266 GENERAL SCIENCE 



being in closed circuit responds to the operator in Pitts- 

 burgh. The operator in Pittsburgh opens his switch 

 and calls the operator in Chicago and then closes his 

 switch and waits for a response. The operator in Chicago 

 opens his switch and responds to the Pittsburgh call and 

 then closes his circuit again. The Pittsburgh operator 

 now opens his switch and proceeds to send his message, 

 which the operator in Chicago writes as he hears the 

 dots and dashes. 



185. Electric Lights. Most electric lights in houses 

 are made to take a current with an E. M. F. of no volts. 

 The filament in the lamp resists the flow of the current 

 to such an extent that it is made white-hot as a result, 

 and thus radiates light and some heat. A lamp with a 

 tungsten filament takes about only one-third as much 

 current as a lamp with a carbon filament, both making 

 the same amount of light. 



Since the E. M. F. of the current delivered by a 60- 

 cycle alternating generator is 2200 volts, it is necessary 

 to have some way of reducing this 

 voltage so that the lamps can use 

 it and so it will be safe to run it 

 into the houses. A very simple 

 apparatus, called a transformer, is 

 used for the reduction of voltage. 

 The transformer consists of a 

 laminated iron ring with two coils 

 wrapped on it. These two coils 

 A xHI^MER are not connected, and when a 

 current passes through one coil an 

 induced current passes through the other coil. 



A transformer will work only when there is an alternat- 

 ing current, because it does not rotate through a mag- 



