1895.] on the Effects of Electric Currents in Iron, &c. 



541 



more slowly. The dip of the curve for the large battery beyond the 

 axes of the ordinates is not a real part of the phenomena. It is 

 caused by the momentum of the moving part of the galvanometer. 



In these experiments the iron has been divided in order to get rid 

 of the particular effect about which I wish to speak this evening. 

 Iron, as everybody knows, is a good conductor of electricity, not so 

 good as copper, but still much better than any other substance than 

 the metals. In a magnet, then, with a solid iron core, the outer 

 portions of the iron are in a similar position to the copper coils 

 surrounding the iron. On reversing the magnetising current, currents 

 will be induced in the iron, and these currents will delay the changes 



_^y 



SECTION ON LINE AA 



Scale 1 in. to lfoot. 



Fig. 2. 



of magnetic induction within them, and they will delay them the 

 more the deeper in the iron is the point under consideration. I have 

 in the room below a large magnet, which has been expressly con- 

 structed for the purpose of investigating the changes of induction 

 which occur at different depths in the iron when the magnetising 

 current is reversed. It is shown diagrammatically in Fig. 2. One 

 view represents the vertical section of the magnet, the other the 

 horizontal section. The magnet consists of a central cylinder of iron 

 surrounded by an annulus with a large slab of iron at each end for 

 the purpose of completing the magnetic circuit. Between the central 

 cylinder and the annulus are placed the magnetising copper coils In 



