Hence, to answer this question, I will first give you a true explanation of how the light makes the colors appear.
For the colors to appear, the light must experience on its way of propagation a resistance of a more or less transparent matter. The most impressive example is an opalite stone:

This is a real photograph of a longish opalite stone (it’s two centimeters long in reality) illuminated with white LED lamp from the right.
When you look at this stone in a half dark room, you see no colors at all. You see approximately this image:

The opalite is a kind of glass. With no additives in it, it is colorless and transparent just like the ordinary glass. But when additives in the form of fine powder are added, then colors appear in it. The last drawing shows how the powder is distributed in the stone from the photograph. On the right there are very little additives. As we go to the left, the concentration increases.

Let me jump for a moment to something else. When a body moves through space filled with air, then higher pressure is created in front of it, while lower pressure behind it. The higher pressure is Plus, the lower pressure is Minus. I use to call it the ‘principle of the arrow’ (− >—> +).

The greater the velocity of the body is, the stronger is the Plus in front of it as well as the Minus behind it.
This principle can also be found in the so-called Bernoulli’s principle (please see THE GREATEST HOAX IN THE HISTORY OF SCIENCE! (Part 2) ).

Moreover, the same principle is also present in the light when it encounters resistance of a more or less transparent matter on its way of propagation. The front of the light turns yellow-red which is the result of a higher light-pressure, while the rear of the light turns violet-cyan which is the result of a lower light-pressure.

Look at these beams obtained by means of a diffraction grating. The arrow in the image is added by me (source Wikimedia):

Regarding this image, I claim that these colors wouldn’t have appeared if there is no air surrounding the diffraction grating. In other words, the air is also an inevitable actor in this performance. If the grating is placed in an absolute vacuum, then no colors will emerge. The white light will remain white after passing through it. An absolute vacuum is hard to achieve. But it would be a sufficient proof if the colors get paler in conditions close to an absolute vacuum.

Look also at this drawing:

This is what is happening when white light passes through a chromatic convex lens. Before the focal point there is a friction between the light-front and the air, therefore higher yellow-red light-pressure here (marked with arrows without tails). After the focal point there is friction between the light-rear and the air, hence lower violet-cyan light-pressure (marked with arrows without heads).

That is the truth about the convex lens. The image you can see in the textbooks when the so-called chromatic aberration is discussed is a shameless lie of the contemporary physics.

Let me ask you something regarding the things I just said. How come the colors suddenly change their frequencies (colors are EM-waves with different frequencies, aren’t they???!!!) in a point far from the refraction surface?! Let me hear your explanation of this experiment!

Now, let’s get back to the Benham’s disk. First, let’s consider this kind of disk:

I call it double plane spiral, because there are two spirals that interwine. If we place it on a spinning top and then spin it clockwise, we see only circles which move outward like waves when we drop a stone in still water (figure a below). However, when we spin it counter-clockwise, then we see circles which move toward inside (figure b below).

If we look carefully at the disc during its spinning clockwise, we see reddish circles appear along the outer edges of the white circles as long as the disk is spinning fast, and as the spinning is slowing down, the circles turn into yellow (the experiment should be carried out near a window through which daylight comes in, but not directly from the sun). Along the inner edges of the white circles appear cyan circles as long as the spiral is turning fast, and as the speed decreases, the circles turn into blue-violet (in the figure (a) below is shown what is happening when the disk is turning slower).

But look, the same is happening when the disk is spinning counter-clockwise, only it is now reversed. The reddish and the yellow circles appear along the inner edges of the white circles, while the cyan and the blue-violet circles appear along the outer edges of the white circles (figure (b) below).

We see that here recurs the same principle as in the previous phenomena: the yellow and the red color appear at the front, while the blue-violet and the cyan at the rear side. For, when the circles are moving outwards, then the outer edges of the white circles are the front and the inner edges are the rear of the light.

We know that the light is reflected only from white surfaces. Thereby the angle of reflection is equal to the angle of incidence. However, this applies only when the surface is still. If it is moving, then the light gets an additional angle in the direction of motion, thus the angle of reflection becomes greater. So the light gets an additional friction with the air which is responsible for the reddish color at the front and the cyan color at the rear when the disk is spinning faster, but also for the yellow color at the front and the blue-violet at the rear when the disk is spinning slower.

Finally, this is the Benham’s disk in its usual form:

When the disk is spinning clockwise, then we see approximately the colors as shown in the figure (a) below. However, when the disk is spinning counter-clockwise, then we see the colors as shown in the figure (b).

Look please at this segment of the disk:

It is like a fork entering the darkness (i.e. the air without light in it). From the “tines” of this fork the light is reflected with an increased angle of reflection toward the air of the darkness when the disk is turning clockwise.

So, what is my assertion regarding this disk?
There will be no colors if there is no air around the disk. The emergence of colors follows here, as everywhere else, the principle of the arrow.

See also:
WHY IS THE SKY BLUE? HOW DOES LIGHT MAKE COLORS APPEAR?

P.S. YOU CANNOT SEE ANY COLORS either on a computer-generated rotation of a Benham's disk. Do you know why? Because the computer screen itself is the source of the light.

The light has to come from outside the disk and get reflected from it if one wants to see colors. So, you must have a real disk.

Why does the electricity flow in an open circuit? Because there is no such thing as moving particles. The electric current is just a vibration of the inner EM-forces of the matter. If you “dissolve” the matter completely, at the end you end up with nothing, more precisely, you end up with invisible immaterial EM-forces - just as the forces of an electric or a magnetic field are. Therefore, the matter is a kind of condensation of immaterial forces.

As Bill Gaede rightly made an analogy, you can think of the electric current as of a drill bit twirling in place. That twirling in the case of the electric current is vibrational.

Another good analogy is the kids string telephone:

Just as the sound can propagate vibrationally through the string in this open circuit, so the electricity can propagate through the wire in these open circuit experiments.

Instead of the sound generating the vibrations of the string, they can also be generated by rubbing the string in the middle with a violin bow. Exactly that we are actually doing in the case of electricity generation when we are moving a magnet close to a metal wire. The movement of the magnet incites the EM-forces in the wire which begin to vibrate.

The amplitude of the vibrations is the largest at the middle part of the string and it is smaller at its ends. If we prolong the string on both ends, then in the parts where recently the vibrations were weak, they are now stronger.

Let me tell you a simple electrical experiment. Take an ordinary screwdriver phase tester with a neon lamp. Connect one of its ends to the phase wire (220–240 V). Connect to the other end a piece of wire. The other end of the wire piece is floating in the air. The lamp shines. The longer the wire piece, the stronger the lamp shines.

By adding the wire piece, we have prolonged the string and thus we have made the vibrations in the lamp stronger.

Let’s get back to the title of this post. In every house there are long wires from the electricity meter to the electrical appliances. These wires are causing electricity consumption even when the appliances are not in use. So, if we insert three switches after the electricity meter, we may open these switches when we are many days not at home. (figure below)

These switches can be installed at many other places except at homes.

See also:

Electric current flows in an open circuit, too!

Please look at these two circuits:

The upper circuit has two NPN transistors (for NPN I have used the model BC547), while the lower circuit has two PNP transistors (for PNP the model BC557). They form the so-called Darlington pair.

The experiment can be also carried out with only one transistor per circuit, but with two transistors it is more effective. The number of transistors doesn’t change anything principally.

We now take a vinyl gramophone plate, a thin-walled glass, and a piece of woolen and silk fabric. We rub the vinyl plate with the woolen cloth and bring it close to the loose end of the wire of the lower circuit ( we will call it Minus-circuit). We will see that the LED will light up for a moment. It will also light up if we bring it close to the wire’s loose end of the upper circuit ( We will call it Plus-circuit). But if we play a little bit, we will notice that there is a fundamental difference between the two cases: the LED in the Minus-circuit lights up when we move the vinyl plate toward the wire, while the LED in the Plus-circuit lights up when we move the plate away from the wire. If we bring the loose ends of the wires close to each other and swing the plate, then the LEDs light up alternately.
Now, if we take the glass, rub it with the silk cloth, we will notice that the reverse happens: the LED in the Plus-circuit lights up when we move the glass toward the wire, while the LED in the Minus-circuit lights up when we move it away from the wire. If we don’t move the electrified objects, absolutely nothing happens, no matter how close they are to the wires. As mentioned before, this is quite feasible with only one transistor per circuit, yet the movements of the vinyl plate and the glass have to be much more energetic. But even in this experiment with two transistors per circuit we can notice that the faster we move the electrified objects, the stronger the lamps light up.

The cloths after the rubbing produce the reverse effect from the rubbed objects. Still, their effect dies out much faster than that of the vinyl plate and the glass.

From this it becomes clear that vinyl and glass act completely opposite: vinyl actuates the minus-transistor (PNP) by moving toward, while glass by moving away from the wire end; vinyl actuates the plus transistor (NPN) by moving away from the wire end, whereas glass by moving toward it. We see that there are four cases:

Let’s make the next experiment with these circuits. Look please at the diagram below:

( Here I have drawn the transistors with their electronic symbols. )

The circuits are the same as before, with only one difference. Both long wires are in this variant connected either to the emitters or to the collectors of the transistors as shown above, but not to the both at the same time. That’s why one of the two variants is marked with dashed lines. If we now repeat the already described procedure with the electrified glass and the electrified vinyl, then absolutely nothing happens, that is, the LEDs don’t light up at all.

Pay attention now, please. In the following circuits, a piece of wire (15–20cm or longer) is connected to each of the bases as shown below (the pieces are colored in red).

The diagram is taken from a textbook called “Elektronik 1” from the following authors: Helmut Röder, Heinz Ruckriegel, Willi Schleer, Dieter Schnell, Dietmar Schmid, Werner Zieß, Heinz Häberle. All of them are either PhDs or electrical engineers. The diagram refers to a synchronous motor (the word "FALSE" is added by me).

The true diagram should look like this:

What does a DC motor have to do with this? A DC motor works exactly on the same principle as the motor above.

How so?

Please look at the figure below:

There are three permanent magnets, two stationary and one placed on an axle so that it can rotate freely. When the middle magnet comes in line with the other two, then it stops moving. Now, let’s imagine that in that moment the polarity of the middle magnet by some magic becomes reversed. Then the magnet continues to rotate for another half cycle. Then the magic works again and so on. The magnet keeps rotating forever.

So, at which moments should the “polarity reversal magic” occur? At the moments when the middle magnet is exactly in line with the other magnets.

Now imagine that instead of the rotating magnet, a coil of wire is placed in the middle. The coil is connected to a DC source. Same as the permanent magnet, the magnetic field of the coil gets in line with the magnetic field of the two other magnets. If we find a way to reverse the polarity of the battery, then the current in the coil is also reversed, its magnetic field, too. Thus the coil will keep rotating.

How is this achieved? By a commutator. It looks like this:

Two metal half rings are connected via metal wires to a battery. The half rings are additionally connected to the terminals of the coil. The plane of the gap between the half rings must be the same as the plane of the magnetic field of the coil as shown in the screenshot below:

The screenshot is taken from this video (please watch it):

If the author of the video had glued the half metal rings otherwise (let’s say turned for 90 degrees), the motor would not have worked. Why? Because the polarity reversal would not happen at the right moment. What is the right moment? It is when the magnetic field of the coil is in line with the magnetic field of the permanent magnets.

Through the coil actually flows an alternating current, i.e., a square wave AC.

That’s why it is actually an AC motor, although it is fed by a DC source. And there is no essential difference between this AC motor and the one from the beginning of this post. There we had a sine wave AC, here we have a square wave AC.

P.S. I have written several times about this and also other misconceptions regarding the electric motors (such as “magnetic locking”, “rotating magnetic field” etc.). I will cite here only my last article:

All the theory about AC generators and motors is wrong!

 

 

 

Let’s say the grey rubber is more elastic than the black rubber. Therefore, you have to apply less force to bend the grey rod than to bend the black rod to the same extent (pictured on the right in the figure above).

Instead of bending them, let’s say you have to twist them to the same extent. What does it mean “to the same extent” in this case? If you twist them by applying force with both hands, then it means, for example, that you have to turn the rod with the left hand for 90 degrees and also with the right hand for 90 degrees. So, you will twist both rods to the same extent (i.e. 180 degrees). (note: you can turn only one hand for 180 degrees. The result is the same.)
You will do that also easier with the grey rod than with the black rod.

Now, imagine two rubber rods, both are made from the same type of rubber and both are equally long, but the one rod is thicker than the other. The thinner rubber rod is more elastic than the thicker rubber rod. You will have to apply less force for the thinner rod to twist it (or to bend it) to the same extent.

Yet another case: you have two rubber rods, both are made from the same type of rubber, both equally thick, but the one rod is longer than the other rod. The longer rubber rod is more elastic than the shorter. You will have to apply less force to twist the longer rod.

So, you see that the elasticity of a rubber rod depends on the material, on the length and on the thickness (i.e. the cross sectional area):

k - coefficient of elasticity
L - the length of the rod
A - cross sectional area of the rod

But look, we can always speak of the opposite (reciprocal) of a certain quantity. For example, the opposite of speed is slowness (1/v). The cheetah is the world champion in speed, but the snail is the world champion in slowness. Its slowness is 115 s/m.

Similarly, instead of elasticity of the rubber rod, we can speak of its reciprocal quantity. What quantity would that be? It would be resistivity. Instead of saying a rod is more elastic, we could also say it is less resistive and vice versa (less elastic corresponds to the more resistive).

You are probably asking yourself, what all this has to do with capacitance? Look, more than a hundred years ago Oliver Heaviside introduced the term “elastance” as the inverse of capacitance. He made an analogy of a capacitor as a spring, which was not a very good comparison. A true comparison is twisting and untwisting of a rubber rod. If you connect a capacitor to a battery, then the EM-forces of the dielectric get twisted. The process of twisting is actually an electric current through the dielectric in one direction. If you disconnect the capacitor and then connect it to a resistor, the process of untwisting in the dielectric begins (the energy stored in the twist is being released). It is actually an electric current in the opposite direction. The greater the resistance of the resistor is, the slower is the process of untwisting.

So, the dielectric of the capacitor is, in a sense, a rubber rod.
For the capacitance of a capacitor applies the equation:

For the elasticity/elastance of a capacitor would apply the reciprocal equation:

If you compare it to the first equation about the elasticity of the rubber rod, you will notice that they are the same. The length of the rubber rod corresponds to the “d” of the dielectric.

Instead of capacitance of a capacitor, we can speak of resistance of a capacitor. So, instead of capacitance/elastance, we can speak of resistance/elasticity. Why would we do that? Because we can apply the same concept to an inductor.

Just as there is twisting and untwisting of the EM-forces in the dielectric of a capacitor, there is also twisting and untwisting of the EM-forces in the (ferromagnetic) core of an inductor. The difference between the two is in that, that in the first case the dominance is on the electric forces, while in the second case the dominance is on the magnetic forces.

The inductance of a ferromagnetic core is:

The number μ (mu) is called magnetic permeability. It corresponds to the resistivity of the rubber as material. The number A is the cross sectional area of the core, while l is the length of the core.

So, the elasticity of the ferromagnetic core will be:

Oliver Heaviside didn’t coin an opposite term for the inductance as he did it for the capacitance. If he did it, then it should have been the same as for the capacitance.

I can also speak about resistance vs elasticity regarding the flow of electric current through the metals, but I will do it in another post. Here I will only say that silver and copper are electrically the most elastic metals, that is, they conduct the electric current the best.

As you can see, resistance and elasticity are universal terms regarding electromagnetism. They can be applied to conductors, dielectrics (capacitors) and ferromagnetics (inductors).

See also: What is magnetic hysteresis? 

What is electric current? 

The principle of electromagnetic induction can be found in mechanics and in an optical illusion!

The greater the velocity of the body is, the stronger is the plus in front of it as well as the minus behind it.
When a propeller is turning underwater, then it creates a swirling water motion, but at the same time it creates a spiral cavitation motion (video below).

https://youtu.be/Y7k7p1RirkI

The water motion is Plus, while the cavitation motion is Minus.
The electric current is also a swirling motion through the conductive path. This motion also creates a negative spiral motion through it. It is the magnetic current.

When a direct current is flowing through a metal wire, then both currents are flowing from the Plus to the Minus-terminal of the battery: the electric current counter-clockwise, the magnetic current clockwise. They are at angle of 90 degress. It is the same with the water-current and the cavitation-current (figure below).

When we make a loop from the wire, then the spiral magnetic current forms a straight magnetic field. The field is straight, but twisted because of the spirality of the magnetic current. The situation in the loop is similar to this figure:

This is actually an image of a toroidal coil, but I have used it to represent a loop of DC-carrying wire. The wire of the toroidal coil becomes a representation of the magnetic current in the loop (its direction is marked with the red arrow, while the electricity flow is marked with the blue arrow).

When we place a ferromagnetic core in the loop, then the magnetic current is twisting the magnetic forces in the core. Thus the magnetic field gets many times strengthened.
The magnetic field of an empty loop or of an empty solenoid is denoted with H. The strengthened field when a core is added in the solenoid is denoted with B. The magnitude of B is expressed with B=μH. The number μ (mu) is called magnetic permeability.

What is magnetic permeability?
I will answer this question through a comparison because it is the best doing so.
Please look at this image:

This is a solenoid with an iron rod in it connected to a battery. We will compare the iron rod to a rubber rod.
Imagine you have two equal rubber rods, but made of different types of rubber. The different types of rubber have different elasticity. Now imagine that you have to twist both rods to an equal extent. You will do that easier with the rod from the more elastic rubber. It offers a lower resistance, but also less energy will be stored in this rubber. What energy? If you leave hold of the rod, it will untwist by itself releasing the stored twisting.
We can say that the more elastic rubber has a higher elasticity. But we can also say that it has a lower resistivity. The resistivity is the opposite of elasticity, R=1/E. 

The permeability of a ferromagnetic material corresponds to the resistivity of the rubber. The greater the permeability (i.e. the resistivity) of a ferromagnetic material, the more magnetic energy is stored in the core, that is, the magnetic density is greater.

When we connect the solenoid with the core to a battery, then the current doesn’t establish itself in the circuit momentarily, but it needs some time. Not much time, but somewhat more in comparison to the case when there is not a core in the solenoid. Why? The establishing of the electric current in the wire is closely connected with the establishing of the magnetic current in it. If one of them experiences resistance, then it passes over to the other. The magnetic forces in the wire experience resistance during the process of twisting of the magnetic forces in the core. When they are twisted to the extent, which the magnitude of the magnetic current can twist them, then the state becomes stable (as if there is no core in the solenoid).

The whole process of establishing the current in the circuit can be compared (in a sense) to a DC circuit which has a solenoid without a core and a series resistor whose resistance after closing the switch quickly drops to zero.

Look please now at the circuits below:

After closing the switch in the figure (a), the magnetic forces in the core of the solenoid get twisted (the resistor R is added solely to limit the current and thus to save the battery from a quick draining). When we try to move the switch to the position (2), the circuit becomes broken for a moment. There is nothing to hold the magnetic forces in the core twisted anymore, thus they begin abruptly to untwist. It induces a strong current in the wire of the solenoid which is manifested as a great voltage at its ends. This is called “self-induction” and the voltage is called “back EMF”. Since the back EMF can be many times greater than the voltage of the battery, the approaching of the switch to the position (2) produces a spark. This phenomenon is used, among other uses, in the ignition systems of the automobiles to ignite the fuel.

We have already said that the magnitude of B can be expressed by B=μH. However, this is only an approximation because the function is not linear, but exponential (figure below).

When the magnetic field H of the solenoid gets stronger due to increasing electric current through it, the magnetic field B also gets stronger. It asymptotically reaches a maximum. The further increasing of the electric current in the solenoid doesn’t result in strengthening of B. The magnetic field B enters so-called saturation.

Let’s compare it to the rubber rod. When we twist the rubber rod, then we soon arrive at a point when the further increasing of the applied force doesn’t result in further twisting of the rod.
The formula B=μH is actually an approximation of the exponential curve in the interval before it reaches the maximum. In that interval the curve is nearly linear.
Now, please look at the so-called hysteresis loop:

From the point A to the point B we have the same exponential curve from the last graph, only it is shifted a little to the right. The electric current in the solenoid is slowly increasing and in the point B the magnetic field B reaches the maximum. Let’s say now that the current in the solenoid begins slowly to decrease. The field H also decreases and in the point C it drops to zero. But look, although it is zero, the field B is not zero. The core retains some magnetism. Using the rubber rod analogy, we can say that the twisted rubber rod, after releasing it from our hands, does not untwist fully. It is not so elastic to untwist fully, but it retains a certain amount of twist.

From the point C onward, the electric current in the solenoid is reversed and slowly increases. Thus its magnetic field is also reversed. In the point D the field B drops to zero. The reversed magnetic field H between the point C and D has actually done the work to fully untwist the residual twist of the core which was present in the point C. From the point D onward begins the twisting of the core with reversed polarity.
Similarly, when the rubber rod does not fully untwist by itself, then we have to apply some force to untwist it.

P.S. The twisting and untwisting of the magnetic forces in the core is accompanied with twisting and untwisting of electric forces. It is actually an electric current. This current is many times stronger if the magnetic core forms a closed loop (figure below).

When the solenoid is connected to a source of alternating current, then there is such a current in the core, too. Please watch this video, which is part of my article Electricity flows in an open circuit, too!

https://youtu.be/jmzXILw9Vk0

In relation to this article, please see also:

What is an electric current?

The principle of electromagnetic induction can be found in mechanics and in an optical illusion!

You can call me all kinds of names, but it won’t change the truth. And the truth is on my side.

How did the greatest nonsense about the explanation of the working principle of electric generators and motors actually come about? I was thinking quite a lot about it and this is what I have come up with.
In all generators and motors there is one thing which is rotating and another thing which is stationary. An invisible interaction between these two things is thereby going on (figure below).

Now, one intuitively concludes that the force of interaction between the two things is the strongest at the moment when they are exactly opposite to each other as shown in the figure (b) above.

And look now: one considers this intuitive conclusion confirmed by something which is called “Faraday’s law of induction”. Please look at the most exploited figure when the Faraday’s law is discussed:

There are also two things here, one rotating and one stationary. The rotating thing is a wire loop, while the stationary thing is a magnet. According to the Faraday’s law (footnote 1), the magnitude of the induced current is the greatest when the loop is in the vertical position, that is, when the two sides of the loop where the current is actually induced (marked with small “L”) are nearest the magnet. Why? Because the rate of change of the so-called magnetic flux through the loop is the greatest when the loop is in the vertical position.

(footnote 1) Faraday’s law says that E=dΦ/dt; the minus sign before “dΦ/dt” I have left out because it is absolutely not important when one discusses the magnitude of the induced current.

 

But neither in the former nor in the latter picture is the induced current maximal at those moments. Quite the contrary! In both cases at those moments the electric current is zero and thus the magnetic interaction also.

Do you want a proof that the Faraday’s law is wrong and therefore its place is in the garbage of the history? I will give you the simplest proof you can imagine.
Look please at the figures below:

A straight conductor is moving vertically toward the exact middle of a magnet (figure a). No current is induced in this conductor.
In the second variant the conductor is shifted a little to the right and is moving again vertically towards the magnet. A current is induced in it which flows away from us (figure b).
In the third variant the conductor is shifted a little to the left and is moving vertically towards the magnet again. A current is induced in it which flows towards us (figure c).

In the figures below are shown three variants of a wire moving through a magnetic field. No matter how the wire is moving through the field, whenever it crosses the middle line the induced current falls to zero.

So, when the rotating loop is in the vertical position, the two sides where the current is induced cross the middle line. Therefore, at that moment the current drops to zero. The Faraday’s law predicts maximum current at that moment. NONSENSE!

As a consequence of this delusion there are also others: the so-called “rotating magnetic field” and “magnetic locking”.
The latter term is such a great nonsense that it is actually a pure stupidity. I don’t write this to offend anybody, but to incite you to begin to think. ( Why it is a stupidity I have explained here and here).

The first term is not a nonsense per se, but it is also very misleading. Please watch this video about an induction motor:

https://youtu.be/ViapciJ7G3k

Is this an AC induction motor? - Yes, it is.
Is it turning? - Yes, it is turning.
Is there a rotating magnetic field that acts on the aluminum disk? - No, there is not.

Watch now please how this man explains the rotating magnetic field:

https://youtu.be/SiZ-mak4h4s

He takes two-phase AC and then applies it to two double coils at 90 degrees distance.
I have never seen such a “wonderful” instance of cherry-picking.

I would ask him how he would apply the concept presented in the video to these three coils:

P.S. If you want really true explanations about the principle of generators and motors, see these articles:

What is the principle of 3 phase induction motor?

Some widespread misconceptions about the electric motors!

Is the Wikipedia’s explanation of the working principle of an induction motor actually a nonsense?

Is the explanation of the working principle of synchronous generators and motors true?

Is Faraday's law of induction true?

What is Faraday's law of electromagnetic induction?

Will touching a neutral wire render an electric shock? Why?

How does a synchronous motor sense that the mechanical load is increasing?

This is one of the most known optical illusions. When we stand on railway tracks, we have the impression that the tracks in the distance get closer and touch each other.

If we stand on a hill and look down at the town under us, we see the nearby houses bigger than the houses in the distance. Is this an illusion?! How would we otherwise have a real sense, what near to us and what farther away is?! Thanks to this "illusion", we can truly perceive the reality.

The "illusion" with the railway tracks has the same basis as the “illusion” with the houses. And many optical illusions also come down to this: we see the distant objects smaller than the nearby ones.

Please look now at this image:

This is an image of a toroidal coil, but I made use of it to represent the magnetic current of a DC-carrying wire loop.
This image can serve two purposes:
1) as an optical “illusion” and,
2) as a real representation of what is going on inside the loop when we move a magnet in and out of it.

What is the optical “illusion” hidden in this coil? If we look at the helical line of the coil, we see these line segments “ \ “ above as well as below. There is no difference in the tilt of these segments above and below. Both are tilted to the left.
Look now at this Pinna-Brelstaff “illusion”:

(footnote: You can find more effective circles of the Pinna-Brelstaff illusion on the web.)

The inner circle has the same line segments as the toroidal coil.

Look now at, so to speak, the inner segments of the toroidal coil (figure below).

Unlike the previous ones, these segments have different directions. Those above point to the left, while those below to the right.
These segments can serve to represent the induced, or rather, the incited magnetic forces, which create a magnetic current in the loop when we move a magnet in and out of it.

When we move the magnet into the loop with its Plus-pole ahead (I call Plus the pole of the compass needle that points North), then the incited magnetic forces in it point with their plus-poles toward us, that is, they resist the movement of the magnet. In the upper part of the loop they point, as it was already said, toward our left side (figure below).

Since the magnetic current has the same direction as the electric current (see What is an electric current?), we can conclude that the induced, or rather, the incited electric current in the loop is flowing counter-clockwise.

When we move the magnet into the loop with its Minus-pole ahead, then the incited magnetic forces in it point with their minus-poles toward us, that is, they resist the movement of the magnet again. In the upper part of the loop they point toward our left side (figure below).

Thus, the incited electric current in the loop is flowing clockwise.

In this way you can easily determine the direction of the induced (incited) current in a wire by a moving magnet. You don’t need such rules as right hand, middle finger, fore finger, thumb etc., etc.

As we move the magnet toward and into the loop, the magnitude of the current changes. Thereby the form of the magnetic helix also changes. When the current is weak, then the magnetic helix is pretty extended. As the current is increasing, so the helix is becoming more and more compact (figure below).

( I am sorry I didn’t draw real helices. )

Since the magnetic helix is more compact when the current is stronger, it follows that the loop’s magnetic field is also denser.

Hence, the magnetic field of a loop is twisted and the stronger the current through it is, the greater also its density is.

Considering that we can make a permanent magnet with the help of an electromagnet, it follows that the magnetic field of a permanent magnet is also twisted. If we have two equal magnets in dimensions, but the first stronger than the second, it means that the magnetic density of the first is higher.

Please watch now the kid’s toy in this video:

https://youtu.be/Hxfa2NhrYGQ?t=16

Two essential parts of this toy are the twisted wires and the washer. When we push the twisted wire through the washer or vice versa, the latter is turning. In this way a linear motion is converted in a circular motion.
The same working principle is applied in some ashtrays with spinning lids, other kids’ toys (carousels, tops), push-pull screwdrivers and drills etc. 
And exactly the same principle is present also when we push a magnet into and pull it out of a loop. But since that what is going on during this process is partly immaterial, it is much more diverse than that what is going on with pure material objects.

The turning in this manual drill is happening due to these relief lines in the twisted object :

Such relief can be achieved either by drilling a twist in a rod, or by twisting two or more wires as the man did in the video.

So, the washer from the video can turn in two ways:
1) by a linear motion of the twisted wires through the washer;
2) by a linear motion of the washer around the twisted wire.

The same phenomena we encounter also with the magnet and the loop.

But look, we cannot make the washer turn if we only turn the twisted wire. Why? Because in this case there is no linear component of the motion of the relief lines . I mention it because of the Faraday’s paradox. Please see: What is the Faraday paradox?

There is one more way to incite a magnetic current and consequently also an electric current in the loop. How? By a sudden twisting or untwisting of the magnetic field. This happens in the cases of an inductor and a transformer . The linear component is present during the twisting and untwisting.

I believe that we can make the washer also turn if we find a way to abruptly untwist the twisted wires.

If we move two equal, but differently strong magnets with the same speed into and out of a loop, then we certainly get a stronger current with the stronger magnet. As I have already said, the stronger magnet has a denser magnetic field. Look please at the figure below:

The figure (a) represents a lower, while the figure (b) a higher magnetic density. The relief lines which I have already drawn

are denser when the magnetic field is stronger. Hence, when we are moving the magnet toward the loop, those “relief” lines of the stronger magnetic field are hitting (inciting) the magnetic forces in the wire with higher frequency and thus the incited magnetic current is stronger.
As I have already said, the phenomena of electromagnetism have a greater diversity than those with pure material objects. Here is a case which is impossible with material objects. We can move the magnet toward a metal loop which has a smaller circumference than that of the magnet and a current is incited in the loop. In this case only a part of the magnetic field passes through the loop. To understand this, we have to apply the principle of self-similarity. It is a very important principle and this mankind should devote more time to its investigation .

Please read now the first passage of the Albert Einstein’s “On the Electrodynamics of Moving Bodies”:

“It is known that the application of Maxwell's electrodynamics, as ordinarily conceived at the present time, to moving bodies, leads to asymmetries which don't seem to be connected with the phenomena. Let us, for example, think of the mutual action between a magnet and a conductor. The observed phenomenon in this case depends only on the relative motion of the conductor and the magnet, while according to the usual conception, a strict distinction must be made between the cases where the one or the other of the bodies is in motion. If, for example, the magnet moves and the conductor is at rest, then an electric field of certain energy-value is produced in the neighborhood of the magnet, which excites a current in those parts of the field where a conductor exists. But if the magnet be at rest and the conductor be set in motion, no electric field is produced in the neighborhood of the magnet, but an electromotive force is produced in the conductor which corresponds to no energy per se; however, this causes – equality of the relative motion in both considered cases is assumed – an electric current of the same magnitude and the same course, as the electric force in the first case.”

What does this guy say in the first sentence? He says that the experiments contradict the Maxwell’s theory. According to that theory, if a magnet is moving in the void space, then besides the magnetic field, there is also an electric field adhered to it. If a metal wire is placed on its way, then this electric field (which is also present without the wire), appears also in the wire. According to this theory, when the wire is moving and the magnet is stationary, no electric current should appear in the wire.

And this guy, without any understanding of electromagnetism, takes these experiments as a basis for his theory of relativity, that is, he asserts that only the relative motion is what counts. But a simple experiment, called Faraday paradox, can disprove his confusing theory which, except for the confusion, hasn’t brought anything to this mankind. The same applies for the other guy’s theory.

I have begun this post with an illusion to tell you that there are not absolute illusions. In every illusion there is reality. The same principle in the illusion from the beginning of this article recurs also in a mechanical and an electromagnetic sense. The latter are surely not illusions.

I am sure there are many questions which are not explained in this post. For more, please see:

What is electromagnetic induction?

Is Faraday's law of induction true?

Are electromagnetic waves transverse or longitudinal?