256 Messrs. Porter and Morris on the Measurement 



12. When a current is passed through cuprous selenide 

 between platinum electrodes which are covered with moistened 

 paper, red Se is deposited on the anode paper and metallic 

 copper on the kathode paper (Exp. 21). 



13. If cuprous selenide is made the kathode and a strip of 

 Pt the anode in water, red Se mixed with detached particles 

 of the selenide is deposited in the water (Exp. 22). 



14. With a mixture of cuprous selenide and Se the quantity 

 of detached black particles is greatly diminished, but the weight 

 of the separated red Se appears to be greater than that re- 

 quired by the law of electrolytic decomposition. Possibly the 

 current has the effect of converting pure crystalline Se when 

 moistened into the red amorphous variety (Exps. 23, 24). 



15. The hydrogen produced by the action of zinc upon 

 dilute sulphuric acid has no effect upon the cuprous selenide 

 nor upon Se in contact with the zinc (Exps. 25, 26). 



16. The photo-electric currents sometimes set up when 

 light falls upon Se are dependent upon the presence of mois- 

 ture and are no doubt of voltaic or chemical origin. (Exp. 27). 



17. Experiments which have seemed to place well-annealed 

 selenium above platinum in the thermo-electric series were 

 vitiated by the presence of moisture. Dry Se is always below 

 Pt (Exps*. 28-37). 



XX. The Measurement of Varying Currents in Inductive 

 Circuits. By Alfred W. Porter, B.Sc, Demonstrator of 

 Physics, University College, London, and David K. Morris, 

 1851 Exhibition Scholar, University College, London*. 



THE following paper describes an application of the principle 

 of the potentiometer to the measurements of rapidly 

 varying (but not alternating) differences of potential ; and 

 hence, to the measurement of the currents to which they give 

 rise : a problem which presented itself in the course of an 

 investigation into the propagation of magnetic induction in 

 iron which was begun more than a year ago. This investi- 

 gation we hope shortly to publish. 



A non-inductive resistance r (fig. 1) is connected in series 

 with a magnetizing coil the current in which we desired to 

 measure ; so that the potential difference between the terminals 

 of r is at any instant a measure of the current in the coil. 

 When the current is steady, this difference of potential can 



* Communicated by the Authors. 



