MAGNETISM 147 



3. What is a circuit? If a circuit is broken at any point, what 

 is the effect upon the current ? Show the use of this in control- 

 ling electrical energy. 



4. Distinguish internal and external resistance. Upon what 

 does the resistance of a conductor depend, and how? 



5. What is a shunt? When a current is divided, how much 

 does each branch receive ? Show how the supply of current to a 

 motor may be controlled by using resistance. 



6. What is a battery? In what two ways may cells be ar- 

 ranged ? What is gained in each case ? 



7. Name some- common uses of battery currents. 



8. What is measured in units of coulombs ? of amperes ? of 

 watts? of ohms? of volts? 



9. Compare the value of one watt with one horse power. 



SECTION IV 

 MAGNETISM 



170. Magnets. We have learned that one of the 

 most important electrical effects is the magnetic effect, 

 and that it is owing largely to this that electricity can 

 be so generally used to cause motion ( 154). When 

 it is used to cause motion, the magnetic force is com- 

 monly applied by means of a magnet. A magnet may be 

 described as a body which can attract iron. In other 

 words, if a magnet is brought near a bit of iron, a force 

 will act between them and cause them to move toward 

 each other, if they are free to move. 



Certain other substances may be attracted by magnets. 

 A kind of iron ore found in the earth is one of these ; 

 also steel, cobalt, and nickel. Some substances are 

 repelled (pushed away) by a magnet ; for example, zinc 

 and bismuth. It has been found that the nature of this 



