312 THE SO-CALLED SECONDARY ELECTROMOTIVE 



Before passing on to the extrapolar phenomena shown by the 

 conducting- axis schema, it is first necessary to supplement in some 

 points the theory and experiments previously set forth. 



The polarisation after-currents in the conducting axis schema, 

 (Kern leitermodell) . 



(a). EXPERIMENTAL. 



Several circumstances made it desirable that the experiments? 

 carried out in 1871-72, upon the after-currents of the conducting 

 axis schema, should be repeated and supplemented. First, that 

 these had been made only with moderate strengths and long closures 

 of current ; consequently, having regard to du Bois-Reymond's experi- 

 ments, the effects of strong currents and short closures had still to be 

 investigated. In the second place, in my former experiments I broke 

 the polarising current and then closed the galvanometer circuit by 

 a key ; it was therefore desirable to arrange for a more rapid succes- 

 sion than this gave, so that the conditions immediately following 

 the break might be determined. Finally, it remained to be seen 

 whether the use of the galvanometer with light magnet, which I 

 now employ, would bring into view new phenomena, such, for 

 instance, as change of direction of after-currents. 



For most of my experiments I used tubes filled with solution 

 of sulphate of zinc, and provided with side openings ; the tubes 

 were 4\ mm. in internal diameter, and from 40 to 196 cm. in length 1 . 

 Through the tubes were drawn platinum wires either very fine or of 

 ordinary thickness. The four electrodes were amalgamated zinc 

 wires with bayonet-shaped bent ends, thrust into the side tubes ; the 

 bayonet-shaped bends gave such support that the wires only reached 

 as far as the upper surface of the main tube. The alternate closure 

 of battery and galvanometer circuits was effected by means of the 

 previously described paraffin commutator. The battery was I 18 

 zinc carbon elements. The delicacy of the galvanometer was dimi- 

 nished to the requisite degree by distancing its coils. 



I have previously stated that, with a combination polarisable in 

 two ways, the extrapolar after-current is opposed in direction to 



1 In the latter case they were made up of several shorter pieces of tube joined by 

 elastic tubing. Some of my tubes have many side openings close to one another, 

 others have a few some distance apart. Long tubes were generally so made up that 

 the side openings were near one another at the two ends, far apart in the 

 middle. 



