288 



Prof. J. J, Thomson. The 'Resistance of [Jan. 17, 



The thicknesses of the films are by equation (7) proportional to the 

 specific resistances, so that the numbers in the fourth line give the 

 relative resistance of the electrolytes to currents whose directions are 

 reversed from 1C 8 to 2 X 10 8 times per second. In order to see 

 whether these resistances are the same as those with an almost 

 infinitely slower reversal, I determined the resistance of the electro- 

 lytes by using a commutator which reversed the current through the 

 electrolyte about 120 times a second, and kept the direction of the 

 current through the galvanometer constant. The electrodes were 

 platinised, and no polarisation could be detected. The numbers are 

 given in the last line of the above table, and agree sufficiently well to 

 enable us to say that the relative resistance of electrolytes is the same 

 when the current is reversed a hundred million times a second as for 

 steady currents. 



It was not possible to compare in this way the resistances of electro- 

 lytes and metals, as the thinnest metallic film which could be obtained 

 was evidently much thicker than was necessary to completely stop all 

 induction. I succeeded, however, in comparing by this method the 

 resistances of graphite and sulphuric acid. The graphite film was 

 prepared by placing a sheet of glass at the bottom of a trough filled 

 with water, holding a large quantity of finely powdered graphite in 

 suspension; after the graphite had deposited itself uniformly on the 

 glass plate, the water was syphoned off, and the graphite film allowed 

 to dry gradually. When quite dry it was hard and compact, and 

 could be rubbed down by emery to any required thickness. By 

 diminishing the thickness of the film and adjusting the distance 

 between the coils, a film of graphite was obtained which just stopped 

 the sparks ; a film of H 2 S0 4 was then substituted for the graphite, 

 and its thickness adjusted until it, too, just stopped the sparks. In 

 this case, by formula (7) the resistance of equal and similar areas of 

 the two films to currents parallel to their surface must be the same, 

 the currents being reversed 10 8 times per second. I determined the 

 resistances to steady currents parallel to the surface, and found that 



