If you look up at a high voltage power line you will see that the cables are suspended from cool rods with fins along their lengths.  These weight supporting insulators are standoffs to prevent the electricity from arcing/short circuiting to the tower or pole.  You can determine the voltage on the transmission line by the length and number of these fins on the insulators, the higher the voltage the longer the insulator. 1 
 
The shape of the ridges increase the surface path length of the insulator between the wire and the structure holding it.  Electricity has a tendency to flow along the surface rather than through the air due to the accumulation of normal airborne contamination which causes a lower resistance than air.  They increase the spacing over the surface of an insulator and create a longer path for electricity to jump without increasing its physical length.  They also help with preventing the rain from falling in a continuous path along its surface and casing a short circuit.  The flashover voltage can be more than 50% lower when the insulator is wet. 2
 
Fun fin fact: 
The fins are actually called “sheds”, spacing through the air is called "clearance" and spacing over the surface of an insulator is called "creepage."3  A design with sheds greatly increases the creepage distance.

Generating energy from the wind to do work is nothing new.  For example, 1000 year old windmills in Iran are still in use today 1. These technologies have evolved from simple machines used to pump water and crush grain to the standard 3 blade design used to generate electricity today.   Electricity is made when wind causes each rotor blade to add a force which turns a shaft that rotates a magnet past a coil of wires. 
 
Are more blades better?  More rotor blades will indeed increase the rotational force of the rotor and produce more power.  Consider though that rotor blades are one of the most expensive parts of a wind turbine, making up 20% of its material cost 2.  A 2.5MW wind turbine can cost 2 million dollars.  Adding another heavy blade would also require costly structural upgrades. A rotor for a typical wind turbine (model SWT2.3-108) weighs 60 tons and each blade is 53 meters long.  Further, more blades do not add more power in equal proportion.  Two blades are 10% more efficient than a single blade and three blades are only 5% more efficient than two blades.  Four blades will add an even smaller marginal gain in efficient but at a huge cost.  The benefit to cost analysis will show that the extra weight, money and diminishing performance is not economically worth it 3.  PS: The reasons why we use a 3 phase electrical system vs more or less phases is for similar economic considerations and law of diminishing returns.
 
Are less blades better?  Two bladed designs can match the performance of a 3 bladed design in two ways: increase the blade width by 50% or increase the rotation speed by 22%.  Increasing the width also increases its costs and weight which defeats the purpose of only having two blades in the first place!   Increasing its speed creates more noise and centrifugal force which also adds apparent weight.  Some wind turbine rotors spin on average 15rpm.  The tip speed can reach 190mph 4.  This video shows what happens to the blades if they spin faster than they can structurally support.

Thomas Edison and his DC (Direct Current) powered grid completely lost out to Nikolas Teslas AC (Alternating Current) power… or did it.   Tesla’s AC power won hands down because it is safer and cheaper to deliver.  But…. new solid-state technology, not available to Edison, is making DC a better choice in cases of long distance or underwater high voltage transmission.  Grid planners are taking another look at HVDC (High Voltage Direct Current) lines as we push for greater energy efficiency, join energy markets like EIM, include more renewables and have advances in technology.  Is Edison getting his revenge? *evil laugh*
 
Breaking it down
The cost of transmission in either system depends on 4 main factors.  Who is the winner, AC or DC?
1) Cost of transformers:                    Winner – AC

• AC’s main advantage is its ability to change its voltage with the use of cheap iron core transformers. 
• DC uses expensive rectifiers to change its voltage.  Modern systems use solid-state semiconductor technologies 1 that Edison didn’t have available. 

2) Cost of cable and towers:             Winner – DC

• DC lines can carry at least twice as much power as an AC line of the same voltage and require less insulation or clearance.
• DC towers are smaller - AC transmissions towers are 1.5 larger
• DC only requires 2 cables compared to an equivalent double AC 3-phase circuit with 6 conductors needed.

3) Cost of land:                                    Winner – DC

• The right of way for an AC line is 70% wider than that of a DC line with equivalent capacity 2

4) Cost of line losses:                          Winner – DC

• DC cables can be thicker and thus more efficient 3 since they experience no skin effect 4
• DC lines generally have a loss rate of around 3%, as compared to the 6% rate for AC lines 5

Summary
There is a breakeven point of distance where DC is the better choice to use over AC.

Iceland is the country that uses the most electricity per capita in the world, 2.5 times that of the U.S.   One of the benefits of being located in one of the most tectonically active places in the world is that they have an abundant supply of geothermal sources.  Geothermal power facilities currently generate 25% of the country's total electricity production 2 and the rest of the country’s energy comes from Hydro power.  Only .1% of electricity is generated from fossil fuels.  This is the world’s highest share of renewable energy in any national total energy budget.

China is the largest producer of electricity in the world producing 31% more electricity than the U.S.  It is no surprise then that it is also the largest emitter of greenhouse gasses and 73% of  China’s electricity is generated from coal.   Factoring in its population, however, its annual CO2 emissions ranks only 43rd in the world per capita 6 and has half of the emissions per person than in the U.S. 7  In 2015 they invested $100 billion in clean energy which was more than double the U.S. investment.  Five of the six largest solar module manufacturing firms globally are Chinese.
 
France has chosen Nuclear energy as its primary source of electricity generation.  75% of its electricity comes from nuclear plants and 15% from Hydro.  France has nearly the lowest cost of electricity in Europe and one of the lowest levels of CO2 emissions.  However the future of Frances electrical energy mix may not be nuclear as many have been shut down with more to come due to low public opinion  and inferior maintenance.   In 2015 France passed a mandatory renewable energy target requiring 40% electricity production to come from renewable sources which would double its current renewable energy mix.
 
Google searches account for about 0.013% of the worlds’ energy usage which equals enough electricity to power 200,000 homes continuously.  The energy it takes to conduct 100 searches on Google is the equivalent of a running a standard 60W light bulb for 17 seconds and translates to roughly .2g of carbon CO2 emissions.    more Google energy facts

A volt (V) is just the Standard International unit of measurement for electric potential energy.  It is the force that opposite charges have for doing work if they are pulled apart and then released to fly at each other.  In other words if I pull a charge one direction and the opposite charge the other direction how bad do they want to get back together (like magnets or old relationships)?  This unit of measurement was named in honor of an Italian guy that invented the first battery in 1799, Count Alessandro Guiseppe Antonio Anastasio Volta.  Aren’t you glad they only used his last name?1
 
Digging deeper
Pressure, gravity and elasticity are other examples of potential energy each with their own unit of measurement.  Voltage is a lot like pressure. For example, a unit of measurement of pressure potential is PSI (pounds per square inch).  I put 90 PSI of potential energy into my road bike tires.  If I were to release that 90 PSI of potential energy through the valve stem I could do work like spin a fan, blow up a balloon, or transfer the energy and partially blow up another tire.   Likewise I have 9 volts of potential energy in my 9 volt battery.  If I were to release that 9 volts of stored potential energy I could do work like spin a motor, shock my friend, or transfer the energy and partially charge another battery.   So a volt is just a way to measure potential energy. 
 
So how did they come up with that random unit of measurement?  Way back when one volt was set equal to the potential across a resistance of 1 ohm when a current of one ampere flows through that resistance.  (V=IR)  Or one could also say that it is the potential difference across a wire when a current of one ampere dissipates one watt of power.  There are lots of ways to d efine it all meaning the same thing. 2
 
Volt list
Nerve cell                                                       .070 volts
Batteries (AA, AAA, C, D)                             1.5 volts
Computer USB port                                      5 volts
9 volt battery                                                 9 volts
Car battery                                                     12 volts
Home outlet in North America                   120 volts
Home outlet everywhere else                    230 volts
Business and Industry                                  480 volts
Distribution Lines                                           12,470 volts
Sub-Transmission Lines                                 115,000 and some 57,000 volts
Transmission Lines                                         500,000 and 230,000 volts
Lightning                                                         100,000,000,000 volts

Are you asking how fast the actual electron or the electric field is moving? 
 
Speed of electron
The average speed an electron is drifting 1 down a conductor is agonizingly slow, less than an inch per hour. 
 
Speed of electric field
The speed that the electric force field moves down the wire is close to the speed of light. 
​
The picture above of the tube and marbles shows why this force moves fast but the electron does not.  If you put a marble (electron) in a tube (wire) the effect of one marble knocking onto the next happens at the speed of light.  This will happen until you stop inserting marbles or there is nowhere to go.  Its energy is transferred to the end quickly even if it takes a long time for the actual marble to make its way to the end.  This same property is what allows for landline telephone’s and other signals to transfer information almost instantly.  This property is also why customers don’t have to wait years for their electrons to be delivered. 
​
The story of Bud the electron who could
Bud the electron lives inside of a of piece of aluminum wire with all of his closest electron friends.  All day long, even when there is no electricity flowing, Bud and his friends are actively moving around in different random directions depending on their mood.  One sunny morning Bud was awoken to find all of his friends slowly making their way in more or less the same direction.  Some of his friends weren’t going with the flow but darting up and down and back and forth but everyone was basically drifting in one direction. Little did they know they were being acted on by a generators electromagnetic field and far away someone had just turned on a switch.  Bud traveled about .00001 inches which was a long way to travel for a little electron in a big world.  Just when Bud was getting used to traveling in that direction he noticed that everyone was starting to go the opposite direction.  Wanting to fit in with his cool friends he went along with the crowed and traveled another .00001 inches back to where he started from.  1/60th of a second into the morning and he had already traveled .00002 inches! 2  Just when Bud though his little legs couldn’t take it anymore his friends behind him started pushing, crowding and knocking him to do it all over again… and again.  When Bud decided to follow his friends that morning and start walking up the conductor he became part of a long chain reaction like a relay race.  The influence of the chain reaction traveled close to the speed of light towards the switch that set it off.  Unfortunately neither Bud the electron nor any of his friends actually ever even made it very far in the end. 
 
Moral of the story:
1.Electron peer pressure travels at the speed of light.
2.Little did Bud know that he was just playing one small part in a much larger and quicker plan.
3.DC (direct current) electrons continuously move down a wire.  However, electrons in our AC (alternating current) system only travel down a conductor a thousandths of an inch before they are pulled back to where they started 60 times every second.  AC electrons don’t see very much of the world.
4.Just like DC power their energy (effect on each other) still moves close to the speed of light. 

A typical lightning strike contains 5 billion joules or 1400 kilowatt-hours of energy1.  This is more than enough energy to power a house for a month or send a time traveling DeLorean back to 1985.  Unfortunately there are a number of big problems with practically putting this energy on the grid the greatest of which is the insignificant amount of energy actually available compared to the large cost to capture it.
 
To capture all of this lightning we would need to construct Eiffel Tower size structures every square mile covering the whole 200 million square mile earth, even floating one’s for the oceans.  Even if you decided to focus on key lightning areas the amount of towers would be mind boggling.  Each tower would need a transformer, super conductor and other expensive equipment.  They would need to be connected together with high voltage transmission lines and an army to maintain the system.  Energy storage technology and super capacitors would need to be invented which could handle the 30 millisecond surge of 5 billion joules.  An entire system would be necessary to convert it to AC power, equalize current and voltage and maintain power quality.  Sometimes lightning is negatively charged and sometimes it is positively charged.  A system would have to be developed to make sure that strikes don’t cancel themselves out2.  Efficiency would also be lost in all of the conversions, transformations, storage and transportations of each strike.   Much of the power in a strike is lost before it even hits the ground when it is converted into heat and light.   The energy instantly heats the surrounding air to 50,000°F, 5 times greater than the surface of the sun, and produces a shock wave known as thunder.  The estimated 350 million lightning ground strikes around the world per year account for 490,000,000,000kWh’s of energy.  But in 2015 the world used 40 times this amount: 20,000,000,000,000 kWh’s of energy3.  Therefor all of the hypothetical harnessable land strikes, if every strike on earth is harnessed with 100% efficiency, would only power the world for 9 days.
 
On the bright side there are much easier ways of harnessing natural energy.   For example, the amount of energy we get from one hour of the sun hitting the earth can cover all of our power needs on earth for a year.  Working out the solar energy challenges seems to be a more efficient use of time.
​

Answer
The hum is being made by Corona Discharge.  This basically happens when the electrons in the wire want to go to the air around it because it has such a high electromagnetic field.  What’s going on is that the electric field gets so strong in the air around a wire that the electrons start to get pulled off their atoms and into the air.  Usually air is an insulator around the wire but in this case it ionizes and becomes conductive plasma!  So the noise happens when the voltage is too high for the air around it to handle before it breaks down and becomes a conductor.

Facts:

• It is loudest at sharp points, corners, bolts, or any surface irregularities. 
• It can be affected by the condition of the temperature or air such as humidity, air density, wind, fog, rain, snow because they all change the electrical resistance of the air around the wire.
• Dry air changes its mind about being an insulator and becomes a conductor at about 10 thousand volts per inch.
• It’s not just loud but it actually wastes power, like a leaky pipe.
• The voltage on distribution lines is low enough that this doesn’t happen, only transmission lines.
• It happens all the time in nature.  You can see it during thunderstorms as a faint glow around sharp rocks, trees or bushes.
• Sometimes you can even see it at night or smell the ozone it creates.

A high pitch alien whine sound is caused by something else called Aeolian vibration.

Geek summary:  In other words the audible noise emitted from high-voltage lines is caused by the discharge of energy that occurs when the electrical field strength on the conductor surface is greater than the 'breakdown strength' (the field intensity necessary to start a flow of electric current) of the air surrounding the conductor.
​

Power (KW) is the instantaneous usage and Energy (kWh) is power that is used over time.  Toasters and things are rated in watts which is the power that they draws at any given moment.  A customers power consumption can change quickly as loads are turned on and off but energy accumulates gradually.  We bill customers based on the energy they use over time in these energy buckets called kilowatt hours (kWh). 
 
Power = Watts
Energy = Kilowatt hours (kWh)
* For you Americans remember that K = 1000 so 1 KW = 1000 W
 
Water Analogy
Power = Gallons per minute, flow rate.
Energy = Total gallons consumed.
If you stop using water the flow rate (power) goes to zero but your total energy stays the same, it just doesn’t get any higher.  The total gallons consumed (energy) is what we bill for.
 
Speedometer Odometer Analogy
Power = Speedometer (current speed)
Energy = Odometer (how far you’ve gone)
The speedometer (power) can quickly go from zero to sixty and back to zero, but your odometer (energy) will only slowly count up.  The odometer counts faster as your speed is faster over time.  The speedometer tells you how fast you are going right now (how much power you are using right now), while the odometer tells you how far you’ve gone (how much energy you’ve used in total).   So speed is to power (kw) as distance is to energy (kWh).
 
Graph Example
The Blue line is power (speedometer) and the Red line is energy (odometer).  Notice as the power (blue) increases the energy (red) line goes steeper.  If the power (blue) was zero the energy (red) line would be flat (no increase).
 
Grid Scale Battery Example

1. How much total energy can the system store? (Think watt-hours)  1250 kWh  / 1,250,000 Watt Hours

How much power can it deliver at any moment? (Think watts)  5 MW / 5,000,000 Watts

Only 2% of the energy used in an incandescent bulb comes out as visible light and the other 98% is converted to heat. 1
 
Digging deeper:
Incandescent light bulbs work by sending a lot of electricity through a thin wire until friction causes it to heat up so much that it glows and gives off light.  The definition of incandescence is “the emission of light  as a consequence of raising its temperature”. 2  When electricity passes thru a tungsten wire filament in a light bulb it is heats up to about 1500°F.  At this temperature most of the electric energy (about 98%) is released as infrared radiation which we can feel as heat, the rest of energy, only 2%, is released as visible light.
Here is a close-up of the Tungsten wire filament:

The industrial revolution saw a drastic increase in new inventions to do work using new methods involving electricity, steam and combustion. 1  This just confused all of us and now the terms are often used interchangeable in casual discourse and even in our own industry.  All three are machines that convert energy from one form to another but here are some simple delineations so that you can use the terms correctly. 
 
GENERATOR = Converts mechanical energy into electrical energy.  This is the opposite of a motor.  

• Examples: Power plants, backup generators, car alternator, regenerative braking, tachometers, crank flashlights, etc.
• The word generator comes from the Latin “generator” meaning begetter or producer. 2

MOTOR = Converts electrical energy into mechanical energy.  This is the opposite of a generator.  

• Examples: Electric car motor, ceiling fan, electric pumps, blender, drills, washing machine, AC compressor and blower fan, etc. 
• The word motor comes from the Latin “moto” meaning to set in motion. 3

ENGINE = Converts potential energy into mechanical energy.  This potential energy could be supplied by the combustion of fuel, another chemical reaction, or potential energy stored in water, air or steam. 

• Example: Reciprocating engines (Backup generators, cars), steam engines, gas turbines, airplanes, rockets, muscles, etc.

The word engine comes from the Latin “ingenium” meaning innate talent. 4
​

August 14, 2003.  At the time it was the WORLDS second largest blackout in history.  50 million people in southeast Canada and eight Northeastern U.S. states were without power for two days to a week due to an untrimmed tree, human error and a software bug.  265 power plants shut down leaving 11 people dead and $6 billion in damages.  Incidents like this reminds us all of the social responsibility that we have to play our role in reliably delivering this essential energy source.
 
Here is the order of events as it occurred: 1 

1. Tree trimming was not done sufficiently in stretches of various transmission line easements
2. 12:15PM:  The alarm system at FirstEnergy’s control room failed due to a software bug 2
3. The system operators in Ohio were not aware of an overloaded transmission line and the need to re-distribute power
4. 2:02 PM:  An overloaded transmission line sagged and made contact with trees
5. As current found other paths to their loads more major transmission lines began to overload and fail
6. Power plants began to disconnect to protect themselves from a collapsing grid
7. 4:13 PM:  The final cascading failure of grids and power plants stops leaving 265 power plants shut down, 50 million people out of power, 11 people dead and $6 billion in damages.
8. Power is restored 2 days later except for some areas which took a week

 
Due to technical and human error what would have been a manageable local blackout cascaded into massive widespread outage.  Two years later the Energy Policy Act of 2005 was approved which required a non-governmental, self-regulatory organization to develop and enforce compliance with mandatory reliability standards to prevent this from happening again.  Having learned from our mistakes the National Electric Reliability Council 3 or NERC was born 4. 

World Record Outages

Different electrical devices prefer different frequencies, and each has its advantages.  Early engineers calculated that any frequency between 50-67ish hertz was the best compromise between a list of pro’s and con’s for higher or lower frequencies.  Basically, this range was as low as early engineers could go benefiting motors and transmission efficiency without causing flicker in lights. 
 
Read on for more details and history.
Benefits of higher frequency

1. Smaller, lighter and cheaper transformers and generators
2. Better for early light bulb technology (less flicker)

 
Benefits of lower frequency

1. Better for transmission  (less loss to electromagnetic fields and skin effect)
2. Better for motors (especially for early electric railways)

 
Early Frequencies
Early AC systems were isolated unlike our connected grid today.  Each system used their own frequency depending on the type of generator and load it was powering.  Early loads were electric railways, city lighting and motors so the frequencies that these loads preferred dominated the debate.   Most railways preferred and used low frequencies such as 25 Hz in the US and 16.7 Hz in Europe.  Lighting standardized on 133 Hz because it was high enough for no flicker and *boring stuff warning* an 8-pole machine operating at a comfortable 2000 RPMs would output this frequency.  So at the time the frequencies were determined by engineers for its use and not as a business exploitation to limit competition like some think.  However, they didn’t want to have separate distribution systems for each type of load, so a compromise frequency needed to be found.  That range was determined at the time to be anything between 50 Hz and 67 Hz.
 
Road to Standardization
As early as 1891 the high-volume manufacturer Westinghouse had standardized on 60 Hz and the German company AEG on 50 Hz but many frequencies continued to be produced and used.  As the new technology, international trade, and the interconnected ‘grid’ grew people saw a greater need for compatible electrical equipment.  However, scrapping a large infrastructure of perfectly working equipment and buying all new equipment is easier said than done.  WW2 marked a turning point for standardization because of the growth of affordable electrical equipment and probably because most of the infrastructure was blown up and needed to be rebuilt anyway.
 
The most probably theory why European companies chose 50 Hz from the range of 50 Hz to 67 Hz was to better fit their metric standards.  The most possible reason why Westinghouse (and therefore the US) chose 60 Hz over 50 Hz comes from the personal account of L.B. Stillwell, a principle Engineer at Westinghouse, of an informal committee in 1890 to recommend to his company the best frequency to use.  According to him this committee was ready to standardize on 50 Hz but the widespread Arc Lights in use at the time operated with less observable flicker at 60 Hz.   The early electrical pioneers didn’t document their deeds in an organized manner before some of the finer points of their decisions were lost. Today we are left with conflicting personal accounts from the dead who are unable to clarify themselves. 
 
Rest of the world
The rest of the world began purchasing generators from Europe and the United States and who they purchased from mostly decided what frequency they would standardize to.  Some countries, like Brazil and Japan, purchased equipment from both. Brazil didn’t fully standardize on 60hz until 1978.  Japan never standardized and to this day the country is split down the middle with a frequency conversion station in the middle to connect the two halves.  Southern California Edison, which supplied power to most of southern California, used 50hz and didn’t change their standard until 1948. 
 
Fun Bonus Facts:

• Before the word ‘Frequency’ was standardized early inventors had a list of other names for it such as “Alternations” and “Periodicity”.
• Aircraft operate at 400 Hz due to the smaller and lighter weight generators and transformers.

DC has a frequency of 0 which means that most of the low frequency benefits of AC apply to DC, such as transmission efficiency.  Transformers allowed for safer and more efficient AC transmission but if the war of the currents had played out with today’s semiconductor technology DC may have won.

EMF’s (Electromagnetic fields) have always existed and come from both the natural environment and human technology.  They exist in a variety of forms categorized by their frequency such as; Visible Light, Cosmic Rays, Radio Waves, Microwaves, Cellular, Wi-Fi, etc.  EMFs that come from the electric power grid have low frequencies of 60 Hz meaning that they are lower on the electromagnetic spectrum and have less energy.   It is a generally known that low frequency EMF’s, of the type generated by power lines, are not harmful.  “Based on a recent in-depth review of the scientific literature, the World Health Organization concluded that current evidence does not confirm the existence of any health consequences from exposure to low level electromagnetic fields.” 1 2. 
 
What are EMF’s?
EMFs are part of the electromagnetic spectrum which organizes radiation by the frequency that it is wiggling back and forth (wavelength).  The EMF of the US power grid is 60 Hz which as you can see in the chart is rated as “Extremely Low Frequency”.  EMF’s are actually two different types of fields of force, Electric and Magnetic.  Although the two exist together they are caused by something different and act on materials differently so let’s separate them out.
Electric fields are always present along wires even when the device on the other end is turned off.  Electric fields are created by an imbalance of electrons (imbalance of positive and negative charge) called voltage, which is electrical pressure like water pressure in a hose.  If the voltage goes up so does the electric field which is rated in volts per meter (V/m).
Magnetic fields are only present along wires when a device is turned on.  Magnetic fields are created by current, which is the flow of electricity like water down a hose.  If the current goes up so does the magnetic field which is rated in Gauss (G). 
​
Are they dangerous?
Electric fields cause electric charges on the surface of materials including people and cats.  When you feel your hair standing on end when you rub a balloon on your head, scuff your feet on carpet, during a lightning storm or when you stand under a transmission line you are feeling an electric field.  Many of the studies examining the biological health effect of electric fields were negative.  4 bottom of page 5 
Magnetics fields of higher frequency radiation called ionizing radiation (like ultraviolet light) can cause damage to cells because they have enough energy to knock electrons out of atoms or molecules and thereby create ions.  Lower frequency magnetic fields called non-ionizing radiation (like AM/FM Radio, WiFi and high voltage power lines) don’t have enough energy. 5  “There is no known mechanism by which magnetic fields of the type generated by high voltage power lines can play a role in cancer development.” 6 
The media attention started in 1979 when a publicized paper suggested a possible correlation in Colorado between power lines and child leukemia. 7  The criticized study had a small sample size, made hypothetical associations and un-measured assumptions nevertheless the fear took root. 8   Almost 40 years, thousands of studies and 30,000 scientific articles later, most agree there are is no conclusive evidence of health consequences from exposure to low frequency EMF’s.

Interesting Facts Bonus Section:

• Hz (Hertz) is named after the German physicist Heinrich Hertz who proved the existence of electromagnetic waves and had an awesome combover. 9
• The frequency of a permanent magnet is 0 but if you spun it around once per second the frequency of its magnetic field would be 1 cycle per second, 1 Hz.   That would be ELF (Extremely Low Frequency).
• Electric fields exist anytime there is an imbalance of positive and negative charge or more particularly an imbalance in the number of electrons and protons.  Capacitors store energy by creating an electric field (charge imbalance) between two plates.  Technically your wall outlet is a capacitor because it has a charge imbalance between the hot and neutral prong inserts.
• Magnetic fields are created any time electrons are moving.  The electrons orbiting around the proton in atoms produces a magnetic field.  If you put enough atoms together and with aligned orbitals you would make a permanent magnet.
• Physicists sometimes explain the properties of EMF using waves and sometimes as particles called photons.  In particular the energy delivered by a single photon is proportional to its wave frequency, f, in cycles per second commonly annotated as Hz. 
• Do you like differential equations?  If so check out Maxwell’s equations, named after James Maxwell, who in 1861 described how electric and magnetic fields are generated by charges, currents and changes of the fields. 10

What causes Skin Effect?

1. Alternating current creates an alternating magnetic field (blue H)
2. This magnetic field creates loops of electrical current called eddy currents (red - Iw)
3. In the center of the wire the direction of the eddy currents is opposite the direction of the main current flow and so it slows it down.  Notice how in the diagram the red eddy current loops point opposite the current flow in the center of the wire.  Main current (I) ↑ and Eddy current  (Iw) ↓
4. In the outside of the wire the direction of the electric field is the same direction as the current and so it reinforces it
5. Therefore, skin effect is caused by eddy currents which point opposite current flow in the middle and with it towards the surface.
6. Would you rather walk up a stream along the banks or in the middle?  My 3 young boys would rather walk up the stream in the middle but they are not normal electrons.

 
How does frequency change the skin effect?

Skin effect depth is only effected by frequency and material 2.  As frequency increases the skin depth decreases and with it the useful cross sectional area of the wire.  On the other hand, as frequency decreases so does the skin effect.  In AC the current changes directions and therefore so does the polarity of the electromagnetic field it creates.  The dissipation of this field between cycles is not instantiations.   Lower frequency has less of an impedance to current flow because this field is given more time to discharge between cycles.  On the other hand, skin effect is more exaggerated at higher frequencies because the field is given less time to dissipate between cycles.  Direct Current, which has a frequency of 0, uses the entire cross section of a conductor.  This efficiency is one of the benefits of a high voltage direct current transmission system.

​Transmission Engineers hate him, find out why…
 
Skin effect losses become more apparent in transmission lines due to the conductors size and weight.  You have to make the wire bigger and bigger to get ever diminishing amounts of current carrying capacity.  Sure, increasing the wire size slightly increases the useful current carrying surface area near the circumference but it also significantly (quadratically) increases the cross sectional area which in turn drastically increases weight and cost.   A lot of cost for diminishing benefits is the law of diminishing returns.   This is why you see transmission cables have multiple individual cables in a bundle (2-6) in opposed to just using a thicker wire. 3  Some new underground transmission lines have isolated segments insulated from each other.  Similar to an overhead ‘bundle’ this reduces the required size of the wire by providing more “surfaces” for current to travel along.  Why have a middle at all?  Some applications just use tubes like in switchyards or flat bus bars. 4
Last fact… you might think that since each actual conductor has multiple strands of interwoven wires each would have their own skin effect.  However they touch so much that they act similar to a single solid core in terms of skin effect. 5   Most high voltage lines use an aluminum cable with a steal core in the middle of the interwoven wires to reinforce it called ACSR. 6  Since almost no current is traveling down the middle of the wire the higher resistance of steel is an easy tradeoff for its superior tensile strength.
This is such a deep topic, 8.4mm deep!  * I’ll see myself out *