Scientific Lectures. 179 



bar. Close behind the magnet is this large screen. I now place in 

 another ordinary lantern this glass plate, on which I have fixed the 

 curves of a small magnet, whose form was exactly proportioned to the 

 length and diameter of that long iron core of the electro-magnet. 

 Yon now see on the screen the greatly magnified image of these lines, 

 which appear to me actually as if formed by that electro-magnet, on 

 filings which have been spread over the screen, but to those at a 

 greater distance they seem to emanate from the core of the magnet, 

 for the axis of the image of the lines and the axis of the magnet 

 coincide. These lines, therefore, can serve as accurate guides in work- 

 ing with the magnet's actual influence at a distance. 



I hold in my hand a light wooden rod, attached to whose end is a 

 wing of eighteen inches in diameter, formed of thick copper wire. 

 Part of a long wire was bent into this ring, and the remaining lengths 

 were carried clown the sides of the rod and firmly tied to it. On the 

 ends of the wire I fasten these binding-screws, from which wires can 

 be led to the galvanometer lantern. Thus I have made an electric 

 wand, as wonderful as «that of Prospero, for with it I also can catch 

 the stuff that lightning is made of, and send it where I will. We 

 will now begin work on these lines, or rather on what they so clearly 

 map out, the rays of directive influence surrounding this huge magnet. 



There are two ways in which I can cut through these " lines of 

 force." First, we can, as you see, do so by moving this ring's center 

 over the line which is the prolongation of the axis of the magnet ; 

 thus, or second!} 7 , by rotating this ring around an axis, thus — and I 

 cut the lines fore and aft. By rotating this ring around any axis I 

 can cut these lines as well ; but I beg you to remark that during the 

 following experiments, in which I will explore the magnetic condi- 

 tion of the space surrounding this bar — I will only rotate the mass I 

 iise around a vertical axis. The reason of this will soon be made 

 evident to you. The plane of the ring now stands at right angles, 

 square with the length of the magnet, and observe the lines appearing 

 to go through it. My assistant will now connect the ends of my wire- 

 wand with the conductor leading to the galvanometer. I will now 

 move the plane of the ring toward the pole of the magnet. See 

 how the galvanometer needle responds; its marked end moving 

 upwards. The needle has now come to rest at its first position, 

 though the magnet is active and the ring is, as during its motion, 

 connected with the galvanometer, I wish you to remember this 

 fact. I now move the ring away from the magnet ; and observe how 

 the needle moves as I recede, and it now rotates in the opposite direc- 



