SUMMARY 147 



stances together. Conductors can be electrified only if held by means 

 of an insulator. 



An electrified body attracts one not electrified, then charges it with 

 its own kind of electric charge, and then repels it. 



The space around a charged body is an electric field. An insulated 

 body brought into the field is electrified by induction. 



FIG. 140. 



Generator Room of the Mississippi River Power Co. at Keokuk. A dam 4,649 feet long 

 extends across the river, and raises the water behind the dam. Some of the water 

 rushes through turbines, generating 200,000 horse power (c/. 26) of electricity. 

 About a third of this is carried to St. Louis, 137 miles away. 



When the strain in the electric field between two oppositely charged 

 bodies becomes sufficiently great, a discharge takes place, usually as 

 a spark. Discharge takes place most easily from points. 



An electric condenser consists of two conductors, such as metal 

 foil, separated by a non-conductor, such as glass. The common form 

 is the Leyden jar. 



Franklin proved that lightning is a huge electric spark. His kite was 

 electrified by induction from the charged cloud. 



Electric currents are usually generated in cells or by dynamos. 



The simple cell consists of zinc, copper (or carbon), and dilute 

 sulphuric acid. The current flows from zinc to copper inside the 



