On the Origin of Kaluza's Idea of Unification
  We argue that the starting point of Kaluza's idea of unifying electrodynamics and gravity was the analogy between gravitation and electromagnetism which was pointed out by Einstein and Thirring. It seems that Kaluza's attention was turned to this point by the three papers on the Lense-Thirring effect and the analogy between gravitation and electromagnetism which were published a short time before Kaluza's paper was submitted. We provide here an English translation of the third of these papers. 
Y. Verbin, N.K. Nielsen