CH. XIII] SWITCHES, FUSES, CIRCUIT BREAKERS 519 



717. Fuses. A fuse is a wire of low melting point forming 

 part of the circuit. If the current becomes too great this fuse is 

 melted, thus making a gap in the line. The fuse is then said to 

 burn out or to "blow." If the current becomes much too great as 

 in a short circuit the fuse will "blow" instantly, if however, the 

 current is only slightly larger than the fuse is designed for as for 

 example, when striking the arc in an arc lamp the fuse will not 

 "blow" instantly, and if the overload is only for a short time it will 

 not melt at all. If the overload continues for some time, however, 

 the fuse will get hotter and hotter until its melting point is reached, 

 when it will melt and open the circuit. This property of the fuse is 

 of great advantage when using arc lamps, for the temporary over- 

 load in lighting the arc lamp is unavoidable. 



718. Location and installation of fuses. Like the switch, the 

 fuses should be placed in the path of all the wires of a circuit i. e., 

 with a two-wire system two fuses, and with a three-wire system, 

 three fuses, etc. The wiring of a fuse block is the same as for a 

 switch ( 712). 



There is always a switch in the supply box from the electric 

 lighting system or from the private dynamo. In this box are also 

 fuses to open the circuit in case of accidental short-circuiting. The 

 fuse block, whether for cartridge fuses or for plug fuses should be 

 selected with care to make sure that it is of the right capacity 

 for the maximum current and conforms to the standard code. The 

 fuses are plainly marked, so there need be no mistake. 



One should not use fuses of higher capacity than the line was 

 designed for, for fear of fire or other accident. 



If the supply box is some distance from the arc lamp, many 

 careful operators have fuses as well as a switch at the supplemen- 

 tary supply box in the operating room, when a conduit or fixed 

 wires are carried from the main supply to the operating room. The 

 fuse nearest the arc lamp is preferably of somewhat less capacity 

 than the ones farther away, then if a fuse is blown it will be the 

 handiest one to renew. 



719. Fuses and the wattmeter. If but a single meter is used 

 to measure the current for arc lights, house lights, heating appara- 



