Understanding Maxwell’s equations (the equations of electromagnetism) involves pictures aplenty. This first equation simply states that electric field lines ( ) only start at positive charges ( ), and only end at negative charges ( ). If a region contains no charges, then the number of field lines entering it is the same as the number of field lines exiting it. If there were such a thing as “magnetic charge”, then you could have “magnetic current”, and there would be a magnetic current term in equation (iv). The speed of light: Around the turn of the last century (1900) some smart people were noticing that Maxwell’s equations implied that changing electric fields created magnetic fields (iii) and that changing magnetic fields created electric fields (iv).