250 



Common Science 



FIG. 140. Connecting up a real telegraph instrument. 



In this experiment it does not matter how long the wires 

 are if the batteries are strong enough. Of course it makes 

 no difference where you break the circuit. So you could 

 have the batteries in the laboratory and the cigar box a 

 hundred miles away, with the wire going from the batteries 

 to the bolt and back again. Then if you made and broke 

 the circuit at the laboratory, the instrument would click a 

 hundred miles away. If you want to, you may take the 

 cigar-box telegraph out into the yard, leaving the batteries 

 in the laboratory, while you try to telegraph this short 

 distance. 



Examine a regular telegraph instrument. Trace the wire 

 from one binding post, around the coil and through the key, 

 back to the other binding post, and notice how pushing down 

 the key completes the circuit and how raising it up breaks 

 the circuit. 



Experiment 77. Connect two regular telegraph instru- 

 ments, leaving one at each end of the long laboratory table. 

 Make the connections as follows : 



