On the other hand, already in 1899 Lorentz assumed that the electrons undergo length contraction in the line of motion, which leads to results for the acceleration of moving electrons that differ from those given by Abraham. Lorentz obtained factors of formula_14 parallel to the direction of motion and formula_15 perpendicular to the direction of motion, where formula_16 and formula_17 is an undetermined factor. Lorentz expanded his 1899 ideas in his famous 1904 paper, where he set the factor formula_17 to unity, thus: