32 8 Messrs. Guthrie and Boys on 



On the same page of the Galvanismus which treats of the 

 research of Mach, as mentioned above, Wiedemann describes, 

 as a means of showing that no action takes place between per- 

 manent electric currents as such, almost the exact arrange- 

 ment of apparatus with which the discovery was finally made. 

 Who first used this apparatus for this purpose I cannot say, 

 unless it may have been Wiedemann himself. The same plan 

 was hit upon by Professor Rowland* (quite independently, I 

 believe) ; and he experimented to some extent in this direction 

 about the year 1876. The same arrangement was finally 

 adopted by me after another method of attacking the problem 

 had been unsuccessfully tried. 



I desire to express my sense of obligation to the Professors 

 and students of the Physical Department of the Johns Hopkins 

 University for the generous assistance which they have ren- 

 dered me during the progress of this research. 



XXXIX. On Magneto-Electric Induction. — Part II. Conduc- 

 tivity of Liquids. By Frederick Guthrie and C. V. Boys, 

 Assoc. JR. Sch. Mines\. 



[Plate VIII.] 



IN a previous communication % we showed experimentally 

 that, other things being equal, a conductor in a moving 

 magnetic field is urged to move by a force which varies as 

 the product of the conductivity into the relative speed ; so 

 that by observing the torsion of a wire supporting successively 

 a variety of conductors of the same form and size in a re- 

 volving magnetic field, a measure of their relative conduc- 

 tivity may be obtained. In the case of most metals, this 

 method of determining conductivity cannot be compared, at 

 least for convenience, with the usual one with the bridge, 

 galvanometer, &c. ; but with less-perfect conductors, with 

 such as cannot be drawn into wire, and especially with electro- 

 lytes, our method seemed very promising ; for, whatever be 

 the actual course of the electrical currents induced in the 

 liquid, they are closed, no electrodes are present, there is no 

 electrolysis, and there is no polarization. 



The preliminary experiments, performed with the apparatus 

 described in our last paper, clearly showed that with suitable 

 means a measurable and even a large effect might be produced. 



* Amer. Journ. of Math. vol. ii. p. 289. 



f Communicated by the Physical Society, having been read at the 

 Meeting on June 26th. 



\ Proceedings of the Physical Society of London, vol. iii. part iii. p. 127 ; 

 Phil. Mag. December 1879, vol. viii. p. 449. 



