542 



Dr. John Ropkinson 



[April 26, 



the solid cylinder holes are drilled at right angles to each other, and 

 these holes meet within the iron. A strand of copper wire has been 

 drawn in through the holes, and has subsequently been connected up 

 so that the coils of the strand are in series with each other. In this 

 way we have a coil surrounding a portion of the iron at 3, at 2, and 

 at 1, of approximately the same area. If these coils then are con- 

 nected to a galvanometer they will measure the changes of induction 

 which occur in the areas 3, 2 and 1 enclosed, and we shall be able to 

 show you the times at which the currents in the iron permit the effect 

 of reversal in the copper coils to be manifest. We have, iD addition, 

 a coil, marked 4, in the outer magnet. Now, when the currents in 



__. 000. a 140 100 120 



Time in Secorids after reversal 



Fig. 3. 



the large copper coils are reversed, the magnetism or the induction in 

 the coils 1, 2, 3 and 4 will also be reversed, but more slowly, and 

 currents will be induced in these coils. I propose to show you the 

 way in which these currents occur and are modified by the currents 

 in the iron. We shall see that the changes in induction induce 

 currents in the iron, and that these in their turn delay the changes in 

 induction. 



In order to give you a better idea of what is happening, we have 

 provided three galvanometers. The galvanometers throw their images 

 upon the screen at different altitudes. Now, what I want you to 

 observe is this— that the disturbance of induction in the outermost 



