314 THE SO-CALLED SECONDARY ELECTROMOTIVE 



wire the extrapolar currents are -f on both sides of the polarising 

 current. For experiments of this kind I used Matteucci's arrange- 

 ment, i.e. platinum wires surrounded with twisted cotton soaked in 

 sulphate of zinc ; leading in and off was effected by means of 

 amalgamated zinc wires bent into hooks and applied to the stretched 

 wire. I have also used two cables made up of many fine platinum 

 wires which behave exactly like the single covered wire. A very 

 long, fine platinum wire, thickly covered with fine silk, was cut 

 into a number of equal lengths, and these bound together so as to 

 make a cable, which was then itself covered with silk ; one such cable 

 contained 30, the other 100 wires (those of the first were 28, of 

 the second 57 cm. long). Both cables, after soaking well in sulphate 

 of zinc, gave on each side of the polarising current -f extrapolar 

 after-currents. The same results were obtained after soaking the 

 single wire, or the cable, in other fluids. Kaolin electrodes were 

 used for leading in and off. 



(4) The difference, stated above, in the behaviour of thin and 

 thick fluid envelopes, and the theoretical considerations connected 

 with this difference, made it desirable to observe the changes in the 

 behaviour of the extrapolar after-currents with varying thick- 

 nesses of fluid envelope. 



I have not as yet been able to accomplish this in the case of the 

 tubes and wires. The end seemed more attainable in those cases in 

 which the conducting combination had a wide-spread surface. To 

 obtain this, I covered the floor of a Bunsen's mercurial bath with 

 mercury, and poured upon this a thin film of sulphate of zinc. For 

 leading off and in the zinc wires dipping into the fluid were not 

 used, as these were liable to disturb the mercury, but tube electrodes 

 were employed, the kaolin points of which, saturated with zinc 

 sulphate, rested upon the fluid. 



With axially arranged leading in and off contacts, the entire 

 extrapolar surface of the fluid showed an after-effect similarly 

 directed to the polarising current -f . As the film of fluid was 

 gradually increased in thickness, the after-effect in the neighbour- 

 hood of the electrodes became or -f , at a distance from the 

 electrodes it remained -f , by further increase in thickness, the after- 

 effect everywhere became + , or only. There is, therefore, a 

 certain thickness of fluid with which the extrapolar after-current 

 shows at a certain distance from the electrodes a * turning-point,' or 

 ' line,' inside which it is , outside + . The thickness in question 

 is dependent upon the strength of the polarising current ; reversal 



