PROFESSOR IN BERLIN 321 



be manifested in the retardation of the oscillatory motions of 

 the electricity, as appears with sudden interruptions of the 

 electrical currents in wires of high conductivity. Since it was 

 to be expected that it would be possible by this means to 

 determine an upper limit to the value of this inertia, Helmholtz, 

 when he had to set a prize subject in physics for his students 

 at the end of the summer term, gave them the task of devising 

 experiments which should demonstrate the strength of extra- 

 currents, 'with the certain and subsequently confirmed ex- 

 pectation ' that Heinrich Hertz, who by von Bezold's advice had 

 been working in the Physical Laboratory of the University 

 under Helmholtz's direction since the autumn of 1878, and 

 whom the latter had recognized even in the elementary course 

 as a student of quite extraordinary promise, would interest 

 himself in it, and attack the problem successfully. 



The magnitude of the extra-current was to be used in 

 ascertaining an upper limit for the mass in motion; and the 

 extra-currents from double-wound coils with their branches 

 traversed by the current in opposite directions were specially 

 recommended. The precise answer given by Hertz showed 

 that at most -fa to ^ of the extra-current in a double-wound 

 coil was due to the inertia of the electricity; investigations of 

 the influence of the centrifugal force of a rapidly rotating plate 

 upon the motion of the electrical current flowing through it 

 led the gifted young investigator to a far lower superior limit 

 of the value of inertia in electricity. 



These researches of Hertz, the results of which were plainly 

 foreseen by Helmholtz, gave substantial support to the 

 Faraday- Maxwell hypothesis of the nature of electricity, and 

 confirmed Helmholtz in his opinion of the accuracy of Faraday's 

 conceptions. The phenomena of diamagnetism were explained 

 in the simplest manner, on the assumption that diamagnetic 

 bodies are such as are less magnetizable than the surrounding 

 media with which space is filled, so that even the space that 

 is free from all ponderable masses, or the luminiferous ether 

 contained in it, must be magnetizable. According to Clerk 

 Maxwell it was, however, of essential importance for Faraday's 

 Theory of Dielectric Polarization and the elimination of action 

 at a distance, to know whether the origin and passage of 

 dielectric polarization in an insulator would give rise to the 



