INDUCED CURRENTS 161 



the coarse wire coil; a current is induced in the fine 

 wire coil during each separate change. This induced 

 current has high potential, and because the changes 

 are so rapid the effect is nearly that of a constant 

 (though not direct) flow. Coils of this sort are used 

 in " medical batteries," Rontgen ray apparatus, wireless 

 telegraphy, etc. 



QUESTIONS 



1. How, in general, are induced currents produced? How does 

 the electro-motive force of these currents vary? 



2. What is a dynamo ? What are its important parts ? 



3. How is the energy for running a dynamo applied? In what 

 part of the dynamo is the current produced ? How is the current 

 taken from the armature to the outside part of the circuit ? 



4. What is an alternating current? a direct current? Why 

 should a dynamo current alternate ? Can such a current be used ? 

 How is it changed to a direct current ? 



5. What advantage have electro-magnet dynamos over those 

 using permanent magnets ? What advantage have the latter over 

 the former ? 



6. Name uses of currents from electro-magnet dynamos. For 

 what are permanent-magnet dynamos used ? How do the coils of 

 the electro-magnets in a dynamo receive their current? 



7. What is a transformer ? Describe its structure. For what 

 purposes are transformers used ? 



8. Through which coil in a transformer would you pass a cur- 

 rent that was to be changed to a higher potential? to a lower 

 potential ? What sort of a current is used in either case ? 



9. Describe the induction coil. For what purpose is it gener- 

 ally used ? Show how an induction coil may use a direct current, 

 as the transformer uses an alternating current. 



10. In which of the two coils is the current induced ? State 

 carefully the condition under which a current will be induced in 

 this coil. 



