The understanding of electromagnetic phenomena is based on the historic landmark A Treatise on Electricity & Magnetism of 1873 by James Clerk Maxwell. The form in which the theory is most commonly expressed is in the form of the vector equations introduced by Gibbs.There is another more general formulation due to Élie Cartan, in terms of external differential forms. We will develop the description of electromagnetic phenomena using these concepts. It is a pre-metric description of conservation laws, that will highlight the basic properties of the electromagnetic phenomena. We will in particular demonstrate the fundamental role of the flux quantum Φ₀ h/2e, and point to ways to calibrate magnetic fields.