260 SCIENCE OF HOME AND COMMUNITY 



may be produced by these dynamos, the alternating current 

 and the direct current. In alternating currents the direction 

 in which the electricity is passing changes with each half 

 revolution of the coil of wire. In direct currents the current 

 is passing in the outside circuit in the same direction all the 

 time. 



In order to produce strong currents, very powerful electro- 

 magnets are used with many coils of wire in the armature. 

 It does not matter whether the magnets are stationary and 

 the armature revolves, or whether the armature is stationary 

 and the magnets revolve. Dynamos of both forms are used. 



DEMONSTRATION 19 



Purpose. To illustrate the principle of the dynamo. 



Apparatus. Galvanometer, bar magnet, primary and second- 

 ary coil (the inner one removable), wire. 



Directions. Connect a coil of wire with a galvanometer. 

 Thrust a bar magnet into the center of the coil and watch the 

 needle. After the bar comes to rest, is there any current? 

 Remove the magnet and notice the needle. Connect a primary 

 coil of wire with a cell. Thrust this coil into the center of the 

 secondary coil used in the previous experiment. Watch the 

 needle. Remove the coil. Under what conditions is a current 

 generated in the secondary coil of wire ? 



Power to turn dynamos. Some power must be used to 

 turn the dynamos. For this purpose, water wheels, steam 

 engines, and gas engines are used. Water is the cheapest 

 of these and is very widely used. The power plants at 

 Niagara Falls are the best-known examples of the use of 

 water power. The electricity here generated may be carried 

 many miles to points where it is used. The electric current 

 is taken from Niagara a distance of 154 miles to the city of 

 Syracuse, where it is used to drive the street cars. In other 

 localities it is being carried even longer distances than this. 

 At the present time the most common means used for turn- 



