The Principle of Relativity

by Albert Einstein; H. Minkowski

(iii) Curl e + δm/δt = 0

(iv) div m = 0

I shall now write (x₁ x₂ x₃ x₄) for (x, y, z, t) and (ρ₁, ρ₂, ρ₃, ρ₄) for

i.e. the components of the convection current ρu, and the electric density multiplied by √ -1

Further I shall write

f_{2 3}, f_{3 1}, f_{1 2}, f_{1 4}, f_{2 4}, f_{3 4}.

for

m_x, m_y, m_z, -ie_x, -ie_y, -ie_z.

i.e., the components of m and (-i.e.) along the three axes; now if we take any two indices (h. k) out of the series