Two paths towards circulation time derivative (Maxwell's $\mathfrak E$ revisited)
The time derivative of the circulation of a vector field $\boldsymbol{A}$ over a moving and deforming closed curve, $\frac {\mathrm{d}}{\mathrm{d} t}\oint \boldsymbol{A} \cdot \mathrm{d} \boldsymbol{r}$, is computed in two ways, with and without bringing the time derivative under the integral sign. As a by-product, the computations reveal that the conceptualization of Faraday's law of electromagnetic induction may depend on which of the two methods is employed. The discussion presented provides an unexpected argument in favor of Maxwell's mysterious choice for his electromotive intensity $\mathfrak E$, made in Article 598 of his Treatise.
physics.hist-ph↗