Poincaré’s Note and Memoir

The aim of this chapter is to show that, in these two publications, Poincaré first set out the principle of relativityPrinciple of relativity, which he applied to electromagnetic phenomena. He will then give a complete formulation of the transformation that leaves MaxwellMaxwell, James Clerk’s four equations invariant, and demonstrate that it forms an invarianceInvariance group of Dynamics. Last but not least, he corrected Lorentz’s erroneous expressions for velocity and charge densityCharge density. This led him to state the new relativistic velocity addition lawRelativistic velocity addition law. This analysis shows that Poincaré’s aim was not to develop a new theory, but to “save Lorentz’s theory of electrons”.