380 Appendk 



The fuse gap (D) consists either of two parts of an old 

 knife switch, the knife removed, or of two brass binding 

 posts. Across it a piece of 4-ampere fuse wire is always 

 kept as a protection to the more expensive plug and cart- 

 ridge fuses. Between the resistance wire (7, /, K) and the 

 wall should be either slate or sheet asbestos, double thick- 

 ness. Under the fuse gap the table should be protected 

 by galvanized iron so that the melted bits of fuse wire can 

 set nothing on fire when the fuse wire burns out. 



B. CONSTRUCTION OF THE CIGAR-BOX TELEGRAPH 



The " cigar-box telegraph" shown on page 381 is made 

 as follows : An iron machine bolt (A ) is wound with about 

 three layers of No. 24 insulated copper magnet wire, the 

 two ends of the wire (B, B) projecting. The threaded 

 end of the bolt (C) is not wound. A nut (D) is screwed 

 on the bolt as far down as the wire wrapping. The threaded 

 end is then pushed up through the hole in the top of the 

 cigar box as that stands on its edge. Another nut (E) 

 is then screwed on to the bolt, holding it in position. The 

 bolt can now be raised or lowered and tightened firmly in 

 position by adjusting the two nuts (D and ), one above 

 and one below the wood. 



A screw eye (F), large enough to form a rest for the head 

 of another machine bolt (<7), is screwed into the back of the 

 box about three fourths of an inch below the head of the 

 suspended bolt (^4). Two or three inches away, at a slightly 

 higher level, another screw eye (H) is screwed into the back 

 of the cigar box. This screw eye must have an opening 

 large enough to permit an iron machine bolt (G) to pass 

 through it easily. A nut (/) is screwed down on the threaded 

 end of a machine bolt until about an inch of the bolt pro- 

 jects beyond the nut. This projecting part of the bolt is 



