282 HERMANN VON HELMHOLTZ 



moved to Heidelberg and then for a second time to Italy : but 

 all was in vain. She returned from Italy in 1877, and died at 

 her home in Dahlem on April 25. Her coffin stood before the 

 altar in the village church, where her parents had been married/ 



Helmholtz, as an Ordinary Member of the Academy, to which 

 he had been elected on April i, contributed a Paper ' On the 

 Rate of Transmission of Electrodynamic Action', on May 25, 

 1871. He connected this with the researches of Blaserna, 

 and in it discussed a question, then of great moment in the 

 development of Electrodynamics, to which he had already 

 alluded in the great electrodynamic memoir cited above. Ac- 

 cording to C. Neumann, and on the hypothesis of Faraday and 

 Clerk Maxwell, which assumes that electrodynamic action at a 

 distance is caused by changes in the medium with which space 

 is filled, this action must be produced by forces which are pro- 

 pagated through space with finite velocity, and this velocity 

 must approximate to that of light. Helmholtz had, however, 

 shown in his earlier criticism of the electrodynamical theories 

 that, on the assumptions made as to the susceptibility of the air 

 to magnetic or dielectric ^polarization, other values of the 

 velocity of propagation are compatible with the facts. After 

 Blaserna had convinced himself by experiments that the pro- 

 pagation at least of the inductive action of electrical currents 

 proceeded at a very moderate velocity in air, Helmholtz, who 

 had long been occupied with experiments on the course of very 

 brief electrical currents, felt compelled to test the accuracy of 

 these experiments as regards the propagation of the action in 

 air. He arrived at the result that ' the separation of the two 

 coils to the considerable distance of 136 cm. does not alter the 

 position of the zero-point of the induced current by one division 

 of the micrometer, i. e. not by ^Ay^ second. So that if the 

 inducing currents are really propagated at any calculable speed, 

 this must be greater than 314,400 m. or about 42-4 (German) 

 geographical miles er second '. 



In the same summer (July 6, 1871) Helmholtz, at the Leibniz 

 Session of the Academy of Sciences, delivered a beautiful and 

 reverent address ' In Commemoration of Gustav Magnus ', whose 

 successor he was, and whose personality and conduct he felt 

 himself the more bound to do justice to, since from Magnus's 

 somewhat chilling reception of his 'Conservation of Energy' 



