STUDYING THE PROPERTIES OF ELECTRIC ARC USING JACOB'S LADDER

2.1. Electric arc in devices

An electric arc between two electrodes in air at atmospheric pressure is formed as follows: when the voltage between two electrodes increases to a certain level, an electrical breakdown occurs between the electrodes in the air. The electrical breakdown voltage depends on the distance between the electrodes and other factors. The ionization potential of the first electron of metal atoms is approximately 4.5 - 5 V, and the arcing voltage is twice as high (9 - 10 V). It is necessary to expend energy to release an electron from the metal atom of one electrode and to ionize the atom of the second electrode. The process leads to the formation of plasma between the electrodes and the burning of an arc (for comparison: the minimum voltage for the formation of a spark discharge is slightly higher than the electron output potential - up to 6 V) [3].

The interaction of electrodes with arc plasma leads to their heating, partial melting, evaporation, oxidation and other types of corrosion. It is the partial melting of the electrode that is used in electric arc welding and steel smelting. Another application of the electric arc: in arc lamps.

When operating high-voltage electrical installations, in which the appearance of an electric arc is inevitable, it is combated using electromagnetic coils combined with arc extinguishing chambers.

How to make a powerful Jacob's ladder from a microwave transformer with your own hands

The desire to write this post was prompted by this photo from a student conference at a technical university.

— Did you see what the students showed at the conference? - asked a friend, showing a photograph. - Wow! This is complete *****! And were you there? - Yes, I approached them and talked, but they didn’t listen to me. What could I do???

And really, what can be done. After talking with friends, we decided to write this post, in which we will tell a little about “Jacob’s Ladder”, our own experience in making these installations, and explain why this photo caused so many emotions.

Jacob's Ladder is an installation consisting of a high-voltage power supply and two electrodes located in a vertical plane at an angle to each other. So the distance between them from below is smaller than from above, and was chosen depending on the voltage of the power source, so that air breakdown between the electrodes occurs independently. During its operation, one can observe the formation and movement of an electric arc from the bottom (bottom) to the top of the electrodes. This effect is explained by the fact that the arc consists of plasma heated to 5000-7000 ° C; it heats the air around it and, thanks to convection currents of heated air, rises upward, pulling the arc cord along with it. If you photograph a discharge taking off with a long shutter speed, you can get an unusual photo (see above) that will resemble a staircase from a biblical story:

Landscape with a Dream (from a Dream) of Jacob, Michael Wilman, c.
1691 Jacob's Ladder effectively demonstrates a whole chain of physical phenomena. And I cannot remain indifferent to this attitude, because I love physics, since my student days I have been a convinced popularizer of science, and in recent years I have been developing exhibits for science museums. Some readers of the hub probably remember that we made the largest Tesla coil in Ukraine. Those who read the article noticed that most attention was paid not so much to the manufacture of the reel itself, but to safety issues.

In the photo at the beginning of the post we see how exposed electrodes are in direct access to conference participants. Anyone who accidentally (out of curiosity) touches the electrodes will most likely die. However, Sergei Aleksandrovich, a hobbyist with vast experience in manufacturing Tesla coils, Jacob's ladders, ionophones, voltage multipliers, and simply a big fan of transformers of all shapes and sizes, will tell you better than me about the obvious and non-obvious dangers of high-voltage installations

On the hub he goes under the nickname Frost273, by the way, he is the author of the largest Tesla coil in Ukraine. The following will be his text.

We can confidently say that every modern person, in one situation or another, has encountered the phenomena of electricity, and sometimes not when he himself wanted it.
If we take into account young (if not in body, then in soul) and bearded (are there any girls here who are interested in electrical engineering?) techies, and it is they who will be primarily attracted by the title of the article, then there will not be anyone who has not experienced an electric shock, be it a disassembled power supply, or an undischarged capacitor, the fact remains a fact. And since you, friend, are now reading this article, it means you are still alive, and you can tell about your experience in the comments. However, with some operating electrical installations, inattention becomes the last mistake made in life. The experiment with the manufacture of Jacob's ladder also belongs to a similar dangerous class of devices.
Especially Jacob’s ladder, “in the heart” of which there is a high-current power source, a transformer base from a microwave oven, which can deliver up to 500 mA (0.5 amperes) depending on the model of the microwave oven.
And now, for a second, a table to refresh your memory on such a familiar subject as life safety.

Impact of electric current on humans
And I am ready to confirm that even such small amounts of alternating current flowing through the fingers of 0.001 amperes (1 mA) can seem like a pleasant sensation to few people. And the more unpleasant this feeling will be, the more sudden it arises. I found this out when I was a teacher at a higher educational institution, when I had the desire and audacity to diversify electrical engineering lessons for 3rd year students by conducting applied experiments with electricity.
Returning to the table, you can see that with any alternating current flowing through the body greater than 15 mA, there is a threat to the health and life of a healthy person. A person who has heart defects is already at risk with lower values.
I would also like to note that the spread of values in the table, for example, for non-release currents (10...15 mA), depends on a combination of five additional factors, in addition to the value of the current itself:
voltage;
resistance of human skin (this is also affected by the presence of skin damage and the contact area);
duration of impact (counting in fractions of a second);
frequency of electric current;
flow path through the body.
From the last point it follows that current will flow through the body and vital organs only when there are two points of contact with different potentials.
As an example, I think you can all remember how you saw birds sitting on the wires of power lines, despite the fact that there could be a voltage of 380 V, 10 kV, or higher. Such invulnerability of objects of the animal world that are in direct contact with wires is explained by the fact that they acquire the electrical potential of the wires and current does not flow through them. And due to the size of birds, the amount of leakage current through the surface of their bodies into the environment is safely small. Mortality occurs only when the bird wants to clean its beak on the traverse. Unfortunately, such incidents are of frightening proportions. We got distracted... I wanted to convey the idea that in this topic there is no place for romanticizing death from electric shock. There is a 100% chance that the experimenter will not become Doctor Manhattan.
And most likely he will no longer become anyone, although this is not certain. Because the chance to stay alive, although small, is there. But there is a high probability of what is:
Experience disturbances in the functioning of the cardiovascular system, respiratory system, nervous system...
Get an electrical burn (a harmonious combination of thermal and chemical burns). When receiving an electric shock, the resistance of our body plays against us, since a sufficiently large amount of current flowing in the tissues is converted into heat, which causes coagulative necrosis at the entry and exit points on the skin, as well as in the muscles and blood vessels through which the current flows. Which in turn leads to the development of thrombosis, often in places remote from the surface of the body, which significantly slows down the process of tissue regeneration. Moreover, the blood plasma itself undergoes electrolysis when current flows, which makes it unsuitable for performing its direct functions of transporting oxygen.
In addition, there is a danger of clothing igniting when in contact with an electric arc. Just imagine how inconvenient it is to extinguish your clothes with paralyzed limbs.
Photograph of an area of the human body affected by electric current (censored)
One should not expect that an electric arc will leave a beautiful burn in the shape of a Lichtenberg figure on the surface of the body, since this form of damage is only possible when struck by static electricity, like thunderstorm lightning.
I would like to remind you that the voltage level used in the “Jacob’s Ladder” experiment was initially aimed at the formation of an electric arc.
Receive burns to the retina of the eyes due to exposure to hard ultraviolet radiation produced by an electrical discharge.
I must admit that I myself dabbled with this device at home, of course, in compliance with safety measures, and with additionally equipped glasses with UV protection, but the unprotected areas of my face, which did not see the sun during the long winter months, remained covered with a completely unaesthetic patchy tan.
Something like this.
Now, reader, weigh your curiosity on one side of the scale, and on the other, the above-listed features of the Jacob's ladder-building. Isn't it safer to watch a video with electricity on a cozy YouTube?

Preparing for assembly

If you are still determined to try your hand at making this wonderful installation, then here are the first necessary points:
Point 0 (main).
Do not use MOT This item was added on the recommendation of Neuromantix, lorc and Firelander.
The MOT (Microwave Oven Transformer, or microwave oven transformer) is an incredibly dangerous device. Never use it!

Using other high voltage sources with lower current will significantly reduce the likelihood of killing someone. However, the danger of causing irreparable harm to health remains.

Point 1. Selecting insulation

It may not be obvious, but conventional insulating materials that can handle voltages up to 500 V stop working at kilovolts. At high voltages, paper, wood, plywood, chipboard, fiberboard, cardboard, and even dust on the surface of the PCB can become conductors! Wires must also be selected with insulation designed for the appropriate voltages. So, the cable from the extension cord is absolutely not suitable!

Point 2. Prepare the room

The installation must be assembled on a dry, non-flammable, non-conductive floor. There should be empty space around the installation so that if the electrodes fall, no one or anything will be harmed. Here is an instructive video where the setup was assembled on a table and fell directly on the experimenter:

Fortunately, due to the flimsy fastening of the electrodes, the circuit broke and the hero of the video did not have time to die. However, not everyone is so lucky; just recently there was news about how a schoolboy was killed while independently assembling a Jacob’s ladder in the garage. Also, no cats, dogs, birds or other animals can be in the room. At the most inopportune moment they can snag/overwhelm the installation, distract you, or simply reach for the wires.

And most importantly

so that no outsider has the physical ability to enter the premises, and not only for the period of work, but also for the time while the installation is there.

Point 3. Body

It is necessary to think through the housing design in advance so that no one can physically touch the electrical components of a working installation, especially the electrodes. When we made the first Jacob's ladder for the science center, we placed it behind bars with a lock and behind glass:

When there is no separate room, we assemble the installation in a protected enclosure, following the following rules:

A source with a current of up to 30 mA is used.
The installation is removed from observers; they cannot touch the body.
A responsible person is always present near the installation.

Point 4. Do not work alone

Firstly, it is necessary that during all test runs there should be a person present who is ready to de-energize the installation and provide qualified emergency medical care. Secondly, there must be an adult who is ready to take responsibility for fire, injury, or even death that may occur under unfavorable circumstances. This may not be a complete list, I invite you to add to it in the comments, I will add important points to the article.

Diagrams and assembly instructions

Sorry dear reader, I deceived you - there will be no instructions or diagrams, and I’m not at all ashamed.
I am sure that you can easily find all this on the Internet. The design is so simple that any student can assemble it. However, before you start assembling, ask yourself, are you ready for the fact that your loved one may get hurt? PS

Yes, the title is yellow, and the post does not contain new information for Habr’s target audience. Nevertheless, I hope that the article will take its rightful place in search results and will be read by many enthusiasts, and perhaps even save a novice experimenter from death.

UPD 1: First conclusions from the survey and comments

As of 23:59 on June 14, the survey results are as follows:

It may seem that everything is not so scary - people are hit by high voltage and everyone is alive and well, especially since this confirms it. However, this is not quite true. Because there is a survivor's fallacy, because only survivors were able to talk about their experience. Moreover, I carefully read the comments and could not find a single case where someone was actually hit by high voltage. The overwhelming majority of comments share their experience of contact with 100-220 V. Only a few experienced a very short-term shock from a capacitor discharge, which is not as scary as continuous contact with the secondary of the MOT. I will add that only one person I know had experience interacting with high voltage. When we were children, he snuck into a substation... ...He died very painfully from a huge number of burns within 15 minutes. And it looked terrible. Perhaps this is why I am sensitive to the harsh violation of TB.

Job

While the builders were dismantling the last putty, physicists from the Landau Center were assembling the Tesla coil and Jacob's Ladder.

Nikola Tesla patented the coil back in 1891. Since then, its design has remained virtually unchanged. You can build a simple transformer using this video:

Physicists from Landau argue about the primary circuit of the coil:

Now let's see how the Tesla transformer

There is a voltage source - 220 volts. The current is supplied through a 200-watt incandescent light bulb, which serves as a current-limiting resistor. If there were no light bulbs, turning on the Tesla coil would trip the circuit breakers.

The current goes to 4 transformers. They are in a plastic container and filled with synthetic machine oil. Oil is a dielectric and protects against electrical breakdown, and also cools transformers during operation.

At the output of the transformers we get 8,400 volts. The current is completed through the air gap between the two contacts that rotate on the motor.

When the engine is running, the contacts close and an oscillatory circuit is formed. The primary winding has inductance, and the capacitors in which energy is stored have capacitance.

After this, a current with a frequency of over 200 kilohertz begins to resonate in the primary circuit, and part of the energy is absorbed by the secondary winding.

As a result, we get the largest coil in Ukraine and almost half a million volts. For a reel 80 centimeters high, the result is decent. But to cause some kind of Tunguska meteorite, this, of course, is not enough.

"Jacob's Ladder"

– these are 2 copper electrodes, each length is 2 meters. They also carry high voltage current – about 18,000 volts. An air breakdown and an electric arc occurs between the electrodes, which rises upward.

An electric arc in the air turns into plasma, the temperature of which is 5000 ºС. Warm air is lighter than cold air, so due to convection it rises, carrying an arc with it. When the arc reaches the widest gap between the electrodes, it breaks. At the same moment, a new arc discharge is ignited at the bottom of the electrodes, and the process is repeated.

In the photo “Jacob’s Ladder” on the right:

It is called "Jacob's Ladder" because of the ranks that resemble steps. This is a reference to the biblical story of the patriarch Jacob, who in a dream saw a ladder leading to heaven.

The "staircase" consists of:

Two arcs, which are fixed at the top to the Faraday cage through insulators.
Four neon transformers. They are not in oil, as for a Tesla coil, but are filled with solid epoxy resin at the factory.
Start duration control boards.

Planning

We thought we could use one of the empty spaces at the center for a fun project that would get kids interested in science and technology. But they didn’t know what exactly could be implemented. I wanted to make something useful, but at the same time spectacular, like an Ames room or an RGB room.

Scientists from the Landau Center have long dreamed of building a large Tesla coil. They said they had a mini reel and gave a little demonstration. We really liked it and decided to implement this idea.

At the beginning of 2016, we came to the Landau Center to once again inspect and measure the premises.

The room was not suitable for our project: the plastic suspended ceiling could easily catch fire, it was too light, the walls needed to be put in order - the facing layer should be removed and, possibly, a new one made.

To make it interesting for visitors, you need to give them the opportunity to run devices - but in such a way that there is no danger to their life and health. We consulted with architects and physicists and decided to divide the room into 2 zones: for visitors and for exhibits.

We decided that the visitor area would accommodate 25 people. For safety, we will separate the zones with a Faraday cage. Let's position it so that it falls into the external electric field emitted by the devices.

A Faraday cage works like this: free electrons move around it and are concentrated in a certain way. Due to this, the opposite sides of the cell receive different charges, which create a new field directed opposite to the external one. As a result, the charge is compensated, and there is no charge inside the cell - that is, the cell completely absorbs electromagnetic waves from the coil.

In addition to the cage, we also isolate the second zone from visitors with glass, which we’ll talk about a little later. It will have a Tesla coil, a Jacob's ladder, some fluorescent lights and a large fan.

Now we had the drawings and we began construction.

We decided to leave the parquet. We wanted the room to resemble a university auditorium and to deliver the project on time. The top layer of wood was removed along with the old varnish and covered with new.