OK ELECTROLTSIS IN ITS PHYSICAL AND CHEMICAL BEARINGS. 341 



Oti tlie Accuracy of Olmi's Laiv in Electrolytes. 

 By Professor Fitzgerald and F. Trouton. 



Experiments have been made with the faster fork alluded to ia last year's 

 report, with the result of bringing the determination to about twice the degree of 

 refinement then mentioned as attained, or ' h' to be less than 3 x 10~^. The rate 

 of this fork (about 290 vibrations per second) is nearly twice that of the previously 

 employed fork (160 per second) . From the experiment described below the possible 

 increase in refinement obtainable by doubling the rate appears to be about fourfold, 

 or twice that obtained. 



Considering that the difiiculties of working increase greatly with the rate of the 

 fork, it was thought more advisable for the further prosecution of the investigation 

 to try some other method of breaking the contacts than to prepare a still faster 

 fork. 



A commutation arrangement attached to the spindle of a small magneto has 

 been employed with fairly satisfactory results. As great a refinement, however, 

 has not yet been reached with it as was attained with the fork method,' chiefly 

 through the difficulties connected with running the magneto uniformly ; but we 

 hope soon to have storage cells in the laboratory, and thus to get over this 

 difficulty. 



The magneto arrangement, however, on account of the facilities for changing 

 the speed, was found admirably adapted for confirming the supposition that the 

 observed deviation from Ohm's law is due to ' heating efl'ect;',' for the deviation 

 always tended to disappear as the speed of contact-breaking was gradually increased ; 

 also for investigating the maimer in which the minimum value of ' A ' obtainable 

 varies with the speed of contact-breaking. This was found to be, approximately, 

 inversely as the square of the speed. 



Is the Velocity of Light in an Electrolytic Liquid influenced hy an Electric 

 Current in the direction of propagation ? By Lord Ratleigh, Sec.B.S. 



The question here proposed has been considered by Roiti (Pogg. ' Ann.' 150, 

 p. 164, 1873) and by Zecher (' Rep. de Phys ' 20, p. 151, 1S84).^ My experiments 

 were made in ignorance of the work of these observers, and the results would 

 scarcely be worth recording were it not that the examination seems to have been 

 pushed further than hitherto. It may be well to say at once that the result is 

 negative. 



The interference fringes were produced by the method of JNIichelson, as used in 

 his important investigation respecting 'The Influence of Motion of the Medium 

 upon the Velocity of Light.' ^ The incident ray ab meets a half-silvered surface at 

 b, by which part of the light is reflected and part is transmitted. The reflected 

 ray follows the course ahcdefby, being in all twice reflected at 6. The transmitted 

 ray takes the course abfedcby, being twice transmitted at b. These rays, having 

 pursued identical paths, are in a condition to form the centre of a system of fringes, 

 however long and far apart may be the courses cd, ef. 



There is here nothing to distinguish the ray ab from a neighbouring parallel 

 ray. The incident plane ivave-froni perpendicular to nb gives rise eventually to 

 two coincident wave-fronts perpendicular to bg. With a wave incident in another 

 direction the case is different. The two emergent wave-fronts remain, indeed, neces- 

 sarily parallel, both having experienced an even number of reflections (four and 

 six). But there will exist in general a relative retardation, of amount (for wave- 

 fronts perpendicular to the plane of the diagram) proportional to the deviation from 

 the principal wave-front. Hence, if the incident light comes in all directions, a 



' A very unsatisfactory experiment with the magneto at a speed of about 400 

 vibrations per second gave, as the value of h, 1-25 x lO-s. 



^ Mr. Wilberforce has experimented upon displacement currents in a dielectric 

 ^Camh. Phil. Trans, t. xiv. p. 170, 1887). 



' Am. Journ. xxxi. p. 377, 1886. 



