the same order of magnitude. Definitely more accurate than most people's memories of "discussing this in freshman physics lab". A measurement that is very dependent on the environment and test conditions but not so much that the answer is meaningless. Let's put it this way, definitive enough to answer the OP's question unless he/she gives exact test conditions they are looking for.

 

I was thinking the same thing. It's hardly definitive, especially considering it was written 90 years ago....

But does it change so much? If it does, how much will it be in error? How old is Einstein's theory of relativity? Should we assume it is wrong as well just because it is old? ... And probably written on paper instead of on your screen?

 

Here, from the 1989 version of the CRC handbook. The table looks the same as my older version.

Data at lower voltages



At higher voltages



And correction factors for temp and pressure



Don't let your world be a mystery, look it up in a 90 year old data table instead of debating, masters.

 

But does it change so much? If it does, how much will it be in error? How old is Einstein's theory of relativity? Should we assume it is wrong as well just because it is old? ... And probably written on paper instead of on your screen?

Understanding of the physical world has increased exponentially since that article was written, so even back then they may have said "It takes 40kV to jump 1cm", but did not know enough at the time to consider how that might change with different environmental factors.

 

Understanding of the physical world has increased exponentially since that article was written, so even back then they may have said "It takes 40kV to jump 1cm", but did not know enough at the time to consider how that might change with different environmental factors.

CRC has decided that spark gaps are either archaic or specialized information. It has not been in the last 25 years of updates to this general handbook. It may appear in other field-specific handbooks they publish.

 

In MIG and stick welding you are correct, the arc is initiated by shorting the welding wire to the metal. Although, In modern TIG welding, the arc is initiated using a high frequency, high voltage between the electrode and metal. No contact is required to start the arc. The older TIG machines, without high frequency start, required the welder to "scratch" start the arc, which is rough on the electrode.

Thanks for the great discussion!

But the following quote made me second guess something else...

Does AC differ at all to DC, when it comes to the dielectric breakdown voltage?

Won't AC possibly have some heating properties on the dielectric due to the charges moving across the conductor.

 

actually the plug gap is the hardest place to jump a spark, due to the high pressure of compressin in the cylinder.

 

actually the plug gap is the hardest place to jump a spark, due to the high pressure of compressin in the cylinder.

Part of that is offset by making sure the hotter centre electrode is negative - electrons are more eager to leave something that's hot.

 

how do you make it negative? the coil has no rectifyer, the high voltage for the spark in engines is ac.

 

how do you make it negative? the coil has no rectifyer, the high voltage for the spark in engines is ac.

The spark is DC. You charge a coil up and then open-circuit it. The field then collapses creating a high enough voltage to create the spark as it keeps the current flowing in the coil. There's no mechanism for it to go the other way.

 

The spark is DC. You charge a coil up and then open-circuit it. The field then collapses creating a high enough voltage to create the spark as it keeps the current flowing in the coil. There's no mechanism for it to go the other way.

Technically the coil is AC because its a transformer, but the "forward transformation" is insignificant compared to the flyback pulse that occurs when the LT circuit breaks and the field collapses.