Blondel's experiments

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.)

Normally experiments to verify the first case consist of measuring the induced current in a closed conducting circuit, concatenated to the magnetic induction field

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

The apparatus consists of an electromagnet E, whose U-shaped core terminates in two large parallel plates P and P' .

Two induction coils B generate the magnetic field in E. Between the two plates there is a rotating wooden drum T on which an insulated electric wire is wound.

A sliding contact f electrically connects the wire to a galvanometer G, by means of a resistor R so that current can flow even when the drum is rotating.

When motor M starts up, it can increase the number of coils wrapped around T' by decreasing those around T or vice versa.

The wire wound on T is connected directly to the rotation shaft on which rests the sliding contact f, through the conducting ring b, of negligible diameter, as shown in the figure.

[10] The wire wound on T is connected to a conducting ring of diameter equal to that of the drum T and integral with it.

Since Blondel feared that it could be objected that the result is due to the fact that, during rotation, the circuit between f and the point of attachment of the coil wire to the ring may follow two different paths that partially neutralize each other, he makes a third experiment.

The wire wound on T is connected to the edge of a solid disk of diameter equal to that of the drum T and integral with it.

Moreover, by varying the point of contact of the coil from the outer edge to the center of the disc, the induced e.m.f.

only if the circuit cuts through the lines of force of the field, as in the first experiment (rotational axis-drum edge section).

Thus "one must reject as inaccurate the too general statements of the law of induction"[13] and to the statement that "An electromotive force originates in a closed circuit when the number of magnetic lines passing through it varies[14]...." should be added "and when the variation is produced either by the conductor sweeping the lines of force or by a variation in the field of the inductor itself".

In fact it is necessary to include also the contribution due to Lorentz force to obtain the general formula.