ELECTRICAL INVENTIONS 



213 



produce a click. When the current is broken, the bar springs back 

 and strikes a post with a click. If the current is made and 

 broken immediately, the two clicks sound almost as one and 

 represent a dot; if the current is allowed to run for a moment, the 

 two clicks are distinctly separate and the signal stands for a 

 dash. By various combinations of dots and dashes the letters 

 of the alphabet are indicated. The Morse Code is given (p. 214) 

 and the Continental 

 Code is shown in paren- 

 thesis where it differs 

 from the Morse Code. 



There is a sending 

 and a receiving instru- 

 ment at each station. 

 When one is receiving 

 a message, he closes a 

 switch in his sending 

 instrument, so the cur- 

 rent can pass through it 

 to battery, ground, and 

 back to the Sending FIG. 86. Telegraph instruments, (a) sending 



station(Fi g s.8 5 and86). key; ^ Diving sounder. 



When the early telegraph instruments were installed, two 

 wires were run from station to station connecting the instruments. 

 Later it was discovered that only one wire was necessary, for 

 the earth would serve to complete the circuit. Now one wire is 

 run from each instrument to a metal plate buried in moist earth; 

 this is called the ground wire. At first, too, it was difficult to 

 send messages a very long way, for it took a very powerful cur- 

 rent to overcome the resistance in a long wire. Now, relay 

 batteries that add to the strength of the passing current are 

 introduced along the way. This, of course, is impossible in 

 the long cables that carry the current under the sea from con- 

 tinent to continent, and in these a strong current must be used. 

 In 1857 a wire was laid on the bottom of the sea between Dover, 



