462 FARM MOTORS 



between the poles two times reduces the force acting 

 between them 2X2 = 4 times. In other words, the 

 force is one-fourth as strong. 



606. Magnetic materials. Steel and iron are the only 

 common substances which show magnetic properties to 

 any appreciable degree. 



STATIC ELECTRICITY 



607. Static electricity. If a hard rubber rod be rubbed 

 with flannel and then brought close to a suspended pith 

 ball the ball will jump toward the rod. By rubbing the 

 rod has been electrified and the action of the charge 

 is to attract the ball. This charge of electricity is not 

 within the rod but is on the surface and is known as 

 stationary or static electricity. Another example of this 

 is rubbing a glass rod with silk. 



608. Laws of electrical attraction and repulsion. If a 

 rubber and a glass rod be excited and suspended as shown 



G/QS3 



FIG. 342 



in Fig. 342 and brought close together it will be noticed 

 that they attract each other, but if two rubber rods be 

 suspended in the same way and brought together, they 

 will repel each other. Hence the following law is ad- 

 vanced : Electrical charges of a like kind repel each other 

 and those of an unlike kind attract. 



609. Density of charge varies with form of surface. 



