AT THE PHYSICO-TECHNICAL INSTITUTE 403 



in electromagnetic systems had so far been derived from the 

 energy value only. Helmholtz, however, had previously shown 

 that in cases where the kinetic potential contains terms which 

 are linear in respect of the velocities, these disappear from the 

 energy value, so that the forces due to them cannot be 

 ascertained from the energy. Such linear terms are in fact 

 present in the kinetic potential according to F. E. Neumann, 

 so soon as permanent magnets and closed currents act upon 

 each other. The question whether there may not be others 

 of the same kind cannot be determined without special investi- 

 gation. Helmholtz actually succeeded in formulating a kinetic 

 potential of such a kind that the variation, assumed equal to 

 zero, of its integral, taken between two points in time, gave 

 the Maxwell-Hertz equations, while the pondero-motive forces 

 were shown by the minimal principle to be in complete 

 agreement with Maxwell's theory. Unlike the known forms 

 of the problem, it was found here that quantities which were 

 ultimately characterized as momentum, were treated in the 

 variation as independent variables in accordance with his 

 earlier general investigations, in which the velocities were in 

 the same way treated as independent variables, and the 

 signification of these quantities first appeared from their varia- 

 tions. There are many cases of which it is unknown whether 

 they are states or alterations of velocity of states; similar 

 investigations occupied Helmholtz at the close of his life. 



On June n, at the General Meeting of the Goethe Society 

 at Weimar, Helmholtz delivered the address on i Goethe's 

 Anticipations of Coming Scientific Ideas ', to which reference 

 has already been made. On his return from Weimar, he found 

 an intimation from Paris that he had been elected ' Foreign 

 Associate on June 13, 1892, in place of Don Pedro II D'Alcantara, 

 Emperor of Brazil' ; as also his nomination to be Honorary 

 Member of the German Chemical Society of Berlin. On the 

 other hand he had the immense pleasure of communicating 

 a high mark of distinction to his old friend Lord Kelvin. On 

 July 4, 1892, he writes (in English) to Kelvin : 4 1 don't know 

 if you have already received the information that on last 

 Thursday the Academy of Sciences at Berlin has elected you 

 to be one of the first possessors of the Helmholtz medal. 

 At the same time the medal has been given to Mr. du Bois- 



D d 2 



