Paul Gruner

“ Bern, 19 May 1921.
(Received 21 May 1921.) ↑ The relativity formulas are presented most conveniently, if one choses 1 kilometer as unit of length and 1/300000 second as unit of time. We propose the name light second for this unit of time. ”

“ With this easily constructed coordinate systems, length contraction and clock retardation can be seen without further ado.
Fig. 1 In the "primed" system (Fig. 2) the world-lines parallel to the time axis provide the "world history" of the point resting in this system. always represents the length of a rod resting in it.
Fig. 2 The observers resting in the "unprimed" system can only measure this length , by finding out the location and of the endpoints of at equal clock-indication (thus upon a line parallel to ) ; they find ”