206 ELEMENTS OF ELECTRICAL ENGINEERING. 



ceeds a certain value, the link is heated to its fusing point, is 

 melted, and thereby causes the circuit to be broken. Fuse wire is 

 generally made of an alloy of tin and lead and it is rated at 

 about 80 per cent, of the greatest current it can carry for an 

 indefinite time. Thus a rated lo-ampere fuse wire is one which 

 can carry no more than 12.5 amperes steadily without melting. 



The melting of a fuse link is always followed by an electric 

 arc which, although of short duration, generates a quantity of 

 hot vapor, the sudden expansion of which tends to scatter the 

 melted metal. This blowing of a fuse, as it is called, is likely to 

 set fire to nearby inflammable materials, and therefore a fuse link 

 should always be enclosed in a fire-proof receptacle. This 

 receptacle is usually made of porcelain. Furthermore, when two 

 fuse links are mounted side by side, as shown in Fig. 69, they 

 should be placed in separate compartments in the porcelain re- 

 ceptacle so that the arc cannot flash across and short-circuit the 

 mains. A fuse link is sometimes enclosed in a fire -proof tube of 

 insulating material. 



Fuse wire or a fuse link is more or less sluggish in action 

 inasmuch as a considerable amount of heat is required to raise 

 the temperature of the wire or link to the fusing point. This is 

 especially true of large fuse links. Furthermore, the amount of 

 current that a fuse link will carry steadily without melting varies 

 greatly with the mode of mounting. Thus a fuse link lying flat 

 against the porcelain of a receptacle is cooled thereby, and its 

 current carrying capacity is therefore increased ; again a fuse link 

 which is mounted in a thoroughly ventilated receptacle has a 

 greater current carrying capacity than it would have if mounted 

 in a receptacle which does not permit of the circulation of air. 



The terminals to which the fuse wire or link is connected 

 absorb considerable heat, and therefore influence the rating of a 

 fuse of given size, especially if the fuse is short. 



For the above reasons the rating of a fuse is a somewhat in- 

 definite and uncertain matter. 



The electromagnetic circuit breaker, on the other hand, is very 



