Let us conceive of the world-line of such an electron with the charge (_e_), and let us introduce upon it the “Proper-time” τ reckoned from any possible initial point. In order to obtain the field caused by the electron at any world-point P₁ let us construct the fore-cone belonging to P₁ (_vide_ fig. 4). Clearly this cuts the unlimited world-line of the electron at a single point P, because these directions are all time-like vectors. At P, let us draw the tangent to the world-line, and let us draw from P₁ the normal to this tangent. Let _r_ be the measure of P₁Q. According to the definition of a fore-cone, _r_/_e_ is to be reckoned as the measure of PQ. Now at the world-point P₁, the vector-potential of the field excited by _e_ is represented by the vector in direction PQ, having the magnitude _e_/_cr_, in its three space components along the _x_-, _y_-, _z_-axes; the scalar-potential is represented by the component along the _t_-axis. This is the elementary law found out by A. Lienard, and E. Wiechert.[34] If the field caused by the electron be described in the above-mentioned way, then it will appear that the division of the field into electric and magnetic forces is a relative one, and depends upon the time-axis assumed; the two forces considered together bears some analogy to the force-screw in mechanics; the analogy is, however, imperfect. I shall now describe _the ponderomotive force which is exerted by one moving electron upon another moving electron_. Let us suppose that the world-line of a second point-electron passes through the world-point P₁. Let us determine P, Q, _r_ as before, construct the middle-point M of the hyperbola of curvature at P, and finally the normal MN upon a line through P which is parallel to QP₁. With P as the initial point, we shall establish a system of reference in the following way: the _t_-axis will be laid along PQ, the _x_-axis in the direction of QP₁. The _y_-axis in the direction of MN, then the _z_-axis is automatically determined, as it is normal to the _x_-, _y_-, _z_-axes. Let [:_x_], [:_y_], [:_z_], [:_t_] be the acceleration-vector at P, [._x_]₁, [._y_]₁ [._z_]₁, [._t_]₁ be the velocity-vector at P₁. Then the force-vector exerted by the first election _e_, (moving in any possible manner) upon the second election _e_, (likewise moving in any possible manner) at P₁ is represented by -_e e₁_([._t₁_] - [._x₁_]/_c_)F, _For the components F_{x}, F_{y}, F_{z}, F_{t} of the vector F the following three relations hold_:— _c_F_{_t_} - F_{_x_} = 1/_r²_, F_{_y_} = [:_y_]/(_c²__r_), F_{_z_} = 0, _and fourthly this vector F is normal to the velocity-vector_ P₁, _and through this circumstance alone, its dependence on this last velocity-vector arises_.