Alternating currents in combination with reactive components are a source of confusion. In my previous post I touched upon capacitive components so let’s now look at inductive so that I can move back on the Tesla track.

Capacitors, as I said, are easiest to understand if you look at them as rechargeable batteries that can be charged and discharged in extremely short time intervals. A battery has the tendency to maintain a certain voltage level. When you apply a higher voltage, the battery needs some time to charge and adjust to that higher voltage. Likewise if the voltage drops, the battery will supply current and lower the rate at which the voltage drops.

The other reactive component, a coil, or more generally speaking an inductor does something similar with currents instead of voltages. As a capacitor ‘resists’ changes in voltage, an inductor ‘resists’ changes in current. To avoid confusion with resistors, which are non-reactive, this resistance of reactive components is usually called impedance.
When I send a current through a coil, the coil will build up a magnetic field, in other words; its magnetic field _changes_, and a changing magnetic field induces a current. This current opposes the current that we try to send, hence the impedance. But once a current flows, the magnetic field no longer changes and the coil no longer resists the current. Now, when we try to interrupt the current, the magnetic field changes again and induces a current in the coil. This can lead to excessively high voltages at the place where the current is interrupted, because the coil keeps ‘pushing’ charges in that direction.
![Inductie.png](https://steemitimages.com/DQmPQaFCWCEUEZBDgAbfTYKAdenXXssTGvVSBoZgJnyB9xT/Inductie.png)

__Analogy__
Inductance is often compared to inertia in mechanics. Tesla often uses the term inertia when speaking of inductance. Imagine a long pipe that supplies a large city of water. Let’s assume the pipe has a cross section of half a square meter and a length of 20 km. The pipe then contains 10,000 cubed meters, or 10 million litres of water. Now suppose 200,000 people are having a shower at some time, each taking 0.1 l/s, giving a total of 20,000 l/s. This causes a water flow of 40 m/s in our pipe. The total kinetic energy of the water in the pipe now is E = ½ m v ² = 0.5 x 10,000,000 x 1,600 = 8,000,000,000 Joules. Now in the unfortunate event that everyone closes their shower at the exact same instant, this energy will be delivered to all taps in town and cause chaos.
A similar thing happens in power networks when you try to interrupt a line that carries a current; the inductance of the line may cause massive voltage spikes.



__The LC circuit__
What would happen if we connect a charged capacitor to a coil. The capacitor will try to discharge, but to do so it must first establish a current through the coil. Once this is done and the capacitor is emptied, the current through the coil is no longer pushed by the capacitor, but the coil can not instantly stop the current. The current will take as long to stop as it took to start and thus the coil will charge our capacitor in the opposite direction (what was first positive will now get negative and vice versa). When the current stops our capacitor will again have the same energy but oppositely charged, and so it will want to discharge again and the process will repeat over and over again.

__Self resonance__
As every conductor has its capacitance, a coil doesn’t really need an external capacitor in order to resonate. When charges are displaced in a coil, let’s say toward the right end, then these charges will want to redistribute evenly along the coil. However, the coil’s inductance will cause an overshoot to  the left end as with the LC circuit and this process will repeat over and over again.
The entire coil becomes a mix of capacitance and inductance, with the ends acting mainly as capacitance and the middle mainly as inductance.

__Tesla’s bifilar coil__
In his search for an energy source that does not consume fuel, Tesla attempted to reduce the coil’s impedance while still creating a magnetic field. If a magnetic field could be created without impedance then obviously energy could be created out of thin air, a scheme highly unlikely of success. Still, it is an interesting thought and worth an experiment or two.
Tesla’s idea was to wind a coil in such a way that the turn-to-turn capacitance would be maximized, thus the ‘suction’ of the coil’s internal capacitance would counteract the coil’s magnetic impedance.
Although this does change the coil’s characteristics very dramatically, it is -as expected- not enough to create energy out of thin air. But the unusual characteristics of this coil may find use in some kind of apparatus and therefore Tesla patented this idea (US patent 512,340).