The revolutionary discovery of the electo-magnetic induction was made by Michael Faraday (1791-1867).

ritratto di Faraday

This great English chemist and physicist posed the question: if a current generates a magnetic field, can a magnetic field then generate a current?


In 1831 he saw...

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that by bringing a magnet close to a wire an electromotor force proportional to the induction of the magnetic field crossed by the wire and the velocity of the movement is generated. Thus it is the variation in the magnetic field, and not the magnetic field itself that generates a current in a conductor!

Click on the photo to see the experiment.


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In this case the fundamental apparatus is "Faraday’s Disk":

Click on the photo to see how it is made


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and to see what happens click on this photo...


Faraday’s “induction law" gives the electromotor force f generated by the ends of a conductor when this “sees” a variation velocity of flow dΦ/dt:


The minus sign is important. It expresses "Lenz’s Law": the sign of the induced electromotor force is such as to create a current that has a direction such that the magnetic flow it creates across the surface limited by the circuit tends to diminish the variation in the magnetic flow that originated such a current. In other words, this current creates a magnetic field that opposes the one that generated it.
The induction phenomenon is of fundamental importance. It is the basis of all industrial systems for the generation of electricity.


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Here in action we can see the induction law and Lenz’s law. The black object at the centre is the winding of many coils on an iron core: an inductor.

Click on the photo to see the experiment.


The variation in the current generates a magnetic field which in turn induces in the same circuit a current that opposes the variation that generated it. Thus the total current, the sum of the two, during the variation in flow, tends to cancel itself. This is the autoinduction phenomenon. Concretely, an inductor opposes sharp variations in current. The current’s magnetic field in the circuit creates a magnetic flow Φ across surface S limited by the circuit itself, which is easy to imagine as being proportional to the current itself:

Φ =L . I  

ritratto di Henry

The L coefficient is called the autoinduction coefficient, or inductance, of the circuit, and is measured in henrys, in honour of Joseph Henry (1797-1878), an American scientist who studied under Franklin and Gibbs, here shown in a fine oil portrait...


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Now let’s examine another consequence of autoinduction.

To see what happens click on the photo


To keep the autoinduction current in the (auto)induction circuit, the circuit itself generates at its ends a tension much higher than the tension of the power supply, which in this case is 25 V. This is the extremely important phenomenon of the generation of extratensions, which are at the base of all devices that inductively generate tensions much higher than the starting ones, as in car ignition coils, Rumkhorff coils in high-tension power supplies for television cathode tubes, monitors and so on.
Since Φ=L.I , it is easy to deduce the value of electromotor force E autoinduced in the circuit when the current varies:


which may be enormous if the variation in current takes place in a very short time.

Induction and autoinduction phenomena are at the basis of many other facts as we shall see on the following pages...

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