THE CORRELATION OF THE PHYSICAL FORCES 85 



pectations of farther discoveries, and so go on pursuing, real- 

 izing, establishing, and founding new hopes again and 

 again. 



Now observe this: here is a piece of wire which I am 

 about to make into a bridge of force, that is to say, a com- 

 municator between the two ends of the battery. It is 

 copper wire only, and is therefore not magnetic of itself. 

 We will examine this wire with our magnetic needle (Fie. 

 51), and, though connected with one extreme end of the 

 battery, you see that before the circuit is completed it has 



-. J01 *-^ 



FIG. 51 



no power over the magnet. But observe it when I make 

 contact; watch the needle; see how it is swung round; and 

 notice how indifferent it becomes if I break contact again; 

 so, you see, we have this wire evidently affecting the 

 magnetic needle under these circumstances. Let me show 

 you that a little more strongly. I have here a quantity of 

 wire which has been wound into a spiral, and this will affect 

 the magnetic needle in a very curious manner, because, 

 owing to its shape, it will act very like a real magnet. The 

 copper spiral has no power over that magnetic needle at 

 present ; but if I cause the electric current to circulate through 

 it, by bringing the two ends of the battery in contact with 

 the ends of the wire which forms the spiral, what will 

 happen? Why, one end of the needle is most powerfully 

 drawn to it; and if I take the other end of the needle, it 

 is repelled; so, you see, I have produced exactly the same 

 phenomena as I had with the bar magnet, one end attracting 

 and the other repelling. Is not this, then, curious to 

 see, that we can construct a magnet of copper? Further- 

 more, if I take an iron bar, and put it inside the coil, so 

 long as there is no electric current circulating round, it has 



