  What is the cause of this discrepancy between the idea of congruent  transfer and the behaviour of measuring-rods and clocks? I  differentiate between the determination of a magnitude in Nature by  “persistence” (Beharrung) and by  “adjustment” (Einstellung). I shall make the  difference clear by the following illustration: We can give to the  axis of a rotating top any arbitrary direction in space. This  arbitrary original direction then determines for all time the  direction of the axis of the top when left to itself, by means of a  tendency of persistence which operates from moment to  moment; the axis experiences at every instant a parallel  displacement. The exact opposite is the case for a magnetic needle in a magnetic field. Its direction is determined at each instant  independently of the condition of the system at other instants by the fact that, in virtue of its constitution, the system adjusts  itself in an unequivocally determined manner to the field in  which it is situated. A priori we have no ground for  assuming as integrable a transfer which results purely from the  tendency of persistence. …Thus, although, for example,  Maxwell’s equations demand the conservational equation  \(de\,/\,dt =0\) for the charge \(e\) of an  electron, we are unable to understand from this fact why an electron, even after an indefinitely long time, always possesses an unaltered  charge, and why the same charge \(e\) is associated with all  electrons. This circumstance shows that the charge is not determined  by persistence, but by adjustment, and that there can exist only  one state of equilibrium of the negative electricity, to  which the corpuscle adjusts itself afresh at every instant. For the  same reason we can conclude the same thing for the spectral lines of  atoms. The one thing common to atoms emitting the same frequency is  their constitution, and not the agreement of their frequencies on the occasion of an encounter in the distant past. Similarly, the length  of a measuring-rod is obviously determined by adjustment, for I could not give this measuring-rod in this field-position  any other length arbitrarily (say double or treble length) in place  of the length which it now possesses, in the manner in which I can at will pre-determine its direction. The theoretical possibility of a  determination of length by adjustment is given as a consequence of  the world-curvature, which arises from the metrical field  according to a complicated mathematical law. As a result of its  constitution, the measuring-rod assumes a length which possesses this or that value, in relation to the radius of curvature of the  field.