Nomenclature
In Does the inertia of a body depend upon its energy-content? Einstein used V to mean the speed of light in a vacuum and L to mean the energy lost by a body in the form of radiation. Consequently, the equation E = mc2 was not originally written as a formula but as a sentence in German that meant if a body gives off the energy L in the form of radiation, its mass diminishes by L/V2. A remark placed above it informed that the equation was approximate because the conclusion was only justified if one neglected "magnitudes of fourth and higher orders" of a series expansion. In 1907, Einstein's mass-energy relationship was written as M0 = E0/c2 by Max Planck and, subsequently, was given a quantum interpretation by Johannes Stark, who assumed its validity and correctness (Gültigkeit). However, Stark wrote the equation as e0=m0 c2 which meant the energy bound in the mass of an electron at rest and still was not the present popular version of the equation. In 1924, Louis de Broglie assumed the correctness of the relationship "énergie=masse c2" on page 31 in his Research on the Theory of the Quanta (published in 1925) but he did not write E = mc2. However, Einstein returned to the topic once again after the World War Two and this time he wrote E = mc2 in the title of his article intended as an explanation for a general reader by analogy
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