THE ELECTRIC BELL. 731 



without touching, as in'J?, and also in C and D, the lines represent- 

 ing the circuit are for a short distance replaced by dots. 



In C there are two keys in one room, t l and t 2 . If t } is pressed 

 k l rings in one room, and if t 2 is pressed & 2 rings in another room. 



D shows an arrangement by means of which one bell may be rung 

 in answer to another, a key and a bell being erected in each of the 

 two rooms between which such a communication is to be esta- 

 blished, the key t l rings & 2 , and 2 rings the bell k { . 



With a little attention the student will easily trace the path of 

 the current in each of the preceding cases. 



The crossing of wires without contact is easily accomplished, 

 either by nailing a small flat piece of wood over one wire and lead- 

 ing the second wire over the wood, or by covering each wire at the 

 crossing with a short'piece of india-rubber tubing, and tying both 

 wires together at the intersection if there is any risk of a displace- 

 ment of the tubing. 



An electric alarum like that in fig. 369 the student may easily 

 construct for himself. The electro-magnet is a horse- shoe of iron 

 wire, 5 or 6 mm thick, which is covered with four or six layers of 

 covered copper wire, O mm> 6 thick. It is fastened to a wooden board 

 by means of a long screw, 7^, which passes through a small flat piece 



7? 



< 3 



C 



.w\ /> 



FIG. 368. 



of wood, Z, 3 or 4 mm in thickness. The magnet is first placed in the 

 proper position, and by tightening the screw it is permanently 

 clamped. The keeper is cut from a piece of bar iron, and adjusted 



