Appendix 



381 



then passed through the screw eye (H) and another nut 

 (/) screwed on to it to hold it in place. This nut must 



FIG. 191. The cigar-box telegraph. 



not be so tight as to prevent the free play of the bolt as its 

 head rises and falls under the influence of the vertical bolt. 

 The head of the horizontal bolt rests upon the screw eye 

 which is immediately below the head of the suspended bolt. 

 You therefore have the wrapped bolt hanging vertically 

 from the top of the box, with its head just over the head 

 of the horizontal bolt. There should be about one quarter 

 inch of space between the heads of the two bolts. An 

 electric current passing through the wires of the vertical bolt 

 will therefore lift the head of the horizontal bolt, which 

 will drop back on to the screw eye when the circuit is broken. 



