8o HERMANN VON HELMHOLTZ 



of induced currents are effected so slowly that the strength 

 of current is equalized in the entire circuit, and the intensity of 

 current in a simple circuit is then determined by Ohm's Law, in 

 the form of an exponential function of time, on the assumption 

 that the induced electromotive force lags by a very small interval 

 behind the variations of the inducing current. After Helmholtz 

 had extended this exponential law mathematically to divided 

 circuits, he tested it experimentally by means of a new type 

 of galvanic contact-key, which made it possible to vary the 

 interval between the opening and closing of any current, as 

 required. Instead of measuring the time directly, he calculated it 

 from the action of the currents upon a magnet, by a modification 

 of Pouillet's method. Thus he proved that, subject to the 

 assumption that the induced electromotive force coincided in 

 time with the inducing variations of current, no inducing 

 action was present y^^o- second after breaking the current 

 in a coil. By this means the time-intervals due to the electric 

 current were distinguished with mathematical accuracy from 

 those due to nerve-action, and Helmholtz was free, after this 

 absolute verification of his methods, to devote himself to the 

 complicated experiments on the excitation of nerve that 

 occupied him during the remainder of 1851. 



In the autumn holidays of 1851 Helmholtz carried out his 

 project of inspecting some other physiological laboratories. 

 After taking his wife and child to Dahlem, he went in the 

 first place for a few days' rest to his Berlin friends, Professor 

 Heintz and his family, at Halle. Then after a short stay in 

 Kassel he went on to Gottingen. His first visit was to 

 Professor Ritter, one of his former teachers at the Gymnasium 

 of Potsdam, and his father's faithful friend, in whose company 

 he 'saw the little town, rather better built than Halle and 

 Konigsberg, with the University as its fulcrum ' ; after which 

 he called on the professors, though somewhat hampered by 

 the presence of the King. He describes them in a letter to 

 his wife as 'the aristocracy of the town; one can see that 

 they are alive to their own merits, and a little inclined to 

 over-estimate the accomplishments of their circle not specially 

 in regard to myself, but obviously in referring to any third 

 person'. His lively descriptions show that he much enjoyed 

 this visit to the Gottingen doctors : 



