Blondel's experiments

Blondel's experiments are a series of experiments performed by physicist André Blondel in 1914 in order to determine what was the most general law of electromagnetic induction. In fact, noted Blondel, "Significant discussions have been raised repeatedly on the question of what is the most general law of induction: we should consider the electromotive force (e.m.f.) as the product of any variation of magnetic flux (${\displaystyle \Phi }$) surrounding a conductor or of the fact that the conductor sweeps part of this flux?".

In the first case Blondel referred to Faraday-Neumann law, which is often considered the most general law, while in the second case he referred to Lorentz force.

Normally experiments to verify the first case consist of measuring the induced current in a closed conducting circuit, concatenated to the magnetic induction field ${\displaystyle B}$ of a magnet, with ${\displaystyle B}$ varying in time, while for the verification of the second case usually we measure the induced current in a closed circuit of variable shape or moving by cutting perpendicularly a field ${\displaystyle B}$ constant.

The second case, however, is due to a variation of the magnetic flux ${\displaystyle \Phi =B\cdot S}$, not so much because the intensity of ${\displaystyle B}$ varies, but because the surface ${\displaystyle S}$ crossed by the field varies.

Blondel, on the other hand, devised "a new device which consists in varying the total magnetic flux passing through a coil, by a continuous variation of the number of turns of this coil". In this way ${\displaystyle B}$ and ${\displaystyle S}$ are constant for each coil, but the total flux varies with the number of coils affected by the field ${\displaystyle B}$.

It follows that, given the flux ${\displaystyle \Phi }$ concatenated to a single loop and ${\displaystyle N}$ the total number of loops, by Faraday-Neumann's law, the resulting electromotive force is:

${\displaystyle \mathrm {e.m.f.} =-{d(\mathrm {N} \Phi ) \over dt}=-\Phi {d\mathrm {N} \over dt},}$

i.e. dependent on the variation of the number of turns in time.

Blondel tested four configurations of his apparatus in which he demonstrates that a change in flux does not always generate an e.m.f. in a circuit concatenated to it, concluding that the Faraday-Neumann law cannot be the general law.