38 Prof. H. F. Weber on Electromagnetic and 



millims. ; Its plane was parallel to the turns of the spiral ; its 

 centre was on the axis of the latter. A small magnet of 40 

 millims. length was suspended, by a single cocoon-thread, ex- 

 actly in the middle of the spiral. 



The following was the method of experiment : — The induced 

 circuit being open, a constant current was produced in the 

 inducing circuit, the intensity of which, I, was measured in 

 absolute measure by the action of the ring upon the little 

 magnet. Then the inducing circuit was opened, the magnet 

 brought to rest, the ring taken out of the inducing circuit and 

 the latter again closed. After the path of the induced current 

 was also closed, the inducing current I was opened ; the in- 

 duction-current called forth by the sudden sinking of the in- 

 tensity of the inducing-current to zero was measured by its 

 integral current. Hereupon the inducing current's intensity 

 I was again determined, and so on. Thus were taken from 

 20 to 30 successive measurements of the inducing current's 

 intensity I and of the integral current j, generated by opening- 

 induction. In none of the series of experiments carried out did 

 the intensity I vary, in the course of from one to two hours, 

 more than about ^ per cent. 



The calculation of the induction-processes thus excited was 

 based upon the following assumptions : — 



(1) The course of the induction produced by sudden altera- 

 tion of current-intensity in the inducing circuit is perfectly 

 represented by the general law of induction set forth by F. E. 

 Neumann ; and 



(2) The induced current called forth by this extremely 

 rapidly passing induction fulfils Ohm's law. 



Mr. F. E. Neumann, in his treatise Die mathematischen 

 Gesetze der inducirten electrischen Strome, had not more closely 

 investigated this kind of induction. He says, " So far as these 

 formulas admit of being applied to those cases in which a gal- 

 vanic current suddenly appears or is interrupted, further ex- 

 perimental trial is required ; for they presuppose that the 

 velocity with which the inducing cause enters is inconsiderable 

 in comparison with the velocity of propagation of electricity 

 in an induced conductor. On the assumption of the applica- 

 bility of formulas (16) and (17) to the induction occasioned 

 by the sudden rise or disappearance of galvanic currents, we 

 can say that the current induced in a conductor at rest by the 

 sudden appearance of a galvanic current is the same as if the 

 conductor had moved towards the current, from infinite dis- 

 tance to the place where it is." That currents induced by 

 swiftly-passing fluctuations of a current actually range them- 

 selves under Neumann's general law of induction, and at the 

 same time indeed follow Ohm's law, Helmholtz (in his memoir 



