USES OF ELECTRICAL ENERGY 165 



The circuit is completed by allowing the current to 

 return through the earth. A battery is commonly used 

 on the circuit to overcome the resistance of the wire. 



189. The Telegraph. The sender of a telegraph 

 message uses a key k (Fig. 121), that simply closes and 

 opens a circuit. In the distant office is a sounder s, by 

 which the message is received. Pressing on k closes 

 the circuit; the current then magnetizes the electro- 

 magnet m ; this draws the armature a so that it strikes 



feu 



el 



FIG. 121 



the frame f, making a clicking sound. The key being 

 lifted, m is demagnetized, and a spring pulls a till it 

 strikes the frame above, making a different sound. 



If these two sounds are separated by only an instant, 

 a dot is said to be made; when a longer bit of time 

 comes between them, the report is a dash. Every letter 

 is represented by a different group of dots and dashes ; 

 the sender can make each at will, by pressing his key 

 for a shorter or longer time. Any telegraph operator 

 must learn to know each letter instantly by the sound 

 of the dots and dashes which represent it. The current, 

 as in the telephone, goes through one wire and returns 

 to the battery through the earth (Fig. 121). As a rule, 

 the batteries are composed of a few gravity cells placed 

 at intervals along the circuit. 



