In late 1820, French physicist André-Marie Ampère presented his fundamental experiments to the Académie des Sciences, proving that parallel electrical conductors attract or repel one another magnetically.

Ampère’s Force Law Equation

The magnetic force per unit length $\frac{F}{L}$ between two parallel straight conductors separated by distance $d$ carrying currents $I_1$ and $I_2$ is:

$$\frac{F}{L} = \frac{\mu_0 I_1 I_2}{2\pi d}$$

  • Parallel Currents ($I_1 \uparrow \uparrow I_2$): Attractive force.

  • Antiparallel Currents ($I_1 \uparrow \downarrow I_2$): Repulsive force.

Legacy

Ampère introduced the term electrodynamics and postulated molecular electric current loops to explain ferromagnetism.