218 Dr. F. Auorbach on the Passage of 



That such an influence must be present the following con- 

 sideration shows. In the fundamental laws, among other 

 ideas occurs that of resistance ; this must therefore be fixed 

 once for all, in order that the validity of those laws may have 

 a meaning. For iron, according to the investigation which has 

 been carried out, this is a doubtful problem. Over against 

 the reflection that, in order to make comparisons with other 

 metals possible, the resistance of unmagnetic iron must be 

 taken into consideration, stands another, that this quantity is 

 inaccessible to experience. Since, then, the laws of Ohm, 

 Joule, and Lenz, &c, hold good for it alone, experience, 

 which of necessity makes use of the empiric notion of resist- 

 ance, must give deviations from the laws. 



According to Ohm's law the resistance is independent of 

 the electromotive force*. This of course is true also of the 

 ideal resistance of an iron wire ; its real resistance must, on the 

 contrary, change ichen the electromotive force, and ivith it the 

 current-intensity, is changed. For with the latter the circular 

 magnetization, and with this the resistance, increases up to 

 the limit of saturation. 



In order to test this requirement experimentally, I made 

 use of an arrangement which permitted the electromotive 

 force to be varied instantaneously. This was a plug commu- 

 tator constructed upon a plan of Professor Meyer's. Upon a 

 hardgum plate the brass pieces represented in the shaded part 

 of fig. 5 (PI. I.) are fixed. By inserting metal plugs in the conic 

 apertures they can be connected by means of the binding- 

 screws A, B, &c. with one onother and with the other parts of 

 the circuit. If, for example, a galvanic element (the positive 

 pole is always first mentioned) be connected w T ith E and B, a 

 second with A and H, a third with Gr and F, and of the plugs 

 only those at a, d, e, and h be fixed, while the binding-screws 

 C and D serve merely for carrying off the current, the three 

 elements are inserted one after another. If now the plugs at 

 a and h be taken out, and plugs put in at b, c, f, and g, the 

 three elements are inserted side by sidef . 



For my purpose I inserted the commutator at A and C in 



* According to the last Report made to tlie British Association, for 

 copper this law agrees perfectly, so far as the accuracy of observation ex- 

 tends.— Beibl. ii. p. 267 (1878). 



t Let me be permitted to take this opportunity to recommend for lec- 

 ture-purposes the above-described and another commutator, likewise pro- 

 posed by M. Meyer, of which the drawing fig-. 6 is an illustration. A 

 combination of one specimen of the^rs^ and tico of the second sort makes 

 it possible for the lecturer to pass over at pleasure from any galvanic 

 experiment to any other by merely transposing the plugs. M. Pinzger, 

 of this place, supplies them excellently executed. 



