314 M. P. Riess on the Passage of Electric Currents 



one, naked, at the end of a rod attached to the upper plate. To 

 generate the magneto-induction current, I used a very efficient 

 apparatus constructed by Siemens and Halske, and formerly de- 

 scribed by me*. The induction coil of this apparatus was 2i 

 inches long and 2^ inches broad ; the voltaic current in the pri- 

 mary coil, in which lay a bundle of loose iron wires, was closed 

 and broken by means of the tongue invented by Wagner, and a 

 long time ago introduced by Neef. The circuit of the induction 

 coil was closed after introducing into it a galvanometer with an 

 astatic system of needles, as well as the before- described glass 

 cylinder, in which the air was rarefied to one line of mercury 

 pressure. By means of a commutator, the coated knob in the 

 cylinder could be made positive or negative by connecting it 

 immediately with the one or the other pole of the induction coil. 

 That pole of the induction coil will be called positive which de- 

 vclopes iodine when both poles are connected by a strip of paper 

 moistened with a solution of iodide of potassium, and when the 

 primary current is broken. This pole can be easily detected by 

 first fixing two pointed elastic strips of metal with their points 

 opposite one another, so that the distance between the points 

 can be changed, and then fastening a strip of thin writing-paper 

 between the points. When the points are connected with the 

 poles of the induction coil, and made to approach one another 

 until a continuous series of sparks ensues between the two, the 

 sparks will be seen to pass on one side of the paper only, pro- 

 vided the latter is not perceptibly displaced. It follows from a 

 known property of positive electricity, that the point in contact 

 with this side of the paper is connected with the positive pole of 

 the induction coil. To set the induction apparatus in action, I 

 used one of Grove's elements ; but this current being too strong, 

 I found it necessary to pull the iron wire half-way out of the 

 primary coil. When the coated knob in the cylinder was con- 

 nected with the positive pole of the induction coil, the needle of 

 the galvanometer was instantly deflected towards a certain side, 

 and remained oscillating about 30°. On the other hand, when 

 the naked knob was positive, the needle remained for some mo- 

 ments motionless, and then moved to a few degrees on one or on 

 the other side of the zero of the scale. Hence it really appeared 

 as if the interruption current passed easily and completely from 

 the covered to the naked knob ; but not at all, or only partially 

 and with difficulty, from the naked to the covered knob. This 

 explanation of the experiment, however, was contradicted by the 

 light in the cylinder, which could be seen in both cases, and, in 

 full day with shaded cylinder, ajipeared to have the same form ; 

 further, in the case where the needle was deflected, the light 

 * PoggendorfF's Annalen, vol. xci. p. 290. 



