FARM MOTORS 



will require a temperature of nearly 2,000 to melt the 

 copper. 



Because lead melts at such a low temperature it is 

 used as a fuse. A fuse consists of a leaden wire connected 

 in series with the circuit it is to protect, and when the 

 current becomes too excessive the lead melts out and 

 thus opens the circuit. Fuse wires, as they are called, are 

 always labeled with the number of amperes they are 

 supposed to carry. 



629. Magnetic properties of coils. Let a wire carry- 

 ing a current be formed into a small single coil and bring 

 a compass close to it. When the compass is on one side 

 of the coil it will be noticed that the N pole is attracted 

 and the S pole repelled. Change the compass to the 

 other side and the reverse will be found true. Now re- 

 verse the direction of the current and it will be found 

 that the needle acts in just the opposite manner. From 

 this it is learned that the electric coil is simply a flat 

 disk magnet with a N and a S pole, the same as any other 

 magnet. 



630. Electromagnet. When instead of forming the 

 wire which carries the current into a single loop the wire 

 is formed into several loops in the shape of a helix, a com- 

 pass brought into its field will produce the same actions 

 of the needle as in the single loop, only they will be 

 much more violent. Now, if a soft iron bar, commonly 

 known as a core, be placed within the helix, a very strong 

 magnet known as an electromagnet will be formed. The 

 lines of force of such a magnet are identical with those 

 of the bar magnet. Hence, if the electromagnet is con- 

 structed so that the lines of force can remain in iron 

 throughout their entire length, the magnet will be much 

 stronger. For this reason electromagnets are made in 

 the horseshoe form as shown by Fig. 353. 



