Electricity 



249 



FIG. 139. Sending a message with a cigar-box telegraph. 



have now broken the circuit, and the electricity can no 

 longer flow around the bolt to magnetize it. See if the bolt 

 will pick up the iron filings any more ; it may keep a little 

 of its magnetism even when no electricity is flowing, but the 

 magnetism will be noticeably less. When you disconnect 

 the wire so that the electricity can no longer flow through a 

 complete circuit from its source back to its source again, 

 you are said to break the circuit. 



Experiment 76. Examine the cigar-box telegraph (see 

 Appendix B) and notice that it is made on the same principle 

 as was the magnetized bolt in Experiment 75. Complete the 

 circuit through the electromagnet (the bolt wound with wire) 

 by connecting the two ends of the wire that is wrapped 

 around the bolt, with wires from the two poles of the battery. 

 By making and breaking the circuit (connecting and dis- 

 connecting one of the wires) you should be able to make the 

 lower bolt jump up and down and give the characteristic 

 click of the telegraph instrument. 



